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<chapter id="plpgsql"> 
  <title><application>PL/pgSQL</application> - <acronym>SQL</acronym> Procedural Language</title>

 <indexterm zone="plpgsql">
  <primary>PL/pgSQL</primary>
 </indexterm>

 <para>
  <application>PL/pgSQL</application> is a loadable procedural
  language for the <productname>PostgreSQL</productname> database
  system.  The design goals of <application>PL/pgSQL</> were to create
  a loadable procedural language that

    <itemizedlist>
     <listitem>
      <para>
       can be used to create functions and trigger procedures,
      </para>
     </listitem>
     <listitem>
      <para>
       adds control structures to the <acronym>SQL</acronym> language,
      </para>
     </listitem>
     <listitem>
      <para>
       can perform complex computations,
      </para>
     </listitem>
     <listitem>
      <para>
       inherits all user-defined types, functions, and operators,
      </para>
     </listitem>
     <listitem>
      <para>
       can be defined to be trusted by the server,
      </para>
     </listitem>
     <listitem>
      <para>
       is easy to use.
      </para>
     </listitem>
    </itemizedlist>
   </para>

  <sect1 id="plpgsql-overview">
   <title>Overview</title>

   <para>
    The <application>PL/pgSQL</> call handler parses the function's source text and
    produces an internal binary instruction tree the first time the
    function is called (within each session).  The instruction tree
    fully translates the 
    <application>PL/pgSQL</> statement structure, but individual
    <acronym>SQL</acronym> expressions and <acronym>SQL</acronym> commands
    used in the function are not translated immediately.
   </para>

   <para>
    As each expression and <acronym>SQL</acronym> command is first
    used in the function, the <application>PL/pgSQL</> interpreter
    creates a prepared execution plan (using the
    <acronym>SPI</acronym> manager's <function>SPI_prepare</function>
    and <function>SPI_saveplan</function>
    functions).<indexterm><primary>preparing a query</><secondary>in
    PL/pgSQL</></> Subsequent visits to that expression or command
    reuse the prepared plan.  Thus, a function with conditional code
    that contains many statements for which execution plans might be
    required will only prepare and save those plans that are really
    used during the lifetime of the database connection.  This can
    substantially reduce the total amount of time required to parse,
    and generate execution plans for the statements in a
    <application>PL/pgSQL</> function. A disadvantage is that errors
    in a specific expression or command may not be detected until that
    part of the function is reached in execution.
   </para>

   <para>
    Once <application>PL/pgSQL</> has made an execution plan for a particular
    command in a function, it will reuse that plan for the life of the
    database connection.  This is usually a win for performance, but it
    can cause some problems if you dynamically
    alter your database schema. For example:

<programlisting>
CREATE FUNCTION populate() RETURNS integer AS $$
DECLARE
    -- declarations
BEGIN
    PERFORM my_function();
END;
$$ LANGUAGE plpgsql;
</programlisting>

    If you execute the above function, it will reference the OID for
    <function>my_function()</function> in the execution plan produced for
    the <command>PERFORM</command> statement. Later, if you
    drop and recreate <function>my_function()</function>, then
    <function>populate()</function> will not be able to find
    <function>my_function()</function> anymore. You would then have to
    recreate <function>populate()</function>, or at least start a new
    database session so that it will be compiled afresh. Another way
    to avoid this problem is to use <command>CREATE OR REPLACE
    FUNCTION</command> when updating the definition of
    <function>my_function</function> (when a function is
    <quote>replaced</quote>, its OID is not changed).
   </para>

   <para>
    Because <application>PL/pgSQL</application> saves execution plans
        in this way, SQL commands that appear directly in a
        <application>PL/pgSQL</application> function must refer to the
        same tables and columns on every execution; that is, you cannot use
        a parameter as the name of a table or column in an SQL command.  To get
        around this restriction, you can construct dynamic commands using
        the <application>PL/pgSQL</application> <command>EXECUTE</command>
        statement --- at the price of constructing a new execution plan on
        every execution.
   </para>

   <note>
        <para>
         The <application>PL/pgSQL</application>
         <command>EXECUTE</command> statement is not related to the
         <xref linkend="sql-execute" endterm="sql-execute-title"> SQL
         statement supported by the
         <productname>PostgreSQL</productname> server. The server's
         <command>EXECUTE</command> statement cannot be used within
         <application>PL/pgSQL</> functions (and is not needed).
        </para>
   </note>

   <para>
    Except for input/output conversion and calculation functions
    for user-defined types, anything that can be defined in C language
    functions can also be done with <application>PL/pgSQL</application>.
    For example, it is possible to
    create complex conditional computation functions and later use
    them to define operators or use them in index expressions.
   </para>

  <sect2 id="plpgsql-advantages">
   <title>Advantages of Using <application>PL/pgSQL</application></title>

    <para>
     <acronym>SQL</acronym> is the language <productname>PostgreSQL</>
     (and most other relational databases) use as query language. It's
     portable and easy to learn. But every <acronym>SQL</acronym>
     statement must be executed individually by the database server.
    </para>

    <para>
     That means that your client application must send each query to
     the database server, wait for it to be processed, receive the
     results, do some computation, then send other queries to the
     server. All this incurs interprocess communication and may also
     incur network overhead if your client is on a different machine
     than the database server.
    </para>

    <para>
     With <application>PL/pgSQL</application> you can group a block of computation and a
     series of queries <emphasis>inside</emphasis> the
     database server, thus having the power of a procedural
     language and the ease of use of SQL, but saving lots of
     time because you don't have the whole client/server
     communication overhead. This can make for a
     considerable performance increase.
    </para>

    <para>
     Also, with <application>PL/pgSQL</application> you can use all
     the data types, operators and functions of SQL.
    </para>
  </sect2>

  <sect2 id="plpgsql-args-results">
   <title>Supported Argument and Result Data Types</title>

    <para>
     Functions written in <application>PL/pgSQL</application> can accept
     as arguments any scalar or array data type supported by the server,
     and they can return a result of any of these types.  They can also
     accept or return any composite type (row type) specified by name.
     It is also possible to declare a <application>PL/pgSQL</application>
     function as returning <type>record</>, which means that the result
     is a row type whose columns are determined by specification in the
     calling query, as discussed in <xref linkend="queries-tablefunctions">.
    </para>

    <para>
     <application>PL/pgSQL</> functions may also be declared to accept
     and return the polymorphic types
     <type>anyelement</type> and <type>anyarray</type>.  The actual
     data types handled by a polymorphic function can vary from call to
     call, as discussed in <xref linkend="extend-types-polymorphic">.
     An example is shown in <xref linkend="plpgsql-declaration-aliases">.
    </para>

    <para>
     <application>PL/pgSQL</> functions can also be declared to return
     a <quote>set</>, or table, of any data type they can return a single
     instance of.  Such a function generates its output by executing
     <literal>RETURN NEXT</> for each desired element of the result set.
    </para>

    <para>
     Finally, a <application>PL/pgSQL</> function may be declared to return
     <type>void</> if it has no useful return value.
    </para>
  </sect2>
 </sect1>

 <sect1 id="plpgsql-development-tips">
  <title>Tips for Developing in <application>PL/pgSQL</application></title>

   <para>
    One good way to develop in
    <application>PL/pgSQL</> is to use the text editor of your
    choice to create your functions, and in another window, use
    <application>psql</application> to load and test those functions.
    If you are doing it this way, it
    is a good idea to write the function using <command>CREATE OR
    REPLACE FUNCTION</>. That way you can just reload the file to update
    the function definition.  For example:
<programlisting>
CREATE OR REPLACE FUNCTION testfunc(integer) RETURNS integer AS '
          ....
end;
' LANGUAGE plpgsql;
</programlisting>
   </para>

   <para>
    While running <application>psql</application>, you can load or reload such
    a function definition file with
<programlisting>
\i filename.sql
</programlisting>
    and then immediately issue SQL commands to test the function.
   </para>

   <para>
    Another good way to develop in <application>PL/pgSQL</> is using a
    GUI database access tool that facilitates development in a
    procedural language. One example of such as a tool is
    <application>PgAccess</>, although others exist. These tools often
    provide convenient features such as escaping single quotes and
    making it easier to recreate and debug functions.
   </para>

  <sect2 id="plpgsql-quote-tips">
   <title>Handling of Quotation Marks</title>

  <para>
   Since the code of a <application>PL/pgSQL</> function is specified in
   <command>CREATE FUNCTION</command> as a string literal, single
   quotes inside the function body must be escaped by doubling them
    unless the string literal comprising the function body is dollar
    quoted.
    </para>

    <para>
    Doubling can lead to incomprehensible code at times, especially if
    you are writing a function that generates other functions, as in the
    example in <xref linkend="plpgsql-statements-executing-dyn">.  This
    chart may be useful when translating pre-dollar quoting code into
    something that is comprehensible.
  </para>

  <variablelist>
   <varlistentry>
    <term>1 quotation mark</term>
    <listitem>
     <para>
      To begin and end the function body, for example:
<programlisting>
CREATE FUNCTION foo() RETURNS integer AS '...'
    LANGUAGE plpgsql;
</programlisting>
      Anywhere within the function body, quotation marks <emphasis>must</>
      appear in pairs.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>2 quotation marks</term>
    <listitem>
     <para>
      For string literals inside the function body, for example:
<programlisting>
a_output := ''Blah'';
SELECT * FROM users WHERE f_name=''foobar'';
</programlisting>
      The second line is seen by <application>PL/pgSQL</> as
<programlisting>
SELECT * FROM users WHERE f_name='foobar';
</programlisting>
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>4 quotation marks</term>
    <listitem>
     <para>
      When you need a single quotation mark in a string constant inside the function
      body, for example:
<programlisting>
a_output := a_output || '' AND name LIKE ''''foobar'''' AND xyz''
</programlisting>
      The value actually appended to <literal>a_output</literal> would be:
      <literal> AND name LIKE 'foobar' AND xyz</literal>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>6 quotation marks</term>
    <listitem>
     <para>
      When a single quotation mark in a string inside the function body is
      adjacent to the end of that string constant, for example:
<programlisting>
a_output := a_output || '' AND name LIKE ''''foobar''''''
</programlisting>
      The value appended to <literal>a_output</literal> would then be:
      <literal> AND name LIKE 'foobar'</literal>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>10 quotation marks</term>
    <listitem>
     <para>
      When you want two single quotation marks in a string constant (which
      accounts for 8 quotation marks) and this is adjacent to the end of that
      string constant (2 more).  You will probably only need that if
      you are writing a function that generates other functions.  For
      example:
<programlisting>
a_output := a_output || '' if v_'' || 
    referrer_keys.kind || '' like '''''''''' 
    || referrer_keys.key_string || '''''''''' 
    then return ''''''  || referrer_keys.referrer_type 
    || ''''''; end if;''; 
</programlisting>
      The value of <literal>a_output</literal> would then be:
<programlisting>
if v_... like ''...'' then return ''...''; end if;
</programlisting>
     </para>
    </listitem>
   </varlistentry>
  </variablelist>

   <para>
    A different approach is to escape quotation marks in the function body
    with a backslash rather than by doubling them.  With this method
    you'll find yourself writing things like <literal>\'\'</> instead
    of <literal>''''</>.  Some find this easier to keep track of, some
    do not.
   </para>
  </sect2>
 </sect1>

 <sect1 id="plpgsql-structure">
  <title>Structure of <application>PL/pgSQL</application></title>

  <para>
   <application>PL/pgSQL</application> is a block-structured language.
   The complete text of a function definition must be a
   <firstterm>block</>. A block is defined as:

<synopsis>
<optional> &lt;&lt;<replaceable>label</replaceable>&gt;&gt; </optional>
<optional> DECLARE
    <replaceable>declarations</replaceable> </optional>
BEGIN
    <replaceable>statements</replaceable>
END;
</synopsis>
    </para>

    <para>
     Each declaration and each statement within a block is terminated
     by a semicolon.
    </para>

    <para>
     All key words and identifiers can be written in mixed upper and
     lower case.  Identifiers are implicitly converted to lowercase
     unless double-quoted.
    </para>

    <para>
     There are two types of comments in <application>PL/pgSQL</>. A double dash (<literal>--</literal>)
     starts a comment that extends to the end of the line. A <literal>/*</literal>
     starts a block comment that extends to the next occurrence of <literal>*/</literal>.
     Block comments cannot be nested, but double dash comments can be
     enclosed into a block comment and a double dash can hide
     the block comment delimiters <literal>/*</literal> and <literal>*/</literal>.
    </para>

    <para>
     Any statement in the statement section of a block
     can be a <firstterm>subblock</>.  Subblocks can be used for
     logical grouping or to localize variables to a small group
     of statements.
    </para>

    <para>
     The variables declared in the declarations section preceding a
     block are initialized to their default values every time the
     block is entered, not only once per function call. For example:
<programlisting>
CREATE FUNCTION somefunc() RETURNS integer AS $$
DECLARE
    quantity integer := 30;
BEGIN
    RAISE NOTICE 'Quantity here is %', quantity;  -- Quantity here is 30
    quantity := 50;
    --
    -- Create a subblock
    --
    DECLARE
        quantity integer := 80;
    BEGIN
        RAISE NOTICE 'Quantity here is %', quantity;  -- Quantity here is 80
    END;

    RAISE NOTICE 'Quantity here is %', quantity;  -- Quantity here is 50

    RETURN quantity;
END;
$$ LANGUAGE plpgsql;
</programlisting>
    </para>

    <para>
     It is important not to confuse the use of <command>BEGIN</>/<command>END</> for
     grouping statements in <application>PL/pgSQL</> with the database commands for
     transaction control.  <application>PL/pgSQL</>'s <command>BEGIN</>/<command>END</> are only for grouping;
     they do not start or end a transaction.  Functions and trigger procedures
     are always executed within a transaction established by an outer query
     --- they cannot start or commit transactions, since
     <productname>PostgreSQL</productname> does not have nested transactions.
    </para>
  </sect1>

  <sect1 id="plpgsql-declarations">
    <title>Declarations</title>

    <para>
     All variables used in a block must be declared in the
     declarations section of the block. 
     (The only exception is that the loop variable of a <literal>FOR</> loop iterating
     over a range of integer values is automatically declared as an integer
     variable.)
    </para>

    <para>
     <application>PL/pgSQL</> variables can have any SQL data type, such as
     <type>integer</type>, <type>varchar</type>, and
     <type>char</type>.
    </para>

    <para>
     Here are some examples of variable declarations:
<programlisting>
user_id integer;
quantity numeric(5);
url varchar;
myrow tablename%ROWTYPE;
myfield tablename.columnname%TYPE;
arow RECORD;
</programlisting>
    </para>

    <para>
     The general syntax of a variable declaration is:
<synopsis>
<replaceable>name</replaceable> <optional> CONSTANT </optional> <replaceable>type</replaceable> <optional> NOT NULL </optional> <optional> { DEFAULT | := } <replaceable>expression</replaceable> </optional>;
</synopsis>
      The <literal>DEFAULT</> clause, if given, specifies the initial value assigned
      to the variable when the block is entered.  If the <literal>DEFAULT</> clause
      is not given then the variable is initialized to the
      <acronym>SQL</acronym> null value. 
      The <literal>CONSTANT</> option prevents the variable from being assigned to,
      so that its value remains constant for the duration of the block.
      If <literal>NOT NULL</>
      is specified, an assignment of a null value results in a run-time
      error. All variables declared as <literal>NOT NULL</>
      must have a nonnull default value specified.
     </para>

     <para>
      The default value is evaluated every time the block is entered. So,
      for example, assigning <literal>'now'</literal> to a variable of type
      <type>timestamp</type> causes the variable to have the
      time of the current function call, not the time when the function was
      precompiled.
     </para>

     <para>
      Examples:
<programlisting>
quantity integer DEFAULT 32;
url varchar := 'http://mysite.com';
user_id CONSTANT integer := 10;
</programlisting>
     </para>

    <sect2 id="plpgsql-declaration-aliases">
     <title>Aliases for Function Parameters</title>

<synopsis>
<replaceable>name</replaceable> ALIAS FOR $<replaceable>n</replaceable>;
</synopsis>

     <para>
      Parameters passed to functions are named with the identifiers
      <literal>$1</literal>, <literal>$2</literal>,
      etc.  Optionally, aliases can be declared for
      <literal>$<replaceable>n</replaceable></literal>
      parameter names for increased readability.  Either the alias or the
      numeric identifier can then be used to refer to the parameter value.
      Some examples:
<programlisting>
CREATE FUNCTION sales_tax(real) RETURNS real AS $$
DECLARE
    subtotal ALIAS FOR $1;
BEGIN
    RETURN subtotal * 0.06;
END;
$$ LANGUAGE plpgsql;


CREATE FUNCTION instr(varchar, integer) RETURNS integer AS $$
DECLARE
    v_string ALIAS FOR $1;
    index ALIAS FOR $2;
BEGIN
    -- some computations here
END;
$$ LANGUAGE plpgsql;


CREATE FUNCTION concat_selected_fields(tablename) RETURNS text AS $$
DECLARE
    in_t ALIAS FOR $1;
BEGIN
    RETURN in_t.f1 || in_t.f3 || in_t.f5 || in_t.f7;
END;
$$ LANGUAGE plpgsql;
</programlisting>
     </para>

     <para>
      When the return type of a <application>PL/pgSQL</application>
      function is declared as a polymorphic type (<type>anyelement</type>
      or <type>anyarray</type>), a special parameter <literal>$0</literal>
      is created.  Its data type is the actual return type of the function,
      as deduced from the actual input types (see <xref
      linkend="extend-types-polymorphic">).
      This allows the function to access its actual return type
      as shown in <xref linkend="plpgsql-declaration-type">.
      <literal>$0</literal> is initialized to null and can be modified by
      the function, so it can be used to hold the return value if desired,
      though that is not required.  <literal>$0</literal> can also be
      given an alias.  For example, this function works on any data type
      that has a <literal>+</> operator:
<programlisting>
CREATE FUNCTION add_three_values(anyelement, anyelement, anyelement)
RETURNS anyelement AS $$
DECLARE
    result ALIAS FOR $0;
    first ALIAS FOR $1;
    second ALIAS FOR $2;
    third ALIAS FOR $3;
BEGIN
    result := first + second + third;
    RETURN result;
END;
$$ LANGUAGE plpgsql;
</programlisting>
     </para>
    </sect2>

  <sect2 id="plpgsql-declaration-type">
   <title>Copying Types</title>

<synopsis>
<replaceable>variable</replaceable>%TYPE
</synopsis>

   <para>
    <literal>%TYPE</literal> provides the data type of a variable or
    table column. You can use this to declare variables that will hold
    database values. For example, let's say you have a column named
    <literal>user_id</literal> in your <literal>users</literal>
    table. To declare a variable with the same data type as
    <literal>users.user_id</> you write:
<programlisting>
user_id users.user_id%TYPE;
</programlisting>
   </para>

   <para>
    By using <literal>%TYPE</literal> you don't need to know the data
    type of the structure you are referencing, and most importantly,
    if the data type of the referenced item changes in the future (for
    instance: you change the type of <literal>user_id</>
    from <type>integer</type> to <type>real</type>), you may not need
    to change your function definition.
   </para>

   <para>
    <literal>%TYPE</literal> is particularly valuable in polymorphic
    functions, since the data types needed for internal variables may
    change from one call to the next.  Appropriate variables can be
    created by applying <literal>%TYPE</literal> to the function's
    arguments or result placeholders.
   </para>

  </sect2>

    <sect2 id="plpgsql-declaration-rowtypes">
     <title>Row Types</title>

<synopsis>
<replaceable>name</replaceable> <replaceable>table_name</replaceable><literal>%ROWTYPE</literal>;
<replaceable>name</replaceable> <replaceable>composite_type_name</replaceable>;
</synopsis>

   <para>
    A variable of a composite type is called a <firstterm>row</>
    variable (or <firstterm>row-type</> variable).  Such a variable
    can hold a whole row of a <command>SELECT</> or <command>FOR</>
    query result, so long as that query's column set matches the
    declared type of the variable.
    The individual fields of the row value
    are accessed using the usual dot notation, for example
    <literal>rowvar.field</literal>.
   </para>

   <para>
    A row variable can be declared to have the same type as the rows of
    an existing table or view, by using the
    <replaceable>table_name</replaceable><literal>%ROWTYPE</literal>
    notation; or it can be declared by giving a composite type's name.
    (Since every table has an associated composite type of the same name,
    it actually does not matter in <productname>PostgreSQL</> whether you
    write <literal>%ROWTYPE</literal> or not.  But the form with
    <literal>%ROWTYPE</literal> is more portable.)
   </para>

   <para>
    Parameters to a function can be
    composite types (complete table rows). In that case, the
    corresponding identifier <literal>$<replaceable>n</replaceable></> will be a row variable, and fields can
    be selected from it, for example <literal>$1.user_id</literal>.
   </para>

   <para>
    Only the user-defined columns of a table row are accessible in a
    row-type variable, not the OID or other system columns (because the
    row could be from a view).  The fields of the row type inherit the
    table's field size or precision for data types such as
    <type>char(<replaceable>n</>)</type>.
   </para>

   <para>
    Here is an example of using composite types:
<programlisting>
CREATE FUNCTION use_two_tables(tablename) RETURNS text AS $$
DECLARE
    in_t ALIAS FOR $1;
    use_t table2name%ROWTYPE;
BEGIN
    SELECT * INTO use_t FROM table2name WHERE ... ;
    RETURN in_t.f1 || use_t.f3 || in_t.f5 || use_t.f7;
END;
$$ LANGUAGE plpgsql;

SELECT use_two_tables(t.*) FROM tablename t WHERE ... ;
</programlisting>
   </para>
  </sect2>

  <sect2 id="plpgsql-declaration-records">
   <title>Record Types</title>

<synopsis>
<replaceable>name</replaceable> RECORD;
</synopsis>

   <para>
    Record variables are similar to row-type variables, but they have no
    predefined structure.  They take on the actual row structure of the
    row they are assigned during a <command>SELECT</> or <command>FOR</> command.  The substructure
    of a record variable can change each time it is assigned to.
    A consequence of this is that until a record variable is first assigned
    to, it has no substructure, and any attempt to access a
    field in it will draw a run-time error.
   </para>

   <para>
    Note that <literal>RECORD</> is not a true data type, only a placeholder.
    One should also realize that when a <application>PL/pgSQL</application>
    function is declared to return type <type>record</>, this is not quite the
    same concept as a record variable, even though such a function may well
    use a record variable to hold its result.  In both cases the actual row
    structure is unknown when the function is written, but for a function
    returning <type>record</> the actual structure is determined when the
    calling query is parsed, whereas a record variable can change its row
    structure on-the-fly.
   </para>
  </sect2>

  <sect2 id="plpgsql-declaration-renaming-vars">
   <title><literal>RENAME</></title>

<synopsis>
RENAME <replaceable>oldname</replaceable> TO <replaceable>newname</replaceable>;
</synopsis>

   <para>
    Using the <literal>RENAME</literal> declaration you can change the
    name of a variable, record or row. This is primarily useful if
    <varname>NEW</varname> or <varname>OLD</varname> should be
    referenced by another name inside a trigger procedure.  See also
    <literal>ALIAS</literal>.
   </para>

   <para>
    Examples:
<programlisting>
RENAME id TO user_id;
RENAME this_var TO that_var;
</programlisting>
   </para>

    <note>
     <para>
      <literal>RENAME</literal> appears to be broken as of
      <productname>PostgreSQL</> 7.3.  Fixing this is of low priority,
      since <literal>ALIAS</literal> covers most of the practical uses
      of <literal>RENAME</literal>.
     </para>
    </note>
   </sect2>
  </sect1>

  <sect1 id="plpgsql-expressions">
  <title>Expressions</title>

    <para>
     All expressions used in <application>PL/pgSQL</application>
     statements are processed using the server's regular
     <acronym>SQL</acronym> executor. Expressions that appear to
     contain constants may in fact require run-time evaluation
     (e.g., <literal>'now'</literal> for the <type>timestamp</type>
     type) so it is impossible for the
     <application>PL/pgSQL</application> parser to identify real
     constant values other than the key word <literal>NULL</>. All expressions are
     evaluated internally by executing a query
<synopsis>
SELECT <replaceable>expression</replaceable>
</synopsis>
     using the <acronym>SPI</acronym> manager. For evaluation,
     occurrences of <application>PL/pgSQL</application> variable
     identifiers are replaced by parameters, and the actual values from
     the variables are passed to the executor in the parameter array.
     This allows the query plan for the <command>SELECT</command> to
     be prepared just once and then reused for subsequent
     evaluations.
    </para>

    <para>
     The evaluation done by the <productname>PostgreSQL</productname>
     main parser has some side
     effects on the interpretation of constant values. In detail there
     is a difference between what these two functions do:

<programlisting>
CREATE FUNCTION logfunc1(text) RETURNS timestamp AS $$
    DECLARE
        logtxt ALIAS FOR $1;
    BEGIN
        INSERT INTO logtable VALUES (logtxt, 'now');
        RETURN 'now';
    END;
$$ LANGUAGE plpgsql;
</programlisting>

     and

<programlisting>
CREATE FUNCTION logfunc2(text) RETURNS timestamp AS $$
    DECLARE
        logtxt ALIAS FOR $1;
        curtime timestamp;
    BEGIN
        curtime := 'now';
        INSERT INTO logtable VALUES (logtxt, curtime);
        RETURN curtime;
    END;
$$ LANGUAGE plpgsql;
</programlisting>
    </para>

    <para>
     In the case of <function>logfunc1</function>, the 
     <productname>PostgreSQL</productname> main parser knows when 
     preparing the plan for the <command>INSERT</command>, that the string 
     <literal>'now'</literal> should be interpreted as 
     <type>timestamp</type> because the target column of <classname>logtable</classname>
     is of that type. Thus, it will make a constant from it at this
     time and this constant value is then used in all invocations of 
     <function>logfunc1</function> during the lifetime of the
     session. Needless to say that this isn't what the
     programmer wanted.
    </para>

    <para>
     In the case of <function>logfunc2</function>, the 
     <productname>PostgreSQL</productname> main parser does not know
     what type <literal>'now'</literal> should become and therefore 
     it returns a data value of type <type>text</type> containing the string 
     <literal>now</literal>. During the ensuing assignment
     to the local variable <varname>curtime</varname>, the
     <application>PL/pgSQL</application> interpreter casts this
     string to the <type>timestamp</type> type by calling the
     <function>text_out</function> and <function>timestamp_in</function>
     functions for the conversion.  So, the computed time stamp is updated
     on each execution as the programmer expects.
    </para>

    <para>
     The mutable nature of record variables presents a problem in this
     connection.  When fields of a record variable are used in
     expressions or statements, the data types of the fields must not
     change between calls of one and the same expression, since the
     expression will be planned using the data type that is present
     when the expression is first reached.  Keep this in mind when
     writing trigger procedures that handle events for more than one
     table.  (<command>EXECUTE</command> can be used to get around
     this problem when necessary.)
    </para>
  </sect1>

  <sect1 id="plpgsql-statements">
  <title>Basic Statements</title>

   <para>
    In this section and the following ones, we describe all the statement
    types that are explicitly understood by
    <application>PL/pgSQL</application>.
    Anything not recognized as one of these statement types is presumed
    to be an SQL command and is sent to the main database engine to execute
    (after substitution of any <application>PL/pgSQL</application> variables
    used in the statement).  Thus,
    for example, the SQL commands <command>INSERT</>, <command>UPDATE</>, and
    <command>DELETE</> may be considered to be statements of
    <application>PL/pgSQL</application>, but they are not specifically
    listed here.
   </para>

   <sect2 id="plpgsql-statements-assignment">
    <title>Assignment</title>

    <para>
     An assignment of a value to a variable or row/record field is
     written as:
<synopsis>
<replaceable>identifier</replaceable> := <replaceable>expression</replaceable>;
</synopsis>
     As explained above, the expression in such a statement is evaluated
     by means of an SQL <command>SELECT</> command sent to the main
     database engine.  The expression must yield a single value.
    </para>

    <para>
     If the expression's result data type doesn't match the variable's
     data type, or the variable has a specific size/precision
     (like <type>char(20)</type>), the result value will be implicitly
     converted by the <application>PL/pgSQL</application> interpreter using
     the result type's output-function and 
     the variable type's input-function. Note that this could potentially
     result in run-time errors generated by the input function, if the
     string form of the result value is not acceptable to the input function.
    </para>

    <para>
     Examples:
<programlisting>
user_id := 20;
tax := subtotal * 0.06;
</programlisting>
    </para>
   </sect2>

   <sect2 id="plpgsql-select-into">
    <title><command>SELECT INTO</command></title>

    <indexterm zone="plpgsql-select-into">
     <primary>SELECT INTO</primary>
     <secondary>in PL/pgSQL</secondary>
    </indexterm>

    <para>
     The result of a <command>SELECT</command> command yielding multiple columns (but
     only one row) can be assigned to a record variable, row-type
     variable, or list of scalar variables.  This is done by:

<synopsis>
SELECT INTO <replaceable>target</replaceable> <replaceable>select_expressions</replaceable> FROM ...;
</synopsis>

     where <replaceable>target</replaceable> can be a record variable, a row
     variable, or a comma-separated list of simple variables and
     record/row fields.  The <replaceable>select_expressions</replaceable>
     and the remainder of the command are the same as in regular SQL.
    </para>

    <para>
     Note that this is quite different from
     <productname>PostgreSQL</>'s normal interpretation of
     <command>SELECT INTO</command>, where the <literal>INTO</> target
     is a newly created table.  If you want to create a table from a
     <command>SELECT</> result inside a
     <application>PL/pgSQL</application> function, use the syntax
     <command>CREATE TABLE ... AS SELECT</command>.
    </para>

    <para>
     If a row or a variable list is used as target, the selected values
     must exactly match the structure of the target, or a run-time error
     occurs.  When a record variable is the target, it automatically
     configures itself to the row type of the query result columns.
    </para>

    <para>
     Except for the <literal>INTO</> clause, the <command>SELECT</>
     statement is the same as a normal SQL <command>SELECT</> command
     and can use its full power.
    </para>

    <para>
     If the query returns zero rows, null values are assigned to the
     target(s).  If the query returns multiple rows, the first
     row is assigned to the target(s) and the rest are discarded.
     (Note that <quote>the first row</> is not well-defined unless you've
     used <literal>ORDER BY</>.)
    </para>

    <para>
     The <literal>INTO</> clause can appear almost anywhere in the <command>SELECT</command>
     statement.
    </para>

    <para>
     You can use <literal>FOUND</literal> immediately after a <command>SELECT
     INTO</command> statement to determine whether the assignment was successful
     (that is, at least one row was was returned by the query). For example:

<programlisting>
SELECT INTO myrec * FROM emp WHERE empname = myname;
IF NOT FOUND THEN
    RAISE EXCEPTION 'employee % not found', myname;
END IF;
</programlisting>
    </para>

    <para>
     To test for whether a record/row result is null, you can use the
     <literal>IS NULL</literal> conditional.  There is, however, no
     way to tell whether any additional rows might have been
     discarded.  Here is an example that handles the case where no
     rows have been returned:
<programlisting>
DECLARE
    users_rec RECORD;
BEGIN
    SELECT INTO users_rec * FROM users WHERE user_id=3;

    IF users_rec.homepage IS NULL THEN
        -- user entered no homepage, return "http://"
        RETURN 'http://';
    END IF;
END;
</programlisting>
    </para>
   </sect2>

   <sect2 id="plpgsql-statements-perform">
    <title>Executing an Expression or Query With No Result</title>

    <para>
     Sometimes one wishes to evaluate an expression or query but
     discard the result (typically because one is calling a function
     that has useful side-effects but no useful result value).  To do
     this in <application>PL/pgSQL</application>, use the
     <command>PERFORM</command> statement:

<synopsis>
PERFORM <replaceable>query</replaceable>;
</synopsis>

     This executes <replaceable>query</replaceable>, which must be a
     <command>SELECT</command> statement, and discards the
     result. <application>PL/pgSQL</application> variables are
     substituted in the query as usual.  Also, the special variable
     <literal>FOUND</literal> is set to true if the query produced at
     least one row or false if it produced no rows.
    </para>

    <note>
     <para>
      One might expect that <command>SELECT</command> with no
      <literal>INTO</> clause would accomplish this result, but at
      present the only accepted way to do it is
      <command>PERFORM</command>.
     </para>
    </note>

    <para>
     An example:
<programlisting>
PERFORM create_mv('cs_session_page_requests_mv', my_query);
</programlisting>
    </para>
   </sect2>

   <sect2 id="plpgsql-statements-executing-dyn">
    <title>Executing Dynamic Commands</title>

    <para>
     Oftentimes you will want to generate dynamic commands inside your
     <application>PL/pgSQL</application> functions, that is, commands
     that will involve different tables or different data types each
     time they are executed.  <application>PL/pgSQL</application>'s
     normal attempts to cache plans for commands will not work in such
     scenarios.  To handle this sort of problem, the
     <command>EXECUTE</command> statement is provided:

<synopsis>
EXECUTE <replaceable class="command">command-string</replaceable>;
</synopsis>

     where <replaceable>command-string</replaceable> is an expression
     yielding a string (of type
     <type>text</type>) containing the command
     to be executed.  This string is fed literally to the SQL engine.
    </para>

    <para>
     Note in particular that no substitution of <application>PL/pgSQL</>
     variables is done on the command string.  The values of variables must
     be inserted in the command string as it is constructed.
    </para>

    <para>
    When working with dynamic commands you will have to face escaping
    of single quotes in <application>PL/pgSQL</>.  The recommended method
    is dollar quoting.  If you have legacy code which does
    <emphasis>not</emphasis> use dollar quoting, please refer to the
    overview in <xref linkend="plpgsql-quote-tips">, which can save you
    some effort when translating said code to a more reasonable scheme.
    </para>

    <para>
     Unlike all other commands in <application>PL/pgSQL</>, a command
     run by an <command>EXECUTE</command> statement is not prepared
     and saved just once during the life of the session.  Instead, the
     command is prepared each time the statement is run. The command
     string can be dynamically created within the function to perform
     actions on variable tables and columns.
    </para>

    <para>
     The results from <command>SELECT</command> commands are discarded
     by <command>EXECUTE</command>, and <command>SELECT INTO</command>
     is not currently supported within <command>EXECUTE</command>.
     There are two ways to extract a result from a dynamically-created
     <command>SELECT</command>: one is to use the <command>FOR-IN-EXECUTE</>
     loop form described in <xref linkend="plpgsql-records-iterating">,
     and the other is to use a cursor with <command>OPEN-FOR-EXECUTE</>, as
     described in <xref linkend="plpgsql-cursor-opening">.
    </para>

    <para>
     An example (except where noted, all examples herein assume that
     you are using dollar quoting):

<programlisting>
EXECUTE 'UPDATE tbl SET '
        || quote_ident(colname)
        || ' = '
        || quote_literal(newvalue)
        || ' WHERE ...';
</programlisting>
    </para>

    <para>
     This example shows use of the functions
     <function>quote_ident(<type>text</type>)</function> and
     <function>quote_literal(<type>text</type>)</function>.<indexterm><primary>quote_ident</><secondary>use
     in
     PL/pgSQL</></indexterm><indexterm><primary>quote_literal</><secondary>use
     in PL/pgSQL</></indexterm> For safety, variables containing column and
     table identifiers should be passed to function
     <function>quote_ident</function>.  Variables containing values
     that should be literal strings in the constructed command should
     be passed to <function>quote_literal</function>.  Both take the
     appropriate steps to return the input text enclosed in double or
     single quotes respectively, with any embedded special characters
     properly escaped.
    </para>

    <para>
     Here is a much larger example of a dynamic command and
     <command>EXECUTE</command>:
<programlisting>
CREATE FUNCTION cs_update_referrer_type_proc() RETURNS integer AS '
DECLARE
    referrer_keys RECORD;  -- declare a generic record to be used in a FOR
    a_output varchar(4000);
BEGIN 
    a_output := ''CREATE FUNCTION cs_find_referrer_type(varchar, varchar, varchar) 
                  RETURNS varchar AS '''' 
                     DECLARE 
                         v_host ALIAS FOR $1; 
                         v_domain ALIAS FOR $2; 
                         v_url ALIAS FOR $3;
                     BEGIN ''; 

    -- Notice how we scan through the results of a query in a FOR loop
    -- using the FOR &lt;record&gt; construct.

    FOR referrer_keys IN SELECT * FROM cs_referrer_keys ORDER BY try_order LOOP
        a_output := a_output || '' IF v_'' || referrer_keys.kind || '' LIKE '''''''''' 
                 || referrer_keys.key_string || '''''''''' THEN RETURN '''''' 
                 || referrer_keys.referrer_type || ''''''; END IF;''; 
    END LOOP; 

    a_output := a_output || '' RETURN NULL; END; '''' LANGUAGE plpgsql;''; 

    EXECUTE a_output; 
END;
' LANGUAGE plpgsql;
</programlisting>

And here is an equivalent using dollar quoting.  At least it is more
legible than the above, although both versions show that the design,
rather than merely the formatting, needs to be re-thought.

<programlisting>
CREATE or replace FUNCTION cs_update_referrer_type_proc2() RETURNS integer AS $func$
 DECLARE
     referrer_keys RECORD;  -- declare a generic record to be used in a FOR
     a_output varchar(4000);
 BEGIN 
     a_output := 'CREATE FUNCTION cs_find_referrer_type(varchar, varchar, varchar) 
                   RETURNS varchar AS $innerfunc$ 
                      DECLARE 
                          v_host ALIAS FOR $1; 
                          v_domain ALIAS FOR $2; 
                          v_url ALIAS FOR $3;
                      BEGIN '; 

     -- Notice how we scan through the results of a query in a FOR loop
     -- using the FOR &lt;record&gt; construct.

     FOR referrer_keys IN SELECT * FROM cs_referrer_keys ORDER BY try_order LOOP
         a_output := a_output || ' IF v_' || referrer_keys.kind || ' LIKE $$' 
                  || referrer_keys.key_string || '$$ THEN RETURN $$' 
                  || referrer_keys.referrer_type || '$$; END IF;'; 
     END LOOP; 

     a_output := a_output || ' RETURN NULL; END; $innerfunc$ LANGUAGE plpgsql;'; 
     EXECUTE a_output; 
    RETURN
END;
$func$ LANGUAGE plpgsql;
</programlisting>
    </para>
   </sect2>

   <sect2 id="plpgsql-statements-diagnostics">
    <title>Obtaining the Result Status</title>

    <para>
         There are several ways to determine the effect of a command. The
         first method is to use the <command>GET DIAGNOSTICS</command>
         command, which has the form:

<synopsis>
GET DIAGNOSTICS <replaceable>variable</replaceable> = <replaceable>item</replaceable> <optional> , ... </optional> ;
</synopsis>

     This command allows retrieval of system status indicators.  Each
     <replaceable>item</replaceable> is a key word identifying a state
     value to be assigned to the specified variable (which should be
     of the right data type to receive it).  The currently available
     status items are <varname>ROW_COUNT</>, the number of rows
     processed by the last <acronym>SQL</acronym> command sent down to
     the <acronym>SQL</acronym> engine, and <varname>RESULT_OID</>,
     the OID of the last row inserted by the most recent
     <acronym>SQL</acronym> command.  Note that <varname>RESULT_OID</>
     is only useful after an <command>INSERT</command> command.
    </para>

    <para>
     An example:
<programlisting>
GET DIAGNOSTICS integer_var = ROW_COUNT;
</programlisting>
    </para>

    <para>
     The second method to determine the effects of a command is to check the
     special variable named <literal>FOUND</literal>, which is of
     type <type>boolean</type>.  <literal>FOUND</literal> starts out
     false within each <application>PL/pgSQL</application> function call.
     It is set by each of the following types of statements:
         <itemizedlist>
          <listitem>
           <para>
                A <command>SELECT INTO</command> statement sets
                <literal>FOUND</literal> true if it returns a row, false if no
                row is returned.
           </para>
          </listitem>
          <listitem>
           <para>
                A <command>PERFORM</> statement sets <literal>FOUND</literal>
                true if it produces (and discards) a row, false if no row is
                produced.
           </para>
          </listitem>
          <listitem>
           <para>
                <command>UPDATE</>, <command>INSERT</>, and <command>DELETE</>
                statements set <literal>FOUND</literal> true if at least one
                row is affected, false if no row is affected.
           </para>
          </listitem>
          <listitem>
           <para>
                A <command>FETCH</> statement sets <literal>FOUND</literal>
                true if it returns a row, false if no row is returned.
           </para>
          </listitem>
          <listitem>
           <para>
                A <command>FOR</> statement sets <literal>FOUND</literal> true
                if it iterates one or more times, else false.  This applies to
                all three variants of the <command>FOR</> statement (integer
                <command>FOR</> loops, record-set <command>FOR</> loops, and
                dynamic record-set <command>FOR</>
                loops). <literal>FOUND</literal> is only set when the
                <command>FOR</> loop exits: inside the execution of the loop,
                <literal>FOUND</literal> is not modified by the
                <command>FOR</> statement, although it may be changed by the
                execution of other statements within the loop body.
           </para>
          </listitem>
         </itemizedlist>
     <literal>FOUND</literal> is a local variable; any changes
     to it affect only the current <application>PL/pgSQL</application>
     function.
    </para>

   </sect2>
  </sect1>

  <sect1 id="plpgsql-control-structures">
   <title>Control Structures</title>

   <para>
    Control structures are probably the most useful (and
    important) part of <application>PL/pgSQL</>. With
    <application>PL/pgSQL</>'s control structures,
    you can manipulate <productname>PostgreSQL</> data in a very
    flexible and powerful way. 
   </para>

   <sect2 id="plpgsql-statements-returning">
    <title>Returning From a Function</title>

    <para>
     There are two commands available that allow you to return data
     from a function: <command>RETURN</command> and <command>RETURN
     NEXT</command>.
    </para>

    <sect3>
     <title><command>RETURN</></title>

<synopsis>
RETURN <replaceable>expression</replaceable>;
</synopsis>

     <para>
      <command>RETURN</command> with an expression terminates the
      function and returns the value of
      <replaceable>expression</replaceable> to the caller.  This form
      is to be used for <application>PL/pgSQL</> functions that do
      not return a set.
     </para>

     <para>
      When returning a scalar type, any expression can be used. The
      expression's result will be automatically cast into the
      function's return type as described for assignments. To return a
      composite (row) value, you must write a record or row variable
      as the <replaceable>expression</replaceable>.
     </para>

     <para>
      The return value of a function cannot be left undefined. If
      control reaches the end of the top-level block of the function
      without hitting a <command>RETURN</command> statement, a run-time
      error will occur.
     </para>

     <para>
      If you have declared the function to
      return <type>void</type>, a <command>RETURN</command> statement
      must still be specified; but in this case the expression following
      <command>RETURN</command> is optional and will be ignored if present.
     </para>
    </sect3>

    <sect3>
     <title><command>RETURN NEXT</></title>

<synopsis>
RETURN NEXT <replaceable>expression</replaceable>;
</synopsis>

     <para>
      When a <application>PL/pgSQL</> function is declared to return
      <literal>SETOF <replaceable>sometype</></literal>, the procedure
      to follow is slightly different.  In that case, the individual
      items to return are specified in <command>RETURN NEXT</command>
      commands, and then a final <command>RETURN</command> command
      with no argument is used to indicate that the function has
      finished executing.  <command>RETURN NEXT</command> can be used
      with both scalar and composite data types; in the latter case, an
      entire <quote>table</quote> of results will be returned.
     </para>

     <para>
      Functions that use <command>RETURN NEXT</command> should be
      called in the following fashion:

<programlisting>
SELECT * FROM some_func();
</programlisting>

      That is, the function is used as a table source in a <literal>FROM</literal>
      clause.
     </para>

     <para>
      <command>RETURN NEXT</command> does not actually return from the
      function; it simply saves away the value of the expression (or
      record or row variable, as appropriate for the data type being
      returned).  Execution then continues with the next statement in
      the <application>PL/pgSQL</> function.  As successive
      <command>RETURN NEXT</command> commands are executed, the result
      set is built up.  A final <command>RETURN</command>, which should
      have no argument, causes control to exit the function.
     </para>

     <note>
      <para>
       The current implementation of <command>RETURN NEXT</command>
       for <application>PL/pgSQL</> stores the entire result set
       before returning from the function, as discussed above.  That
       means that if a <application>PL/pgSQL</> function produces a
       very large result set, performance may be poor: data will be
       written to disk to avoid memory exhaustion, but the function
       itself will not return until the entire result set has been
       generated.  A future version of <application>PL/pgSQL</> may
       allow users to define set-returning functions
       that do not have this limitation.  Currently, the point at
       which data begins being written to disk is controlled by the
       <varname>work_mem</> configuration variable.  Administrators
       who have sufficient memory to store larger result sets in
       memory should consider increasing this parameter.
      </para>
     </note>
    </sect3>
   </sect2>

   <sect2 id="plpgsql-conditionals">
    <title>Conditionals</title>

    <para>
     <literal>IF</> statements let you execute commands based on
     certain conditions.  <application>PL/pgSQL</> has four forms of
     <literal>IF</>:
    <itemizedlist>
     <listitem>
      <para><literal>IF ... THEN</></>
     </listitem>
     <listitem>
      <para><literal>IF ... THEN ... ELSE</></>
     </listitem>
     <listitem>
      <para><literal>IF ... THEN ... ELSE IF</></>
     </listitem>
     <listitem>
      <para><literal>IF ... THEN ... ELSIF ... THEN ... ELSE</></>
     </listitem>
    </itemizedlist>
    </para>

    <sect3>
     <title><literal>IF-THEN</></title>

<synopsis>
IF <replaceable>boolean-expression</replaceable> THEN
    <replaceable>statements</replaceable>
END IF;
</synopsis>

       <para>
        <literal>IF-THEN</literal> statements are the simplest form of
        <literal>IF</literal>. The statements between
        <literal>THEN</literal> and <literal>END IF</literal> will be
        executed if the condition is true. Otherwise, they are
        skipped.
       </para>

       <para>
        Example:
<programlisting>
IF v_user_id &lt;&gt; 0 THEN
    UPDATE users SET email = v_email WHERE user_id = v_user_id;
END IF;
</programlisting>
       </para>
     </sect3>

     <sect3>
      <title><literal>IF-THEN-ELSE</></title>

<synopsis>
IF <replaceable>boolean-expression</replaceable> THEN
    <replaceable>statements</replaceable>
ELSE
    <replaceable>statements</replaceable>
END IF;
</synopsis>

       <para>
        <literal>IF-THEN-ELSE</literal> statements add to
        <literal>IF-THEN</literal> by letting you specify an
        alternative set of statements that should be executed if the
        condition evaluates to false.
       </para>

       <para>
        Examples:
<programlisting>
IF parentid IS NULL OR parentid = ''
THEN
    RETURN fullname;
ELSE
    RETURN hp_true_filename(parentid) || '/' || fullname;
END IF;
</programlisting>

<programlisting>
IF v_count > 0 THEN 
    INSERT INTO users_count (count) VALUES (v_count);
    RETURN 't';
ELSE
    RETURN 'f';
END IF;
</programlisting>
     </para>
    </sect3>

     <sect3>
      <title><literal>IF-THEN-ELSE IF</></title>

       <para>
        <literal>IF</literal> statements can be nested, as in the
        following example:

<programlisting>
IF demo_row.sex = 'm' THEN
    pretty_sex := 'man';
ELSE
    IF demo_row.sex = 'f' THEN
        pretty_sex := 'woman';
    END IF;
END IF;
</programlisting>
       </para>

       <para>
        When you use this form, you are actually nesting an
        <literal>IF</literal> statement inside the
        <literal>ELSE</literal> part of an outer <literal>IF</literal>
        statement. Thus you need one <literal>END IF</literal>
        statement for each nested <literal>IF</literal> and one for the parent
        <literal>IF-ELSE</literal>.  This is workable but grows
        tedious when there are many alternatives to be checked.
        Hence the next form.
       </para>
     </sect3>

     <sect3>
      <title><literal>IF-THEN-ELSIF-ELSE</></title>

<synopsis>
IF <replaceable>boolean-expression</replaceable> THEN
    <replaceable>statements</replaceable>
<optional> ELSIF <replaceable>boolean-expression</replaceable> THEN
    <replaceable>statements</replaceable>
<optional> ELSIF <replaceable>boolean-expression</replaceable> THEN
    <replaceable>statements</replaceable>
    ...
</optional>
</optional>
<optional> ELSE
    <replaceable>statements</replaceable> </optional>
END IF;
</synopsis>

       <para>
        <literal>IF-THEN-ELSIF-ELSE</> provides a more convenient
        method of checking many alternatives in one statement.
        Formally it is equivalent to nested
        <literal>IF-THEN-ELSE-IF-THEN</> commands, but only one
        <literal>END IF</> is needed.
       </para>

       <para>
        Here is an example:

<programlisting>
IF number = 0 THEN
    result := 'zero';
ELSIF number &gt; 0 THEN 
    result := 'positive';
ELSIF number &lt; 0 THEN
    result := 'negative';
ELSE
    -- hmm, the only other possibility is that number is null
    result := 'NULL';
END IF;
</programlisting>
       </para>
     </sect3>
   </sect2>

   <sect2 id="plpgsql-control-structures-loops">
    <title>Simple Loops</title>

    <indexterm zone="plpgsql-control-structures-loops">
     <primary>loop</primary>
     <secondary>in PL/pgSQL</secondary>
    </indexterm>

    <para>
     With the <literal>LOOP</>, <literal>EXIT</>, <literal>WHILE</>,
     and <literal>FOR</> statements, you can arrange for your
     <application>PL/pgSQL</application> function to repeat a series
     of commands.
    </para>

    <sect3>
     <title><literal>LOOP</></title>

<synopsis>
<optional>&lt;&lt;<replaceable>label</replaceable>&gt;&gt;</optional>
LOOP
    <replaceable>statements</replaceable>
END LOOP;
</synopsis>

     <para>
      <literal>LOOP</> defines an unconditional loop that is repeated indefinitely
      until terminated by an <literal>EXIT</> or <command>RETURN</command>
      statement.  The optional label can be used by <literal>EXIT</> statements in
      nested loops to specify which level of nesting should be
      terminated.
     </para>
    </sect3>

     <sect3>
      <title><literal>EXIT</></title>

<synopsis>
EXIT <optional> <replaceable>label</replaceable> </optional> <optional> WHEN <replaceable>expression</replaceable> </optional>;
</synopsis>

       <para>
        If no <replaceable>label</replaceable> is given,
        the innermost loop is terminated and the
        statement following <literal>END LOOP</> is executed next.
        If <replaceable>label</replaceable> is given, it
        must be the label of the current or some outer level of nested loop
        or block. Then the named loop or block is terminated and control
        continues with the statement after the loop's/block's corresponding
        <literal>END</>.
       </para>

       <para>
        If <literal>WHEN</> is present, loop exit occurs only if the specified condition
        is true, otherwise control passes to the statement after <literal>EXIT</>.
       </para>

       <para>
        Examples:
<programlisting>
LOOP
    -- some computations
    IF count &gt; 0 THEN
        EXIT;  -- exit loop
    END IF;
END LOOP;

LOOP
    -- some computations
    EXIT WHEN count &gt; 0;  -- same result as previous example
END LOOP;

BEGIN
    -- some computations
    IF stocks &gt; 100000 THEN
        EXIT;  -- invalid; cannot use EXIT outside of LOOP
    END IF;
END;
</programlisting>
       </para>
     </sect3>

     <sect3>
      <title><literal>WHILE</></title>

<synopsis>
<optional>&lt;&lt;<replaceable>label</replaceable>&gt;&gt;</optional>
WHILE <replaceable>expression</replaceable> LOOP
    <replaceable>statements</replaceable>
END LOOP;
</synopsis>

       <para>
        The <literal>WHILE</> statement repeats a
        sequence of statements so long as the condition expression
        evaluates to true.  The condition is checked just before
        each entry to the loop body.
       </para>

       <para>
        For example:
<programlisting>
WHILE amount_owed &gt; 0 AND gift_certificate_balance &gt; 0 LOOP
    -- some computations here
END LOOP;

WHILE NOT boolean_expression LOOP
    -- some computations here
END LOOP;
</programlisting>
       </para>
     </sect3>

     <sect3>
      <title><literal>FOR</> (integer variant)</title>

<synopsis>
<optional>&lt;&lt;<replaceable>label</replaceable>&gt;&gt;</optional>
FOR <replaceable>name</replaceable> IN <optional> REVERSE </optional> <replaceable>expression</replaceable> .. <replaceable>expression</replaceable> LOOP
    <replaceable>statements</replaceable>
END LOOP;
</synopsis>

       <para>
        This form of <literal>FOR</> creates a loop that iterates over a range of integer
        values. The variable 
        <replaceable>name</replaceable> is automatically defined as type
        <type>integer</> and exists only inside the loop. The two expressions giving
        the lower and upper bound of the range are evaluated once when entering
        the loop. The iteration step is normally 1, but is -1 when <literal>REVERSE</> is
        specified.
       </para>

       <para>
        Some examples of integer <literal>FOR</> loops:
<programlisting>
FOR i IN 1..10 LOOP
    -- some computations here
    RAISE NOTICE 'i is %', i;
END LOOP;

FOR i IN REVERSE 10..1 LOOP
    -- some computations here
END LOOP;
</programlisting>
       </para>

       <para>
        If the lower bound is greater than the upper bound (or less than,
        in the <literal>REVERSE</> case), the loop body is not
        executed at all.  No error is raised.
       </para>
     </sect3>
   </sect2>

   <sect2 id="plpgsql-records-iterating">
    <title>Looping Through Query Results</title>

    <para>
     Using a different type of <literal>FOR</> loop, you can iterate through
     the results of a query and manipulate that data
     accordingly. The syntax is:
<synopsis>
<optional>&lt;&lt;<replaceable>label</replaceable>&gt;&gt;</optional>
FOR <replaceable>record_or_row</replaceable> IN <replaceable>query</replaceable> LOOP
    <replaceable>statements</replaceable>
END LOOP;
</synopsis>
     The record or row variable is successively assigned each row
     resulting from the query (a <command>SELECT</command> command) and the loop
     body is executed for each row. Here is an example:
<programlisting>
CREATE FUNCTION cs_refresh_mviews() RETURNS integer AS '
DECLARE
    mviews RECORD;
BEGIN
    PERFORM cs_log('Refreshing materialized views...');

    FOR mviews IN SELECT * FROM cs_materialized_views ORDER BY sort_key LOOP

        -- Now "mviews" has one record from cs_materialized_views

        PERFORM cs_log('Refreshing materialized view ' || quote_ident(mviews.mv_name) || '...');
        EXECUTE 'TRUNCATE TABLE  ' || quote_ident(mviews.mv_name);
        EXECUTE 'INSERT INTO ' || quote_ident(mviews.mv_name) || ' ' || mviews.mv_query;
    END LOOP;

    PERFORM cs_log('Done refreshing materialized views.');
    RETURN 1;
END;
' LANGUAGE plpgsql;
</programlisting>

     If the loop is terminated by an <literal>EXIT</> statement, the last
     assigned row value is still accessible after the loop.
    </para>

    <para>
     The <literal>FOR-IN-EXECUTE</> statement is another way to iterate over
     rows:
<synopsis>
<optional>&lt;&lt;<replaceable>label</replaceable>&gt;&gt;</optional>
FOR <replaceable>record_or_row</replaceable> IN EXECUTE <replaceable>text_expression</replaceable> LOOP 
    <replaceable>statements</replaceable>
END LOOP;
</synopsis>
     This is like the previous form, except that the source
     <command>SELECT</command> statement is specified as a string
     expression, which is evaluated and replanned on each entry to
     the <literal>FOR</> loop.  This allows the programmer to choose the speed of
     a preplanned query or the flexibility of a dynamic query, just
     as with a plain <command>EXECUTE</command> statement.
    </para>

    <note>
    <para>
     The <application>PL/pgSQL</> parser presently distinguishes the
     two kinds of <literal>FOR</> loops (integer or query result) by checking
     whether <literal>..</> appears outside any parentheses between
     <literal>IN</> and <literal>LOOP</>.  If <literal>..</> is not seen then
     the loop is presumed to be a loop over rows.  Mistyping the <literal>..</>
     is thus likely to lead to a complaint along the lines of
     <quote>loop variable of loop over rows must be a record or row</>,
     rather than the simple syntax error one might expect to get.
    </para>
    </note>
   </sect2>

   <sect2 id="plpgsql-error-trapping">
    <title>Trapping Errors</title>

    <para>
     By default, any error occurring in a <application>PL/pgSQL</>
     function aborts execution of the function, and indeed of the
     surrounding transaction as well.  You can trap errors and recover
     from them by using a <command>BEGIN</> block with an
     <literal>EXCEPTION</> clause.  The syntax is an extension of the
     normal syntax for a <command>BEGIN</> block:

<synopsis>
<optional> &lt;&lt;<replaceable>label</replaceable>&gt;&gt; </optional>
<optional> DECLARE
    <replaceable>declarations</replaceable> </optional>
BEGIN
    <replaceable>statements</replaceable>
EXCEPTION
    WHEN <replaceable>condition</replaceable> THEN
        <replaceable>handler_statements</replaceable>
    <optional> WHEN <replaceable>condition</replaceable> THEN
                   <replaceable>handler_statements</replaceable>
    ...
    </optional>
END;
</synopsis>
    </para>

    <para>
     If no error occurs, this form of block simply executes all the
     <replaceable>statements</replaceable>, and then control passes
     to the next statement after <literal>END</>.  But if an error
     occurs within the <replaceable>statements</replaceable>, further
     processing of the <replaceable>statements</replaceable> is
     abandoned, and control passes to the <literal>EXCEPTION</> list.
     The list is searched for the first <replaceable>condition</replaceable>
     matching the error that occurred.  If a match is found, the
     corresponding <replaceable>handler_statements</replaceable> are
     executed, and then control passes to the next statement after
     <literal>END</>.  If no match is found, the error propagates out
     as though the <literal>EXCEPTION</> clause were not there at all:
     the error can be caught by an enclosing block with
     <literal>EXCEPTION</>, or if there is none it aborts processing
     of the function.  The special condition name <literal>OTHERS</>
     matches every error type except <literal>QUERY_CANCELED</>.
     (It is possible, but usually not a good idea, to trap
     <literal>QUERY_CANCELED</> by name.)
    </para>

    <para>
     If a new error occurs within the selected
     <replaceable>handler_statements</replaceable>, it cannot be caught
     by this <literal>EXCEPTION</> clause, but is propagated out.
     A surrounding <literal>EXCEPTION</> clause could catch it.
    </para>

    <para>
     When an error is caught by an <literal>EXCEPTION</> clause,
     the local variables of the <application>PL/pgSQL</> function
     remain as they were when the error occurred, but all changes
     to persistent database state within the block are rolled back.
     As an example, consider this fragment:

<programlisting>
    INSERT INTO mytab(firstname, lastname) VALUES('Tom', 'Jones');
    BEGIN
        UPDATE mytab SET firstname = 'Joe' WHERE lastname = 'Jones';
        x := x + 1;
        y := x / 0;
    EXCEPTION
        WHEN division_by_zero THEN
            RAISE NOTICE 'caught division_by_zero';
            RETURN x;
    END;
</programlisting>

     When control reaches the assignment to <literal>y</>, it will
     fail with a <literal>division_by_zero</> error.  This will be caught by
     the <literal>EXCEPTION</> clause.  The value returned in the
     <command>RETURN</> statement will be the incremented value of
     <literal>x</>, but the effects of the <command>UPDATE</> command will
     have been rolled back.  The <command>INSERT</> command is not rolled
     back, however, so the end result is that the database contains
     <literal>Tom Jones</> not <literal>Joe Jones</>.
    </para>

    <tip>
     <para>
      A block containing an <literal>EXCEPTION</> clause is significantly
      more expensive to enter and exit than a block without one.  Therefore,
      don't use <literal>EXCEPTION</> without need.
     </para>
    </tip>
  </sect2>
  </sect1>

  <sect1 id="plpgsql-cursors">
   <title>Cursors</title>

   <indexterm zone="plpgsql-cursors">
    <primary>cursor</primary>
    <secondary>in PL/pgSQL</secondary>
   </indexterm>

   <para>
    Rather than executing a whole query at once, it is possible to set
    up a <firstterm>cursor</> that encapsulates the query, and then read
    the query result a few rows at a time. One reason for doing this is
    to avoid memory overrun when the result contains a large number of
    rows. (However, <application>PL/pgSQL</> users do not normally need
    to worry about that, since <literal>FOR</> loops automatically use a cursor
    internally to avoid memory problems.) A more interesting usage is to
    return a reference to a cursor that a function has created, allowing the
    caller to read the rows. This provides an efficient way to return
    large row sets from functions.
   </para>

   <sect2 id="plpgsql-cursor-declarations">
    <title>Declaring Cursor Variables</title>

    <para>
     All access to cursors in <application>PL/pgSQL</> goes through
     cursor variables, which are always of the special data type
     <type>refcursor</>.  One way to create a cursor variable
     is just to declare it as a variable of type <type>refcursor</>.
     Another way is to use the cursor declaration syntax,
     which in general is:
<synopsis>
<replaceable>name</replaceable> CURSOR <optional> ( <replaceable>arguments</replaceable> ) </optional> FOR <replaceable>query</replaceable> ;
</synopsis>
     (<literal>FOR</> may be replaced by <literal>IS</> for
     <productname>Oracle</productname> compatibility.)
     <replaceable>arguments</replaceable>, if specified, is a
     comma-separated list of pairs <literal><replaceable>name</replaceable>
     <replaceable>datatype</replaceable></literal> that define names to be
     replaced by parameter values in the given query.  The actual
     values to substitute for these names will be specified later,
     when the cursor is opened.
    </para>
    <para>
     Some examples:
<programlisting>
DECLARE
    curs1 refcursor;
    curs2 CURSOR FOR SELECT * FROM tenk1;
    curs3 CURSOR (key integer) IS SELECT * FROM tenk1 WHERE unique1 = key;
</programlisting>
     All three of these variables have the data type <type>refcursor</>,
     but the first may be used with any query, while the second has
     a fully specified query already <firstterm>bound</> to it, and the last
     has a parameterized query bound to it.  (<literal>key</> will be
     replaced by an integer parameter value when the cursor is opened.)
     The variable <literal>curs1</>
     is said to be <firstterm>unbound</> since it is not bound to
     any particular query.
    </para>
   </sect2>

   <sect2 id="plpgsql-cursor-opening">
    <title>Opening Cursors</title>

    <para>
     Before a cursor can be used to retrieve rows, it must be
     <firstterm>opened</>. (This is the equivalent action to the SQL
     command <command>DECLARE CURSOR</>.) <application>PL/pgSQL</> has
     three forms of the <command>OPEN</> statement, two of which use unbound
     cursor variables while the third uses a bound cursor variable.
    </para>

    <sect3>
     <title><command>OPEN FOR SELECT</command></title>

<synopsis>
OPEN <replaceable>unbound-cursor</replaceable> FOR SELECT ...;
</synopsis>

       <para>
    The cursor variable is opened and given the specified query to
        execute.  The cursor cannot be open already, and it must have been
        declared as an unbound cursor (that is, as a simple
        <type>refcursor</> variable).  The <command>SELECT</command> query
        is treated in the same way as other <command>SELECT</command>
        statements in <application>PL/pgSQL</>: <application>PL/pgSQL</>
        variable names are substituted, and the query plan is cached for
        possible reuse.
       </para>

       <para>
        An example:
<programlisting>
OPEN curs1 FOR SELECT * FROM foo WHERE key = mykey;
</programlisting>
       </para>
     </sect3>

    <sect3>
     <title><command>OPEN FOR EXECUTE</command></title>

<synopsis>
OPEN <replaceable>unbound-cursor</replaceable> FOR EXECUTE <replaceable class="command">query-string</replaceable>;
</synopsis>

         <para>
          The cursor variable is opened and given the specified query to
          execute.  The cursor cannot be open already, and it must have been
          declared as an unbound cursor (that is, as a simple
          <type>refcursor</> variable).  The query is specified as a string
          expression in the same way as in the <command>EXECUTE</command>
          command.  As usual, this gives flexibility so the query can vary
          from one run to the next.
       </para>

       <para>
        An example:
<programlisting>
OPEN curs1 FOR EXECUTE 'SELECT * FROM ' || quote_ident($1);
</programlisting>
       </para>
     </sect3>

    <sect3>
     <title>Opening a Bound Cursor</title>

<synopsis>
OPEN <replaceable>bound-cursor</replaceable> <optional> ( <replaceable>argument_values</replaceable> ) </optional>;
</synopsis>

         <para>
          This form of <command>OPEN</command> is used to open a cursor
          variable whose query was bound to it when it was declared.  The
          cursor cannot be open already.  A list of actual argument value
          expressions must appear if and only if the cursor was declared to
          take arguments.  These values will be substituted in the query.
          The query plan for a bound cursor is always considered cacheable;
          there is no equivalent of <command>EXECUTE</command> in this case.
         </para>

    <para>
     Examples:
<programlisting>
OPEN curs2;
OPEN curs3(42);
</programlisting>
       </para>
     </sect3>
   </sect2>

   <sect2 id="plpgsql-cursor-using">
    <title>Using Cursors</title>

    <para>
     Once a cursor has been opened, it can be manipulated with the
     statements described here.
    </para>

    <para>
     These manipulations need not occur in the same function that
     opened the cursor to begin with.  You can return a <type>refcursor</>
     value out of a function and let the caller operate on the cursor.
     (Internally, a <type>refcursor</> value is simply the string name
     of a so-called portal containing the active query for the cursor.  This name
     can be passed around, assigned to other <type>refcursor</> variables,
     and so on, without disturbing the portal.)
    </para>

    <para>
     All portals are implicitly closed at transaction end.  Therefore
     a <type>refcursor</> value is usable to reference an open cursor
     only until the end of the transaction.
    </para>

    <sect3>
     <title><literal>FETCH</></title>

<synopsis>
FETCH <replaceable>cursor</replaceable> INTO <replaceable>target</replaceable>;
</synopsis>

         <para>
          <command>FETCH</command> retrieves the next row from the
          cursor into a target, which may be a row variable, a record
          variable, or a comma-separated list of simple variables, just like
          <command>SELECT INTO</command>.  As with <command>SELECT
           INTO</command>, the special variable <literal>FOUND</literal> may
          be checked to see whether a row was obtained or not.
         </para>

    <para>
     An example:
<programlisting>
FETCH curs1 INTO rowvar;
FETCH curs2 INTO foo, bar, baz;
</programlisting>
       </para>
     </sect3>

    <sect3>
     <title><literal>CLOSE</></title>

<synopsis>
CLOSE <replaceable>cursor</replaceable>;
</synopsis>

       <para>
        <command>CLOSE</command> closes the portal underlying an open
        cursor.  This can be used to release resources earlier than end of
        transaction, or to free up the cursor variable to be opened again.
       </para>

       <para>
        An example:
<programlisting>
CLOSE curs1;
</programlisting>
       </para>
     </sect3>

    <sect3>
     <title>Returning Cursors</title>

       <para>
        <application>PL/pgSQL</> functions can return cursors to the
        caller. This is useful to return multiple rows or columns,
        especially with very large result sets.  To do this, the function
        opens the cursor and returns the cursor name to the caller (or simply
        opens the cursor using a portal name specified by or otherwise known
        to the caller).  The caller can then fetch rows from the cursor. The
        cursor can be closed by the caller, or it will be closed automatically
        when the transaction closes.
       </para>

       <para>
        The portal name used for a cursor can be specified by the
        programmer or automatically generated.  To specify a portal name,
        simply assign a string to the <type>refcursor</> variable before
        opening it.  The string value of the <type>refcursor</> variable
        will be used by <command>OPEN</> as the name of the underlying portal.
        However, if the <type>refcursor</> variable is null,
        <command>OPEN</> automatically generates a name that does not
        conflict with any existing portal, and assigns it to the
        <type>refcursor</> variable.
       </para>

       <note>
        <para>
         A bound cursor variable is initialized to the string value
         representing its name, so that the portal name is the same as
         the cursor variable name, unless the programmer overrides it
         by assignment before opening the cursor.  But an unbound cursor
         variable defaults to the null value initially , so it will receive
         an automatically-generated unique name, unless overridden.
        </para>
       </note>

       <para>
        The following example shows one way a cursor name can be supplied by
        the caller:

<programlisting>
CREATE TABLE test (col text);
INSERT INTO test VALUES ('123');

CREATE FUNCTION reffunc(refcursor) RETURNS refcursor AS '
BEGIN
    OPEN $1 FOR SELECT col FROM test;
    RETURN $1;
END;
' LANGUAGE plpgsql;

BEGIN;
SELECT reffunc('funccursor');
FETCH ALL IN funccursor;
COMMIT;
</programlisting>
       </para>

       <para>
        The following example uses automatic cursor name generation:

<programlisting>
CREATE FUNCTION reffunc2() RETURNS refcursor AS '
DECLARE
    ref refcursor;
BEGIN
    OPEN ref FOR SELECT col FROM test;
    RETURN ref;
END;
' LANGUAGE plpgsql;

BEGIN;
SELECT reffunc2();

      reffunc2      
--------------------
 &lt;unnamed cursor 1&gt;
(1 row)

FETCH ALL IN "&lt;unnamed cursor 1&gt;";
COMMIT;
</programlisting>
       </para>
     </sect3>
   </sect2>
  </sect1>

  <sect1 id="plpgsql-errors-and-messages">
   <title>Errors and Messages</title>

   <para>
    Use the <command>RAISE</command> statement to report messages and
    raise errors.

<synopsis>
RAISE <replaceable class="parameter">level</replaceable> '<replaceable class="parameter">format</replaceable>' <optional>, <replaceable class="parameter">variable</replaceable> <optional>, ...</optional></optional>;
</synopsis>

    Possible levels are <literal>DEBUG</literal>,
    <literal>LOG</literal>, <literal>INFO</literal>,
    <literal>NOTICE</literal>, <literal>WARNING</literal>,
    and <literal>EXCEPTION</literal>.
    <literal>EXCEPTION</literal> raises an error (which normally aborts the
    current transaction); the other levels only generate messages of different
    priority levels.
    Whether messages of a particular priority are reported to the client,
    written to the server log, or both is controlled by the
    <varname>log_min_messages</varname> and
    <varname>client_min_messages</varname> configuration
    variables. See <xref linkend="runtime-config"> for more
    information.
   </para>

   <para>
    Inside the format string, <literal>%</literal> is replaced by the
    next optional argument's string representation. Write
    <literal>%%</literal> to emit a literal <literal>%</literal>. Note
    that the optional arguments must presently be simple variables,
    not expressions, and the format must be a simple string literal.
   </para>

   <!--
   This example should work, but does not:
        RAISE NOTICE 'Id number ' || key || ' not found!';
   Put it back when we allow non-string-literal formats.
    -->

   <para>
    In this example, the value of <literal>v_job_id</> will replace the
    <literal>%</literal> in the string:
<programlisting>
RAISE NOTICE 'Calling cs_create_job(%)', v_job_id;
</programlisting>
   </para>

   <para>
    This example will abort the transaction with the given error message:
<programlisting>
RAISE EXCEPTION 'Nonexistent ID --> %', user_id;
</programlisting>
   </para>

    <para>
     <command>RAISE EXCEPTION</command> presently always generates
     the same SQLSTATE code, <literal>P0001</>, no matter what message
     it is invoked with.  It is possible to trap this exception with
     <literal>EXCEPTION ... WHEN RAISE_EXCEPTION THEN ...</> but there
     is no way to tell one <command>RAISE</> from another.
    </para>
 </sect1>

 <sect1 id="plpgsql-trigger">
  <title>Trigger Procedures</title>

  <indexterm zone="plpgsql-trigger">
   <primary>trigger</primary>
   <secondary>in PL/pgSQL</secondary>
  </indexterm>

  <para>
        <application>PL/pgSQL</application> can be used to define trigger
        procedures. A trigger procedure is created with the
        <command>CREATE FUNCTION</> command, declaring it as a function with
        no arguments and a return type of <type>trigger</type>.  Note that
        the function must be declared with no arguments even if it expects
        to receive arguments specified in <command>CREATE TRIGGER</> ---
        trigger arguments are passed via <varname>TG_ARGV</>, as described
        below.
  </para>

  <para>
   When a <application>PL/pgSQL</application> function is called as a
   trigger, several special variables are created automatically in the 
   top-level block. They are:

   <variablelist>
    <varlistentry>
     <term><varname>NEW</varname></term>
     <listitem>
      <para>
       Data type <type>RECORD</type>; variable holding the new
       database row for <command>INSERT</>/<command>UPDATE</> operations in row-level
       triggers. This variable is <symbol>NULL</symbol> in statement-level triggers.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><varname>OLD</varname></term>
     <listitem>
      <para>
       Data type <type>RECORD</type>; variable holding the old
       database row for <command>UPDATE</>/<command>DELETE</> operations in row-level
       triggers. This variable is <symbol>NULL</symbol> in statement-level triggers.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><varname>TG_NAME</varname></term>
     <listitem>
      <para>
       Data type <type>name</type>; variable that contains the name of the trigger actually
       fired.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><varname>TG_WHEN</varname></term>
     <listitem>
      <para>
       Data type <type>text</type>; a string of either 
              <literal>BEFORE</literal> or <literal>AFTER</literal>
              depending on the trigger's definition.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><varname>TG_LEVEL</varname></term>
     <listitem>
      <para>
       Data type <type>text</type>; a string of either
       <literal>ROW</literal> or <literal>STATEMENT</literal>
       depending on the trigger's definition.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><varname>TG_OP</varname></term>
     <listitem>
      <para>
       Data type <type>text</type>; a string of
       <literal>INSERT</literal>, <literal>UPDATE</literal>, or
       <literal>DELETE</literal> telling for which operation the
       trigger was fired.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><varname>TG_RELID</varname></term>
     <listitem>
      <para>
       Data type <type>oid</type>; the object ID of the table that caused the
       trigger invocation.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><varname>TG_RELNAME</varname></term>
     <listitem>
      <para>
       Data type <type>name</type>; the name of the table that caused the trigger
       invocation.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><varname>TG_NARGS</varname></term>
     <listitem>
      <para>
       Data type <type>integer</type>; the number of arguments given to the trigger
       procedure in the <command>CREATE TRIGGER</command> statement.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><varname>TG_ARGV[]</varname></term>
     <listitem>
      <para>
       Data type array of <type>text</type>; the arguments from
              the <command>CREATE TRIGGER</command> statement.
       The index counts from 0. Invalid
       indices (less than 0 or greater than or equal to <varname>tg_nargs</>) result in a null value.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>

   <para>
    A trigger function must return either <symbol>NULL</symbol> or a
    record/row value having exactly the structure of the table the
    trigger was fired for.
   </para>

   <para>
    Row-level triggers fired <literal>BEFORE</> may return null to signal the
    trigger manager to skip the rest of the operation for this row
    (i.e., subsequent triggers are not fired, and the
    <command>INSERT</>/<command>UPDATE</>/<command>DELETE</> does not occur
    for this row).  If a nonnull 
    value is returned then the operation proceeds with that row value.
    Returning a row value different from the original value
    of <varname>NEW</> alters the row that will be inserted or updated
    (but has no direct effect in the <command>DELETE</> case).
    To alter the row to be stored, it is possible to replace single values
    directly in <varname>NEW</> and return the modified <varname>NEW</>,
    or to build a complete new record/row to return.
   </para>

   <para>
    The return value of a <literal>BEFORE</> or <literal>AFTER</>
    statement-level trigger or an <literal>AFTER</> row-level trigger is
    always ignored; it may as well be null. However, any of these types of
    triggers can still abort the entire operation by raising an error.
   </para>

   <para>
    <xref linkend="plpgsql-trigger-example"> shows an example of a
    trigger procedure in <application>PL/pgSQL</application>.
   </para>

   <example id="plpgsql-trigger-example">
    <title>A <application>PL/pgSQL</application> Trigger Procedure</title>

    <para>
     This example trigger ensures that any time a row is inserted or updated
     in the table, the current user name and time are stamped into the
     row. And it checks that an employee's name is given and that the
     salary is a positive value.
    </para>

<programlisting>
CREATE TABLE emp (
    empname text,
    salary integer,
    last_date timestamp,
    last_user text
);

CREATE FUNCTION emp_stamp() RETURNS trigger AS $emp_stamp$
    BEGIN
        -- Check that empname and salary are given
        IF NEW.empname IS NULL THEN
            RAISE EXCEPTION 'empname cannot be null';
        END IF;
        IF NEW.salary IS NULL THEN
            RAISE EXCEPTION '% cannot have null salary', NEW.empname;
        END IF;

        -- Who works for us when she must pay for it?
        IF NEW.salary &lt; 0 THEN
            RAISE EXCEPTION '% cannot have a negative salary', NEW.empname;
        END IF;

        -- Remember who changed the payroll when
        NEW.last_date := 'now';
        NEW.last_user := current_user;
        RETURN NEW;
    END;
$emp_stamp$ LANGUAGE plpgsql;

CREATE TRIGGER emp_stamp BEFORE INSERT OR UPDATE ON emp
    FOR EACH ROW EXECUTE PROCEDURE emp_stamp();
</programlisting>
   </example>
  </sect1>

  <!-- **** Porting from Oracle PL/SQL **** -->

 <sect1 id="plpgsql-porting">
  <title>Porting from <productname>Oracle</productname> PL/SQL</title>

  <indexterm zone="plpgsql-porting">
   <primary>Oracle</primary>
   <secondary>porting from PL/SQL to PL/pgSQL</secondary>
  </indexterm>

  <indexterm zone="plpgsql-porting">
   <primary>PL/SQL (Oracle)</primary>
   <secondary>porting to PL/pgSQL</secondary>
  </indexterm>

  <para>
   This section explains differences between
   <productname>PostgreSQL</>'s <application>PL/pgSQL</application>
   language and Oracle's <application>PL/SQL</application> language,
   to help developers who port applications from Oracle to
   <productname>PostgreSQL</>.
  </para>

  <para>
   <application>PL/pgSQL</application> is similar to PL/SQL in many
   aspects. It is a block-structured, imperative language, and all
   variables have to be declared.  Assignments, loops, conditionals
   are similar.  The main differences you should keep in mind when
   porting from <application>PL/SQL</> to
   <application>PL/pgSQL</application> are:

    <itemizedlist>
     <listitem>
      <para>
       There are no default values for parameters in <productname>PostgreSQL</>.
      </para>
     </listitem>

     <listitem>
      <para>
       You can overload function names in <productname>PostgreSQL</>. This is
       often used to work around the lack of default parameters.
      </para>
     </listitem>

     <listitem>
      <para>
       No need for cursors in <application>PL/pgSQL</>, just put the
       query in the <literal>FOR</literal> statement.  (See <xref
       linkend="plpgsql-porting-ex2">.)
      </para>
     </listitem>

     <listitem>
      <para>
       In <productname>PostgreSQL</> you need to escape single
       quotes in the function body. See <xref linkend="plpgsql-quote-tips">.
      </para>
     </listitem>

     <listitem>
      <para>
       Instead of packages, use schemas to organize your functions
       into groups.
      </para>
     </listitem>
    </itemizedlist>
   </para>

  <sect2>
   <title>Porting Examples</title>

   <para>
    <xref linkend="pgsql-porting-ex1"> shows how to port a simple
    function from <application>PL/SQL</> to <application>PL/pgSQL</>.
   </para>

   <example id="pgsql-porting-ex1">
    <title>Porting a Simple Function from <application>PL/SQL</> to <application>PL/pgSQL</></title>

    <para>
     Here is an <productname>Oracle</productname> <application>PL/SQL</> function:
<programlisting>
CREATE OR REPLACE FUNCTION cs_fmt_browser_version(v_name IN varchar, v_version IN varchar)
RETURN varchar IS
BEGIN
    IF v_version IS NULL THEN
        RETURN v_name;
    END IF;
    RETURN v_name || '/' || v_version;
END;
/
show errors;
</programlisting>
    </para>

    <para>
     Let's go through this function and see the differences to <application>PL/pgSQL</>:

     <itemizedlist>
      <listitem>
       <para>
        <productname>PostgreSQL</productname> does not have named
        parameters. You have to explicitly alias them inside your
        function.
       </para>
      </listitem>

      <listitem>
       <para>
        <productname>Oracle</productname> can have
        <literal>IN</literal>, <literal>OUT</literal>, and
        <literal>INOUT</literal> parameters passed to functions.
        <literal>INOUT</literal>, for example, means that the
        parameter will receive a value and return
        another. <productname>PostgreSQL</> only has <literal>IN</literal>
        parameters.
       </para>
      </listitem>

      <listitem>
       <para>
        The <literal>RETURN</literal> key word in the function
        prototype (not the function body) becomes
        <literal>RETURNS</literal> in
        <productname>PostgreSQL</productname>.
       </para>
      </listitem>

      <listitem>
       <para>
        In <productname>PostgreSQL</>, functions are created using
        single quotes as the delimiters of the function body, so you
        have to escape single quotes inside the function body.
       </para>
      </listitem>

      <listitem>
       <para>
        The <literal>/show errors</literal> command does not exist in
        <productname>PostgreSQL</>.
       </para>
      </listitem>
     </itemizedlist>
    </para>

    <para>
     This is how this function would look when ported to
     <productname>PostgreSQL</>:

<programlisting>
CREATE OR REPLACE FUNCTION cs_fmt_browser_version(varchar, varchar)
RETURNS varchar AS $$
DECLARE
    v_name ALIAS FOR $1;
    v_version ALIAS FOR $2;
BEGIN
    IF v_version IS NULL THEN
        return v_name;
    END IF;
    RETURN v_name || '/' || v_version;
END;
$$ LANGUAGE plpgsql;
</programlisting>
    </para>
   </example>

   <para>
    <xref linkend="plpgsql-porting-ex2"> shows how to port a
    function that creates another function and how to handle to
    ensuing quoting problems.
   </para>

   <example id="plpgsql-porting-ex2">
    <title>Porting a Function that Creates Another Function from <application>PL/SQL</> to <application>PL/pgSQL</></title>

    <para>
     The following procedure grabs rows from a
     <command>SELECT</command> statement and builds a large function
     with the results in <literal>IF</literal> statements, for the
     sake of efficiency. Notice particularly the differences in the
     cursor and the <literal>FOR</literal> loop,
    </para>

    <para>
     This is the Oracle version:
<programlisting>
CREATE OR REPLACE PROCEDURE cs_update_referrer_type_proc IS
    CURSOR referrer_keys IS 
        SELECT * FROM cs_referrer_keys 
        ORDER BY try_order;

    a_output VARCHAR(4000); 
BEGIN 
    a_output := 'CREATE OR REPLACE FUNCTION cs_find_referrer_type(v_host IN VARCHAR, v_domain IN VARCHAR, 
v_url IN VARCHAR) RETURN VARCHAR IS BEGIN'; 

    FOR referrer_key IN referrer_keys LOOP 
        a_output := a_output || ' IF v_' || referrer_key.kind || ' LIKE ''' || 
referrer_key.key_string || ''' THEN RETURN ''' || referrer_key.referrer_type || 
'''; END IF;'; 
    END LOOP; 

    a_output := a_output || ' RETURN NULL; END;'; 
    EXECUTE IMMEDIATE a_output; 
END; 
/ 
show errors;
</programlisting>
    </para>

    <para>
     Here is how this function would end up in <productname>PostgreSQL</>:

<programlisting>
CREATE or replace FUNCTION cs_update_referrer_type_proc() RETURNS
text AS $func$
DECLARE
    referrer_keys RECORD;  -- declare a generic record to be used in a FOR
    a_output TEXT;
BEGIN 
    a_output := 'CREATE FUNCTION cs_find_referrer_type(varchar, varchar, varchar) 
                  RETURNS varchar AS $innerfunc$ 
                     DECLARE 
                         v_host ALIAS FOR $1; 
                         v_domain ALIAS FOR $2; 
                         v_url ALIAS FOR $3;
                     BEGIN '; 

     -- Notice how we scan through the results of a query in a FOR loop
     -- using the FOR &lt;record&gt; construct.

     FOR referrer_keys IN SELECT * FROM cs_referrer_keys ORDER BY try_order LOOP
         a_output := a_output || ' IF v_' || referrer_keys.kind || ' LIKE $$' 
                  || referrer_keys.key_string || '$$ THEN RETURN $$' 
                  || referrer_keys.referrer_type || '$$; END IF;'; 
     END LOOP; 

     a_output := a_output || ' RETURN NULL; END; $innerfunc$ LANGUAGE plpgsql;'; 

     return a_output; 
END; 
$func$ LANGUAGE plpgsql;
</programlisting>
    </para>
   </example>

   <para>
    <xref linkend="plpgsql-porting-ex3"> shows how to port a function
    with <literal>OUT</> parameters and string manipulation.
    <productname>PostgreSQL</> does not have an
    <function>instr</function> function, but you can work around it
    using a combination of other
    functions.<indexterm><primary>instr</></indexterm> In <xref
    linkend="plpgsql-porting-appendix"> there is a
    <application>PL/pgSQL</application> implementation of
    <function>instr</function> that you can use to make your porting
    easier.
   </para>

   <example id="plpgsql-porting-ex3">
    <title>Porting a Procedure With String Manipulation and
    <literal>OUT</> Parameters from <application>PL/SQL</> to
    <application>PL/pgSQL</></title>

    <para>
     The following <productname>Oracle</productname> PL/SQL procedure is used to parse a URL and
     return several elements (host, path, and query).
     <application>PL/pgSQL</application> functions can return only one value.  In
     <productname>PostgreSQL</>, one way to work around this is to split the procedure
     in three different functions: one to return the host, another for
     the path, and another for the query.
    </para>

    <para>
     This is the Oracle version:
<programlisting>
CREATE OR REPLACE PROCEDURE cs_parse_url(
    v_url IN VARCHAR,
    v_host OUT VARCHAR,  -- This will be passed back
    v_path OUT VARCHAR,  -- This one too
    v_query OUT VARCHAR) -- And this one
IS
    a_pos1 INTEGER;
    a_pos2 INTEGER;
BEGIN
    v_host := NULL;
    v_path := NULL;
    v_query := NULL;
    a_pos1 := instr(v_url, '//');

    IF a_pos1 = 0 THEN
        RETURN;
    END IF;
    a_pos2 := instr(v_url, '/', a_pos1 + 2);
    IF a_pos2 = 0 THEN
        v_host := substr(v_url, a_pos1 + 2);
        v_path := '/';
        RETURN;
    END IF;

    v_host := substr(v_url, a_pos1 + 2, a_pos2 - a_pos1 - 2);
    a_pos1 := instr(v_url, '?', a_pos2 + 1);

    IF a_pos1 = 0 THEN
        v_path := substr(v_url, a_pos2);
        RETURN;
    END IF;

    v_path := substr(v_url, a_pos2, a_pos1 - a_pos2);
    v_query := substr(v_url, a_pos1 + 1);
END;
/
show errors;
</programlisting>
    </para>

    <para>
     Here is how the <application>PL/pgSQL</> function that returns
     the host part could look like:

<programlisting>
CREATE OR REPLACE FUNCTION cs_parse_url_host(varchar) RETURNS varchar AS $$
DECLARE
    v_url ALIAS FOR $1;
    v_host varchar;
    v_path varchar;
    a_pos1 integer;
    a_pos2 integer;
    a_pos3 integer;
BEGIN 
    v_host := NULL; 
    a_pos1 := instr(v_url, '//');

    IF a_pos1 = 0 THEN 
        RETURN '';  -- Return a blank
    END IF; 

    a_pos2 := instr(v_url,'/',a_pos1 + 2);
    IF a_pos2 = 0 THEN 
        v_host := substr(v_url, a_pos1 + 2); 
        v_path := '/'; 
        RETURN v_host; 
    END IF; 

    v_host := substr(v_url, a_pos1 + 2, a_pos2 - a_pos1 - 2 ); 
    RETURN v_host; 
END; 
$$ LANGUAGE plpgsql;
</programlisting>
    </para>
   </example>

   <para>
    <xref linkend="plpgsql-porting-ex4"> shows how to port a procedure
    that uses numerous features that are specific to Oracle.
   </para>

   <example id="plpgsql-porting-ex4">
    <title>Porting a Procedure from <application>PL/SQL</> to <application>PL/pgSQL</></title>

    <para>
     The Oracle version:

<programlisting>
CREATE OR REPLACE PROCEDURE cs_create_job(v_job_id IN INTEGER) IS
    a_running_job_count INTEGER;
    PRAGMA AUTONOMOUS_TRANSACTION;<co id="co.plpgsql-porting-pragma">
BEGIN
    LOCK TABLE cs_jobs IN EXCLUSIVE MODE;<co id="co.plpgsql-porting-locktable">

    SELECT count(*) INTO a_running_job_count FROM cs_jobs WHERE end_stamp IS NULL;

    IF a_running_job_count > 0 THEN
        COMMIT; -- free lock<co id="co.plpgsql-porting-commit">
        raise_application_error(-20000, 'Unable to create a new job: a job is currently running.');
    END IF;

    DELETE FROM cs_active_job;
    INSERT INTO cs_active_job(job_id) VALUES (v_job_id);

    BEGIN
        INSERT INTO cs_jobs (job_id, start_stamp) VALUES (v_job_id, sysdate);
        EXCEPTION WHEN dup_val_on_index THEN NULL; -- don't worry if it already exists<co id="co.plpgsql-porting-exception">
    END;
    COMMIT;
END;
/
show errors
</programlisting>
   </para>

   <para>
    Procedures like this can be easily converted into <productname>PostgreSQL</>
    functions returning an <type>integer</type>. This procedure in
    particular is interesting because it can teach us some things:

    <calloutlist>
     <callout arearefs="co.plpgsql-porting-pragma">
      <para>
       There is no <literal>PRAGMA</literal> statement in <productname>PostgreSQL</>.
      </para>
     </callout>

     <callout arearefs="co.plpgsql-porting-locktable">
      <para>
       If you do a <command>LOCK TABLE</command> in <application>PL/pgSQL</>, the lock
       will not be released until the calling transaction is finished.
      </para>
     </callout>

     <callout arearefs="co.plpgsql-porting-commit">
      <para>
       You also cannot have transactions in <application>PL/pgSQL</application> functions. The
       entire function (and other functions called from therein) is
       executed in one transaction and <productname>PostgreSQL</> rolls back the transaction if
       something goes wrong.
      </para>
     </callout>

     <callout arearefs="co.plpgsql-porting-exception">
      <para>
       The exception when would have to be replaced by an
       <literal>IF</literal> statement.
      </para>
     </callout>
    </calloutlist>
   </para>

   <para>
    This is how we could port this procedure to <application>PL/pgSQL</>:

<programlisting>
CREATE OR REPLACE FUNCTION cs_create_job(integer) RETURNS integer AS $$
DECLARE
    v_job_id ALIAS FOR $1;
    a_running_job_count integer;
    a_num integer;
BEGIN
    LOCK TABLE cs_jobs IN EXCLUSIVE MODE;
    SELECT count(*) INTO a_running_job_count FROM cs_jobs WHERE end_stamp IS NULL;

    IF a_running_job_count > 0
    THEN
        RAISE EXCEPTION 'Unable to create a new job: a job is currently running.';
    END IF;

    DELETE FROM cs_active_job;
    INSERT INTO cs_active_job(job_id) VALUES (v_job_id);

    SELECT count(*) INTO a_num FROM cs_jobs WHERE job_id=v_job_id;
    IF NOT FOUND THEN  -- If nothing was returned in the last query
        -- This job is not in the table so lets insert it.
        INSERT INTO cs_jobs(job_id, start_stamp) VALUES (v_job_id, current_timestamp);
        RETURN 1;
    ELSE
        RAISE NOTICE 'Job already running.';<co id="co.plpgsql-porting-raise">
    END IF;

    RETURN 0;
END;
$$ LANGUAGE plpgsql;
</programlisting>

    <calloutlist>
     <callout arearefs="co.plpgsql-porting-raise">
      <para>
       Notice how you can raise notices (or errors) in <application>PL/pgSQL</>.
      </para>
     </callout>
    </calloutlist>
   </para>
   </example>
  </sect2>

  <sect2 id="plpgsql-porting-other">
   <title>Other Things to Watch For</title>

   <para>
    This section explains a few other things to watch for when porting
    Oracle <application>PL/SQL</> functions to
    <productname>PostgreSQL</productname>.
   </para>

   <sect3>
    <title><command>EXECUTE</command></title>

    <para>
     The <application>PL/pgSQL</> version of
     <command>EXECUTE</command> works similarly to the
     <application>PL/SQL</> version, but you have to remember to use
     <function>quote_literal(text)</function> and
     <function>quote_string(text)</function> as described in <xref
     linkend="plpgsql-statements-executing-dyn">.  Constructs of the
     type <literal>EXECUTE 'SELECT * FROM $1';</literal> will not
     work unless you use these functions.
    </para>
   </sect3>

   <sect3 id="plpgsql-porting-optimization">
    <title>Optimizing <application>PL/pgSQL</application> Functions</title>

    <para>
     <productname>PostgreSQL</> gives you two function creation
     modifiers to optimize execution: the volatility (whether the
     function always returns the same result when given the same
     arguments) and the <quote>strictness</quote> (whether the
     function returns null if any argument is null).  Consult the description of
     <command>CREATE FUNCTION</command> for details.
    </para>

    <para>
     To make use of these optimization attributes, your
     <command>CREATE FUNCTION</command> statement could look something
     like this:

<programlisting>
CREATE FUNCTION foo(...) RETURNS integer AS $$
...
$$ LANGUAGE plpgsql STRICT IMMUTABLE;
</programlisting>
    </para>
   </sect3>
  </sect2>

  <sect2 id="plpgsql-porting-appendix">
   <title>Appendix</title>

   <para>
    This section contains the code for an Oracle-compatible
    <function>instr</function> function that you can use to simplify
    your porting efforts.
   </para>

<programlisting>
--
-- instr functions that mimic Oracle's counterpart
-- Syntax: instr(string1, string2, [n], [m]) where [] denotes optional parameters.
-- 
-- Searches string1 beginning at the nth character for the mth occurrence
-- of string2.  If n is negative, search backwards.  If m is not passed,
-- assume 1 (search starts at first character).
--

CREATE FUNCTION instr(varchar, varchar) RETURNS integer AS $$
DECLARE
    pos integer;
BEGIN
    pos:= instr($1, $2, 1);
    RETURN pos;
END;
$$ LANGUAGE plpgsql;


CREATE FUNCTION instr(varchar, varchar, integer) RETURNS integer AS $$
DECLARE
    string ALIAS FOR $1;
    string_to_search ALIAS FOR $2;
    beg_index ALIAS FOR $3;
    pos integer NOT NULL DEFAULT 0;
    temp_str varchar;
    beg integer;
    length integer;
    ss_length integer;
BEGIN
    IF beg_index > 0 THEN
        temp_str := substring(string FROM beg_index);
        pos := position(string_to_search IN temp_str);

        IF pos = 0 THEN
            RETURN 0;
        ELSE
            RETURN pos + beg_index - 1;
        END IF;
    ELSE
        ss_length := char_length(string_to_search);
        length := char_length(string);
        beg := length + beg_index - ss_length + 2;

        WHILE beg > 0 LOOP
            temp_str := substring(string FROM beg FOR ss_length);
            pos := position(string_to_search IN temp_str);

            IF pos > 0 THEN
                RETURN beg;
            END IF;

            beg := beg - 1;
        END LOOP;

        RETURN 0;
    END IF;
END;
$$ LANGUAGE plpgsql;


CREATE FUNCTION instr(varchar, varchar, integer, integer) RETURNS integer AS $$
DECLARE
    string ALIAS FOR $1;
    string_to_search ALIAS FOR $2;
    beg_index ALIAS FOR $3;
    occur_index ALIAS FOR $4;
    pos integer NOT NULL DEFAULT 0;
    occur_number integer NOT NULL DEFAULT 0;
    temp_str varchar;
    beg integer;
    i integer;
    length integer;
    ss_length integer;
BEGIN
    IF beg_index > 0 THEN
        beg := beg_index;
        temp_str := substring(string FROM beg_index);

        FOR i IN 1..occur_index LOOP
            pos := position(string_to_search IN temp_str);

            IF i = 1 THEN
                beg := beg + pos - 1;
            ELSE
                beg := beg + pos;
            END IF;

            temp_str := substring(string FROM beg + 1);
        END LOOP;

        IF pos = 0 THEN
            RETURN 0;
        ELSE
            RETURN beg;
        END IF;
    ELSE
        ss_length := char_length(string_to_search);
        length := char_length(string);
        beg := length + beg_index - ss_length + 2;

        WHILE beg > 0 LOOP
            temp_str := substring(string FROM beg FOR ss_length);
            pos := position(string_to_search IN temp_str);

            IF pos > 0 THEN
                occur_number := occur_number + 1;

                IF occur_number = occur_index THEN
                    RETURN beg;
                END IF;
            END IF;

            beg := beg - 1;
        END LOOP;

        RETURN 0;
    END IF;
END;
$$ LANGUAGE plpgsql;
</programlisting>
  </sect2>

 </sect1>

</chapter>

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