<!--
$Header: /cvsroot/pgsql/doc/src/sgml/Attic/plsql.sgml,v 2.57 2002/04/09 03:08:25 momjian Exp $
-->

<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.
 </para>
   
 <para>
  This package was originally written by Jan Wieck. This
  documentation was in part written 
  by Roberto Mello (<email>rmello@fslc.usu.edu</email>).
 </para>

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

   <para>
    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>
   <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 any one backend process).  The instruction tree
    fully translates the 
    <application>PL/pgSQL</> statement structure, but individual
    <acronym>SQL</acronym> expressions and <acronym>SQL</acronym> queries
    used in the function are not translated immediately.
   </para>
   <para>
    As each expression and <acronym>SQL</acronym> query 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).  Subsequent visits
    to that expression or query re-use 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 provide a considerable savings of parsing
    activity.  A disadvantage is that errors in a specific expression
    or query may not be detected until that part of the function is
    reached in execution.
   </para>
  <para>
   Once <application>PL/pgSQL</> has made a query plan for a particular
   query in a function, it will re-use 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 query plan produced for
    the PERFORM statement. Later, if you
    drop and re-create <function>my_function()</function>, then
    <function>populate()</function> will not be able to find
    <function>my_function()</function> anymore. You would then have to
    re-create <function>populate()</function>, or at least start a new
    database session so that it will be compiled afresh.
   </para>

   <para>
    Because <application>PL/pgSQL</application> saves execution plans in this way, queries that appear
    directly in a <application>PL/pgSQL</application> function must refer to the same tables and fields
    on every execution; that is, you cannot use a parameter as the name of
    a table or field in a query.  To get around
    this restriction, you can construct dynamic queries using the <application>PL/pgSQL</application>
    EXECUTE statement --- at the price of constructing a new query plan
    on every execution.
   </para>
   <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>. It is possible to
    create complex conditional computation functions and later use
    them to define operators or use them in functional indexes.
   </para>
  <sect2 id="plpgsql-advantages">
   <title>Advantages of Using PL/pgSQL</title>

   <itemizedlist>
    <listitem>
     <para>
      Better performance (see <xref linkend="plpgsql-advantages-performance">)
     </para>
    </listitem>

    <listitem>
     <para>
      SQL support (see <xref linkend="plpgsql-advantages-sqlsupport">)
     </para>
    </listitem> 

    <listitem>
     <para>
      Portability (see <xref linkend="plpgsql-advantages-portability">)
     </para>
    </listitem>
   </itemizedlist>

   <sect3 id="plpgsql-advantages-performance">
    <title>Better Performance</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 process it,
     receive the results, do some computation, then send
     other queries to the server. All this incurs inter-process 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>
   </sect3>

   <sect3 id="plpgsql-advantages-sqlsupport">
    <title>SQL Support</title>

    <para>
     <application>PL/pgSQL</application> adds the power of a procedural language to the
     flexibility and ease of <acronym>SQL</acronym>. With
     <application>PL/pgSQL</application> you can use all the data types, columns, operators
     and functions of SQL.    
    </para>
   </sect3>

   <sect3 id="plpgsql-advantages-portability">
    <title>Portability</title>

    <para>
     Because <application>PL/pgSQL</application> functions run inside <productname>PostgreSQL</>, these
     functions will run on any platform where <productname>PostgreSQL</>
     runs. Thus you can reuse code and have less development costs.
    </para>
   </sect3>
  </sect2>

  <sect2 id="plpgsql-overview-developing-in-plpgsql">
   <title>Developing in <application>PL/pgSQL</application></title>

   <para>
    Developing in <application>PL/pgSQL</application> is pretty straight forward, especially
    if you have developed in other database procedural languages,
    such as Oracle's <application>PL/SQL</application>. Two good ways of developing in
    <application>PL/pgSQL</application> are:

    <itemizedlist>
     <listitem>
      <para>
       Using a text editor and reloading the file with <command>psql</command>
      </para>
     </listitem>

     <listitem>
      <para>
       Using <productname>PostgreSQL</>'s GUI Tool: <application>PgAccess</>
      </para>
     </listitem>
    </itemizedlist>
   </para>

   <para>
    One good way to develop in <application>PL/pgSQL</> is to simply use the text
    editor of your choice to create your functions, and in another
    console, use <command>psql</command> (PostgreSQL's interactive monitor) to load
    those functions. If you are doing it this way, it is a good
    idea to write the function using <command>CREATE OR REPLACE
    FUNCTION</command>. That way you can 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 <command>psql</command>, 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
    <productname>PostgreSQL</>'s GUI tool: <application>PgAccess</>. It does some
    nice things for you, like escaping single-quotes, and making
    it easy to recreate and debug functions.
   </para>
  </sect2>
 </sect1>

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

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

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

    <para>
     Any <firstterm>statement</> in the statement section of a block
     can be a <firstterm>sub-block</>.  Sub-blocks 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 sub-block
   --
   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 BEGIN/END for
     grouping statements in <application>PL/pgSQL</> with the database commands for
     transaction control.  <application>PL/pgSQL</>'s BEGIN/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>

   <sect2>
    <title>Lexical Details</title>

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

    <para>
     All keywords and identifiers can be written in mixed upper- and
     lower-case.  Identifiers are implicitly converted to lower-case
     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>
   </sect2>
  </sect1>

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

    <para>
     All variables, rows and records used in a block must be declared in the
     declarations section of the block. 
     (The only exception is that the loop variable of a 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;
</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>
     </para>

     <para>
      The DEFAULT clause, if given, specifies the initial value assigned
      to the variable when the block is entered.  If the DEFAULT clause
      is not given then the variable is initialized to the
     <acronym>SQL</acronym> NULL value. 
     </para>

     <para>
      The CONSTANT option prevents the variable from being assigned to,
      so that its value remains constant for the duration of the block.
      If NOT NULL
      is specified, an assignment of a NULL value results in a run-time
      error. All variables declared as NOT NULL
      must have a non-NULL 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 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>

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

     <para>
      Parameters passed to functions are named with the identifiers
      <literal>$1</literal>, <literal>$2</literal>,
      etc.  Optionally, aliases can be declared for <literal>$n</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';
</programlisting>
     </para>
    </sect2>

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

    <para>
<synopsis>
<replaceable>name</replaceable> <replaceable>tablename</replaceable><literal>%ROWTYPE</literal>;
</synopsis>
    </para>

   <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 SELECT or 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>
    Presently, a row variable can only be declared using the
    <literal>%ROWTYPE</literal> notation; although one might expect a
    bare table name to work as a type declaration, it won't be accepted
    within <application>PL/pgSQL</application> functions.
   </para>

   <para>
    Parameters to a function can be
    composite types (complete table rows). In that case, the
    corresponding identifier $n 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 attributes of a table row are accessible in a
    row-type variable, not OID or other system attributes (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(n)</type>.
   </para>
  </sect2>

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

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

   <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 SELECT or 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, <emphasis>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.
    Thus, for example, one cannot declare a function returning
    <literal>RECORD</>.
   </para>
  </sect2>

    <sect2 id="plpgsql-declaration-attributes">
     <title>Attributes</title>

     <para>
      Using the <type>%TYPE</type> and <type>%ROWTYPE</type>
      attributes, you can declare variables with the same
      data type or structure as another database item (e.g: a
      table field).
     </para>

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

        <para>
         By using <type>%TYPE</type> you don't need to know
         the data type of the structure you are referencing,
         and most important, if the data type of the
         referenced item changes in the future (e.g: you
         change your table definition of user_id from INTEGER to
         REAL), you may not need to change your function
         definition.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>
        <literal><replaceable>table</replaceable>%ROWTYPE</literal>
       </term>
       <listitem>
        <para>
	 <type>%ROWTYPE</type> provides the composite data type corresponding
	 to a whole row of the specified table.
	 <replaceable>table</replaceable> must be an existing
	 table or view name of the database.
        </para>

<programlisting>
DECLARE
    users_rec users%ROWTYPE;
    user_id users.user_id%TYPE;
BEGIN
    user_id := users_rec.user_id;
    ...

CREATE FUNCTION does_view_exist(INTEGER) RETURNS bool AS '
   DECLARE
        key ALIAS FOR $1;
        table_data cs_materialized_views%ROWTYPE;
   BEGIN
        SELECT INTO table_data * FROM cs_materialized_views
               WHERE sort_key=key;

        IF NOT FOUND THEN
           RETURN false;
        END IF;
        RETURN true;
   END;
' LANGUAGE 'plpgsql';
</programlisting>
       </listitem>
      </varlistentry>
     </variablelist>
    </sect2>

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

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

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

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

    <note>
    <para>
     RENAME appears to be broken as of PostgreSQL 7.2.  Fixing this is
     of low priority, since ALIAS covers most of the practical uses of
     RENAME.
    </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 SQL 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 NULL keyword. All
     expressions are evaluated internally by executing a query
<synopsis>
SELECT <replaceable>expression</replaceable>
</synopsis>
     using the <acronym>SPI</acronym> manager. In the expression, 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 SELECT to be prepared just once
     and then re-used 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>

     In the case of <function>logfunc1()</function>, the 
     <productname>PostgreSQL</productname> main parser knows when 
     preparing the plan for the INSERT, that the string 
     <literal>'now'</literal> should be interpreted as 
     <type>timestamp</type> because the target field 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
     backend. 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.  (EXECUTE 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 query, and is sent to the main database engine to execute
    (after substitution for any <application>PL/pgSQL</application> variables
    used in the statement).  Thus,
    for example, SQL <command>INSERT</>, <command>UPDATE</>, and
    <command>DELETE</> commands 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>SELECT INTO</title>

    <para>
     The result of a SELECT 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>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. Note that this is quite different from
     <productname>PostgreSQL</>'s normal interpretation of SELECT INTO, which is that the
     INTO target is a newly created table.  (If you want to create a
     table from a 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(s), 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 INTO clause, the SELECT statement is the same as a normal
     SQL SELECT query and can use the full power of SELECT.
    </para>

    <para>
     If the SELECT query returns zero rows, NULLs are assigned to the
     target(s).  If the SELECT 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 ORDER BY.)
    </para>

    <para>
     At present, the INTO clause can appear almost anywhere in the SELECT
     query, but it is recommended to place it immediately after the SELECT
     keyword as depicted above.  Future versions of
     <application>PL/pgSQL</application> may be less forgiving about
     placement of the INTO clause.
    </para>

    <para>
     There is a special variable named FOUND of type
     <type>boolean</type> that can be used immediately after a SELECT
     INTO to check if an assignment had success (that is, at least one
     row was returned by the SELECT).  For example,
  
<programlisting>
SELECT INTO myrec * FROM EMP WHERE empname = myname;
IF NOT FOUND THEN
    RAISE EXCEPTION ''employee % not found'', myname;
END IF;
</programlisting>

     Alternatively, you can use the IS NULL (or ISNULL) conditional to
     test for NULLity of a RECORD/ROW result.  Note that there is no
     way to tell whether any additional rows might have been discarded.
    </para>

    <para>
<programlisting>
DECLARE
    users_rec RECORD;
    full_name varchar;
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 PERFORM statement:

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

     This executes a <literal>SELECT</literal>
     <replaceable>query</replaceable> and discards the
     result. <application>PL/pgSQL</application> variables are substituted
     in the query as usual.
    </para>

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

    <para>
     An example:
<programlisting>
PERFORM create_mv(''cs_session_page_requests_mv'',''
     SELECT   session_id, page_id, count(*) AS n_hits,
              sum(dwell_time) AS dwell_time, count(dwell_time) AS dwell_count
     FROM     cs_fact_table
     GROUP BY session_id, page_id '');
</programlisting>
    </para>
   </sect2>
   
   <sect2 id="plpgsql-statements-executing-dyn-queries">
    <title>Executing dynamic queries</title>
    
    <para>
     Oftentimes you will want to generate dynamic queries inside
     your <application>PL/pgSQL</application> functions, that is,
     queries 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 queries will not work in such
     scenarios.  To handle this sort of problem, the EXECUTE statement
     is provided:

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

     where <replaceable>query-string</replaceable> is an expression
     yielding a string (of type
     <type>text</type>) containing the <replaceable>query</replaceable>
     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 query string.  The values of variables must
     be inserted in the query string as it is constructed.
    </para>

    <para>
    When working with dynamic queries you will have to face
    escaping of single quotes in <application>PL/pgSQL</>. Please refer to the
    table in <xref linkend="plpgsql-porting">
    for a detailed explanation that will save you some effort.
    </para>
     
    <para>
     Unlike all other queries in <application>PL/pgSQL</>, a
     <replaceable>query</replaceable> run by an EXECUTE statement is
     not prepared and saved just once during the life of the server.
     Instead, the <replaceable>query</replaceable> is prepared each
     time the statement is run. The
     <replaceable>query-string</replaceable> can be dynamically
     created within the procedure to perform actions on variable
     tables and fields.
    </para>
  
    <para>
     The results from SELECT queries are discarded by EXECUTE, and
     SELECT INTO is not currently supported within EXECUTE.  So, the
     only way to extract a result from a dynamically-created SELECT is
     to use the FOR-IN-EXECUTE form described later.
    </para>

    <para>
     An example:
<synopsis>
EXECUTE ''UPDATE tbl SET ''
        || quote_ident(fieldname)
        || '' = ''
        || quote_literal(newvalue)
        || '' WHERE ...'';
</synopsis>
    </para>

    <para>
     This example shows use of the functions
     <function>quote_ident</function>(<type>TEXT</type>) and
     <function>quote_literal</function>(<type>TEXT</type>).
     Variables containing field and table identifiers should be
     passed to function <function>quote_ident()</function>.
     Variables containing literal elements of the dynamic query
     string should be passed to
     <function>quote_literal()</function>.  Both take the
     appropriate steps to return the input text enclosed in single
     or double quotes and with any embedded special characters
     properly escaped.
    </para>

    <para>
     Here is a much larger example of a dynamic query and EXECUTE:
<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'''';''; 
 
    -- This works because we are not substituting any variables
    -- Otherwise it would fail. Look at PERFORM for another way to run functions
    
    EXECUTE a_output; 
END; 
' LANGUAGE 'plpgsql';
</programlisting>
    </para>
   </sect2>

   <sect2 id="plpgsql-statements-diagnostics">
    <title>Obtaining result status</title>

    <para>
<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 keyword 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> query 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> query.  Note that <varname>RESULT_OID</>
     is only useful after an INSERT query.
    </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>
<synopsis>
RETURN <replaceable>expression</replaceable>;
</synopsis>

     The function terminates and the value of
     <replaceable>expression</replaceable> will be returned to the
     upper executor.
     The expression's result will be automatically cast into the
     function's return type as described for assignments.
    </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 RETURN statement, a run-time error
     will occur.
    </para>
   </sect2>
    
   <sect2 id="plpgsql-conditionals">
    <title>Conditionals</title>

    <para>
     <function>IF</function> statements let you execute commands based on
      certain conditions.
      <application>PL/pgSQL</> has four forms of IF: IF-THEN, IF-THEN-ELSE,
      IF-THEN-ELSE IF, and IF-THEN-ELSIF-THEN-ELSE.
    </para>

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

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

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

<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>IF-THEN-ELSE</title>

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

        IF-THEN-ELSE statements add to IF-THEN by letting you
        specify an alternative set of statements that should be executed if
	the condition evaluates to FALSE.

<programlisting>
IF parentid IS NULL or parentid = ''''
THEN 
    return fullname;
ELSE
    return hp_true_filename(parentid) || ''/'' || fullname;
END IF;


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>IF-THEN-ELSE IF</title>

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

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

       <para>
<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>

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

       <para>
        The final ELSE section is optional.
       </para>

     </sect3>
   </sect2>

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

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

     <sect3>
      <title>LOOP</title>

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

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

     <sect3>
      <title>EXIT</title>

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

        If no <replaceable>label</replaceable> is given,
        the innermost loop is terminated and the
        statement following 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
        END.
       </para>

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

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

LOOP
    -- some computations
    EXIT WHEN count > 0;
END LOOP;

BEGIN
    -- some computations
    IF stocks > 100000 THEN
        EXIT;  -- illegal. Can't use EXIT outside of a LOOP
    END IF;
END;
</programlisting>
       </para>
     </sect3>

     <sect3>
      <title>WHILE</title>

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

        The 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 > 0 AND gift_certificate_balance > 0 LOOP
    -- some computations here
END LOOP;

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

     <sect3>
      <title>FOR (integer for-loop)</title>

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

        This form of FOR creates a loop that iterates over a range of integer
	values. The variable 
        <replaceable>name</replaceable> is automatically defined as 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 REVERSE is
	specified.
       </para>

       <para>
        Some examples of integer FOR loops:
<programlisting>
FOR i IN 1..10 LOOP
  -- some expressions here

    RAISE NOTICE ''i is %'',i;
END LOOP;

FOR i IN REVERSE 10..1 LOOP
    -- some expressions here
END LOOP;
</programlisting>
       </para>
     </sect3>
   </sect2>

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

    <para>
     Using a different type of FOR loop, you can iterate through
     the results of a query and manipulate that data
     accordingly. The syntax is:
<synopsis>
<optional>&lt;&lt;label&gt;&gt;</optional>
FOR <replaceable>record | row</replaceable> IN <replaceable>select_query</replaceable> LOOP
    <replaceable>statements</replaceable>
END LOOP;
</synopsis>
     The record or row variable is successively assigned all the rows 
     resulting from the SELECT query and the loop body is executed 
     for each row. Here is an example:
    </para>

    <para>
<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 EXIT statement, the last
     assigned row value is still accessible after the loop.
    </para>

    <para>
     The FOR-IN-EXECUTE statement is another way to iterate over
     records:
<synopsis>
<optional>&lt;&lt;label&gt;&gt;</optional>
FOR <replaceable>record | 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 SELECT
     statement is specified as a string expression, which is evaluated
     and re-planned on each entry to the FOR loop.  This allows the
     programmer to choose the speed of a pre-planned query or the
     flexibility of a dynamic query, just as with a plain EXECUTE
     statement.
    </para>

    <note>
    <para>
     The <application>PL/pgSQL</> parser presently distinguishes the
     two kinds of FOR loops (integer or record-returning) by checking
     whether the target variable mentioned just after FOR has been
     declared as a record/row variable.  If not, it's presumed to be
     an integer FOR loop.  This can cause rather nonintuitive error
     messages when the true problem is, say, that one has
     misspelled the FOR variable name.
    </para>
    </note>
  </sect2>
  </sect1>

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

   <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 don't normally need
    to worry about that, since FOR loops automatically use a cursor
    internally to avoid memory problems.) A more interesting usage is to
    return a reference to a cursor that it has created, allowing the
    caller to read the rows. This provides a way to return 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>select_query</replaceable> ;
</synopsis>
     (<literal>FOR</> may be replaced by <literal>IS</> for Oracle
     compatibility.)  <replaceable>arguments</replaceable>, if any,
     are a comma-separated list of <replaceable>name</replaceable>
     <replaceable>datatype</replaceable> pairs 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 int) 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
     four forms of the OPEN statement, two of which use unbound cursor
     variables and the other two use bound cursor variables.
    </para>

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

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

        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 SELECT query is treated
	in the same way as other SELECTs in <application>PL/pgSQL</>:
	<application>PL/pgSQL</> variable names are substituted,
	and the query plan is cached for possible re-use.

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

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

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

        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 EXECUTE command.
	As usual, this gives flexibility so the query can vary
	from one run to the next.

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

    <sect3>
     <title>OPENing a bound cursor</title>

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

        This form of OPEN 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 EXECUTE in this case.

<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 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 useful to reference an open cursor
     only until the end of the transaction.
    </para>

    <sect3>
     <title>FETCH</title>

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

        FETCH 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 SELECT INTO.  As with
	SELECT INTO, the special variable FOUND may be checked to see
	whether a row was obtained or not.

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

    <sect3>
     <title>CLOSE</title>

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

        CLOSE 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.

<programlisting>
CLOSE curs1;
</programlisting>
       </para>
     </sect3>
 
    <sect3>
     <title>Returning Cursors</title>

       <para>

        <application>PL/pgSQL</> functions can return cursors to the
        caller. This is used to return multiple rows or columns from the
        function. The function opens the cursor and returns the cursor
        name 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 cursor name returned by the function can be specified by the
        caller or automatically generated. The following example shows
        how 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 RAISE 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 DEBUG (write the message into the postmaster log),
    NOTICE (write the message into the postmaster log and forward it to
    the client application) and EXCEPTION (raise an error,
    aborting the transaction).
   </para>

   <para>
    Inside the format string, <literal>%</literal> is replaced by the next
    optional argument's external 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>
    Examples:
<programlisting>
RAISE NOTICE ''Calling cs_create_job(%)'',v_job_id;
</programlisting>
    In this example, the value of v_job_id will replace the % in the
    string.
   </para>

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

  <sect2 id="plpgsql-exceptions">
   <title>Exceptions</title>

    <para>
     <productname>PostgreSQL</productname> does not have a very smart
     exception handling model. Whenever the parser, planner/optimizer
     or executor decide that a statement cannot be processed any longer,
     the whole transaction gets aborted and the system jumps back
     into the main loop to get the next query from the client application.
    </para>

    <para>
     It is possible to hook into the error mechanism to notice that this
     happens. But currently it is impossible to tell what really
     caused the abort (input/output conversion error, floating-point
     error, parse error). And it is possible that the database backend
     is in an inconsistent state at this point so returning to the upper
     executor or issuing more commands might corrupt the whole database.
    </para>

    <para>
     Thus, the only thing <application>PL/pgSQL</application> currently does when it encounters
     an abort during execution of a function or trigger
     procedure is to write some additional NOTICE level log messages
     telling in which function and where (line number and type of
     statement) this happened.  The error always stops execution of
     the function.
    </para>
  </sect2>
 </sect1>

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

  <para>
   <application>PL/pgSQL</application> can be used to define trigger
   procedures. A trigger procedure is created with the <command>CREATE
   FUNCTION</command> command as a function with no arguments and a return
   type of <type>OPAQUE</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 INSERT/UPDATE
       operations in ROW 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 UPDATE/DELETE
       operations in ROW 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 is 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 and can be given as an expression. Invalid
       indices (&lt; 0 or &gt;= tg_nargs) result in a NULL value.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>

   <para>
    A trigger function must return either NULL or a record/row value
    having exactly the structure of the table the trigger was fired for.
    Triggers fired BEFORE may return NULL to signal the trigger manager
    to skip the rest of the operation for this row (ie, subsequent triggers
    are not fired, and the INSERT/UPDATE/DELETE does not occur for this
    row).  If a non-NULL value is returned then the operation proceeds with
    that row value.  Note that returning a row value different from the
    original value of NEW alters the row that will be inserted or updated.
    It is possible to replace single values directly
    in NEW and return that, or to build a complete new record/row to
    return.
   </para>

   <para>
    The return value of a trigger fired AFTER is ignored; it may as well
    always return a NULL value.  But an AFTER trigger can still abort the
    operation by raising an error.
   </para>

   <example>
    <title>A <application>PL/pgSQL</application> Trigger Procedure Example</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 ensures that an employee's name is given and that the
     salary is a positive value.

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

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

        -- Who works for us when she must pay for?
        IF NEW.salary < 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;
' LANGUAGE 'plpgsql';

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

  <!-- **** PL/pgSQL Examples **** -->

  <sect1 id="plpgsql-examples">
   <title>Examples</title>

   <para>
    Here are only a few functions to demonstrate how easy it is to
    write <application>PL/pgSQL</>
    functions. For more complex examples the programmer
    might look at the regression test for <application>PL/pgSQL</>.
   </para>

   <para>
    One painful detail in writing functions in <application>PL/pgSQL</application> is the handling
    of single quotes. The function's source text in <command>CREATE FUNCTION</command> must
    be a literal string. Single quotes inside of literal strings must be
    either doubled or quoted with a backslash. We are still looking for
    an elegant alternative. In the meantime, doubling the single quotes
    as in the examples below should be used. Any solution for this
    in future versions of <productname>PostgreSQL</productname> will be
    forward compatible.
   </para>

   <para>
    For a detailed explanation and examples of how to escape single
    quotes in different situations, please see <xref linkend="plpgsql-quote">.
   </para>

   <example>
    <title>A Simple <application>PL/pgSQL</application> Function to Increment an Integer</title>

    <para>
     The following two <application>PL/pgSQL</application> functions are identical to their
     counterparts from the C language function discussion. This
     function receives an <type>integer</type> and increments it by
     one, returning the incremented value.
    </para>

<programlisting>
CREATE FUNCTION add_one (integer) RETURNS INTEGER AS '
    BEGIN
        RETURN $1 + 1;
    END;
' LANGUAGE 'plpgsql';
</programlisting>
   </example>

   <example>
    <title>A Simple <application>PL/pgSQL</application> Function to Concatenate Text</title>

    <para>
     This function receives two <type>text</type> parameters and
     returns the result of concatenating them.
    </para>

<programlisting>
CREATE FUNCTION concat_text (TEXT, TEXT) RETURNS TEXT AS '
    BEGIN
        RETURN $1 || $2;
    END;
' LANGUAGE 'plpgsql';
</programlisting>
   </example>

   <example>
    <title>A <application>PL/pgSQL</application> Function on Composite Type</title>

    <para>
     In this example, we take <literal>EMP</> (a table) and an
     <type>integer</type> as arguments to our function, which returns
     a <type>boolean</type>. If the <structfield>salary</> field of the <structname>EMP</> table is
     <literal>NULL</literal>, we return <literal>f</>. Otherwise we compare with
     that field with the <type>integer</type> passed to the function
     and return the <type>boolean</type> result of the comparison (t
     or f). This is the <application>PL/pgSQL</application> equivalent to the example from the C
     functions.
    </para>

<programlisting>
CREATE FUNCTION c_overpaid (EMP, INTEGER) RETURNS BOOLEAN AS '
    DECLARE
        emprec ALIAS FOR $1;
        sallim ALIAS FOR $2;
    BEGIN
        IF emprec.salary ISNULL THEN
            RETURN ''f'';
        END IF;
        RETURN emprec.salary > sallim;
    END;
' LANGUAGE 'plpgsql';
</programlisting>
   </example>
  </sect1>

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

 <sect1 id="plpgsql-porting">

  <sect1info>
   <date>
    February 2001
   </date>
   <author>
    <firstname>Roberto</firstname>
    <surname>Mello</surname>
    <affiliation>
     <address>
      <email>rmello@fslc.usu.edu</email>
     </address>
    </affiliation>
   </author>
<!--
  Breaks HTML manifest file
   <legalnotice>
    <para>
     Except for portions of this document quoted from other sources,
     this document is licensed under the BSD License.
    </para>
   </legalnotice>
-->
  </sect1info> 

  <title>Porting from Oracle PL/SQL</title>

  <indexterm zone="plpgsql-porting">
   <primary>Oracle</primary>
  </indexterm>

  <indexterm zone="plpgsql-porting">
   <primary>PL/SQL</primary>
  </indexterm>

  <note>
   <title>Author</title>
   <para>
    Roberto Mello (<email>rmello@fslc.usu.edu</email>)
   </para>
  </note>

  <para>
   This section explains differences between Oracle's PL/SQL and
   <productname>PostgreSQL</>'s <application>PL/pgSQL</application> languages in the hopes of helping developers
   port applications from Oracle to <productname>PostgreSQL</>.  Most of the code here
   is from the <ulink url="http://www.arsdigita.com">ArsDigita</ulink>
   <ulink url="http://www.arsdigita.com/asj/clickstream">Clickstream
   module</ulink> that I ported to <productname>PostgreSQL</> when I took an
   internship with <ulink url="http://www.openforce.net">OpenForce
   Inc.</ulink> in the Summer of 2000.
  </para>

  <para>
   <application>PL/pgSQL</application> is similar to PL/SQL in many aspects. It is a block
   structured, imperative language (all variables have to be
   declared). PL/SQL has many more features than its <productname>PostgreSQL</>
   counterpart, but <application>PL/pgSQL</application> allows for a great deal of functionality
   and it is being improved constantly.
  </para>

  <sect2>
   <title>Main Differences</title>

   <para>
    Some things you should keep in mind when porting from Oracle to <productname>PostgreSQL</>:

    <itemizedlist>
     <listitem>
      <para>
       No default parameters in <productname>PostgreSQL</>.
      </para>
     </listitem>

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

     <listitem>
      <para>
       Assignments, loops and conditionals are similar. 
      </para>
     </listitem>

     <listitem>
      <para>
       No need for cursors in <productname>PostgreSQL</>, just put the query in the FOR 
       statement (see example below)
      </para>
     </listitem>

     <listitem>
      <para>
       In <productname>PostgreSQL</> you <emphasis>need</emphasis> to escape single
       quotes. See <xref linkend="plpgsql-quote">.
      </para>
     </listitem>
    </itemizedlist>
   </para>

   <sect3 id="plpgsql-quote">
    <title>Quote Me on That: Escaping Single Quotes</title>

    <para>
     In <productname>PostgreSQL</> you need to escape single quotes inside your
     function definition. This can lead to quite amusing code at
     times, especially if you are creating a function that generates
     other function(s), as in 
     <xref linkend="plpgsql-porting-nastyquote">.  
     One thing to keep in mind
     when escaping lots of single quotes is that, except for the
     beginning/ending quotes, all the others will come in even
     quantity.
    </para>

    <para>
     <xref linkend="plpgsql-quoting-table"> gives the scoop.  (You'll
     love this little chart.)
    </para>

    <table id="plpgsql-quoting-table">
     <title>Single Quotes Escaping Chart</title>

     <tgroup cols="4">
      <thead>
       <row>
        <entry>No. of Quotes</entry>
        <entry>Usage</entry>
        <entry>Example</entry>
        <entry>Result</entry>
       </row>
      </thead>

      <tbody>
       <row>
        <entry>1</entry>
        <entry>To begin/terminate function bodies</entry>
        <entry><programlisting>
CREATE FUNCTION foo() RETURNS INTEGER AS '...'
LANGUAGE 'plpgsql';
</programlisting></entry>
        <entry>as is</entry>
       </row>

       <row>
        <entry>2</entry>
        <entry>In assignments, SELECTs, to delimit strings, etc.</entry>
        <entry><programlisting>
a_output := ''Blah'';
SELECT * FROM users WHERE f_name=''foobar'';
</programlisting></entry>
        <entry><literal>SELECT * FROM users WHERE f_name='foobar';</literal></entry>
       </row>

       <row>
        <entry>4</entry>
        <entry>
         When you need two single quotes in your resulting string
         without terminating that string.
        </entry>
        <entry><programlisting>
a_output := a_output || '' AND name 
    LIKE ''''foobar'''' AND ...''
</programlisting></entry>
        <entry><literal>AND name LIKE 'foobar' AND ...</literal></entry>
       </row>

       <row>
        <entry>6</entry>
        <entry>
         When you want double quotes in your resulting string
         <emphasis>and</emphasis> terminate that string.
        </entry>
        <entry><programlisting>
a_output := a_output || '' AND name 
    LIKE ''''foobar''''''
</programlisting></entry>
        <entry>
         <literal>AND name LIKE 'foobar'</literal>
        </entry>
       </row>

       <row>
        <entry>10</entry>
        <entry>
         When you want two single quotes in the resulting string
         (which accounts for 8 quotes) <emphasis>and</emphasis>
         terminate that string (2 more).  You will probably only need
         that if you were using a function to generate other functions
         (like in <xref linkend="plpgsql-porting-nastyquote">).
        </entry>
        <entry><programlisting>
a_output := a_output || '' if v_'' || 
    referrer_keys.kind || '' like '''''''''' 
    || referrer_keys.key_string || '''''''''' 
    then return ''''''  || referrer_keys.referrer_type 
    || ''''''; end if;''; 
</programlisting></entry>
        <entry>
         <literal>if v_<...> like ''<...>'' then return ''<...>''; end if;</literal>
        </entry>
       </row>
      </tbody>
     </tgroup>
    </table>
   </sect3>
  </sect2>  

  <sect2 id="plpgsql-porting-functions">
   <title>
    Porting Functions
   </title>

   <example>
    <title>
     A Simple Function
    </title>

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

      <listitem>
       <para>
        On <productname>PostgreSQL</> functions are created using single quotes as
        delimiters, so you have to escape single quotes inside your
        functions (which can be quite annoying at times; see <xref
        linkend="plpgsql-quote">).
       </para>
      </listitem>

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

    <para>
     So let's see 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>
 
   <example id="plpgsql-porting-nastyquote">
    <title>
     A Function that Creates Another Function
    </title>

    <para>
     The following procedure grabs rows from a
     <literal>SELECT</literal> statement and builds a large function
     with the results in <literal>IF</literal> statements, for the
     sake of efficiency. Notice particularly the differences in
     cursors, <literal>FOR</literal> loops, and the need to escape
     single quotes in <productname>PostgreSQL</>.

<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 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'''';''; 
 
    -- This works because we are not substituting any variables
    -- Otherwise it would fail. Look at PERFORM for another way to run functions
    
    EXECUTE a_output; 
END; 
' LANGUAGE 'plpgsql';
</programlisting>
    </para>
   </example>
 
   <example>
    <title>
     A Procedure with a lot of String Manipulation and OUT Parameters
    </title>

    <para>
     The following Oracle PL/SQL procedure is used to parse a URL and
     return several elements (host, path and query). It is an
     procedure because in <application>PL/pgSQL</application> functions only one value can be returned
     (see <xref linkend="plpgsql-porting-procedures">).  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>

<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, '//'); -- <productname>PostgreSQL</> doesn't have an instr function

    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>
     Here is how this procedure could be translated for <productname>PostgreSQL</>:

<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>

   <note>
    <para>
     <productname>PostgreSQL</> does not have an <function>instr</function> function,
     so you can work around it using a combination of other functions.
     I got tired of doing this and created my own
     <function>instr</function> functions that behave exactly like
     Oracle's (it makes life easier). See the <xref
     linkend="plpgsql-porting-appendix"> for the code.
    </para>
   </note>
  </sect2>

  <sect2 id="plpgsql-porting-procedures">
   <title>
    Procedures
   </title>

   <para>
    Oracle procedures give a little more flexibility to the developer
    because nothing needs to be explicitly returned, but it can be
    through the use of <literal>INOUT</> or <literal>OUT</> parameters.
   </para>

   <para>
    An example:

<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 <literal>LOCK TABLE</literal> 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> procedures. The
       entire function (and other functions called from therein) is
       executed in a transaction and <productname>PostgreSQL</> rolls back the results if
       something goes wrong. Therefore only one
       <literal>BEGIN</literal> statement is allowed.
      </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>
    So let's see one of the ways 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;
    -- PRAGMA AUTONOMOUS_TRANSACTION;
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
        -- COMMIT; -- free lock
        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, sysdate());
        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>
  </sect2>

  <sect2 id="plpgsql-porting-packages">
   <title>
    Packages
   </title>

   <note>
    <para>
     I haven't done much with packages myself, so if there are
     mistakes here, please let me know.
    </para>
   </note>

   <para>
    Packages are a way Oracle gives you to encapsulate PL/SQL
    statements and functions into one entity, like Java classes, where
    you define methods and objects. You can access these
    objects/methods with a <quote><literal>.</literal></quote>
    (dot). Here is an example of an Oracle package from ACS 4 (the
    <ulink url="http://www.arsdigita.com/doc/">ArsDigita Community
    System</ulink>):

<programlisting>
CREATE OR REPLACE PACKAGE BODY acs
AS
  FUNCTION add_user (
    user_id     IN users.user_id%TYPE DEFAULT NULL,
    object_type     IN acs_objects.object_type%TYPE DEFAULT 'user',
    creation_date   IN acs_objects.creation_date%TYPE DEFAULT sysdate,
    creation_user   IN acs_objects.creation_user%TYPE DEFAULT NULL,
    creation_ip     IN acs_objects.creation_ip%TYPE DEFAULT NULL,
  ...
  ) RETURN users.user_id%TYPE
  IS
    v_user_id       users.user_id%TYPE;
    v_rel_id        membership_rels.rel_id%TYPE;
  BEGIN
    v_user_id := acs_user.new (user_id, object_type, creation_date,
                creation_user, creation_ip, email, ...
    RETURN v_user_id;
  END;
END acs;
/
show errors
</programlisting>
   </para>

   <para>
    We port this to <productname>PostgreSQL</> by creating the different objects of
    the Oracle package as functions with a standard naming
    convention. We have to pay attention to some other details, like
    the lack of default parameters in <productname>PostgreSQL</> functions. The above
    package would become something like this:
 
<programlisting>
CREATE FUNCTION acs__add_user(INTEGER,INTEGER,VARCHAR,DATETIME,INTEGER,INTEGER,...)
RETURNS INTEGER AS '
DECLARE
    user_id ALIAS FOR $1;
    object_type ALIAS FOR $2;
    creation_date ALIAS FOR $3;
    creation_user ALIAS FOR $4;
    creation_ip ALIAS FOR $5;
    ...
    v_user_id users.user_id%TYPE;
    v_rel_id membership_rels.rel_id%TYPE;
BEGIN
    v_user_id := acs_user__new(user_id,object_type,creation_date,creation_user,creation_ip, ...);
    ...

    RETURN v_user_id;
END;
' LANGUAGE 'plpgsql';
</programlisting>
   </para>
  </sect2>

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

   <sect3>
    <title>EXECUTE</title>

    <para>
     The <productname>PostgreSQL</> version of <literal>EXECUTE</literal> works
     nicely, 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-queries">.  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: <literal>iscachable</literal> (function always returns
     the same result when given the same arguments) and
     <literal>isstrict</literal> (function returns NULL if any
     argument is NULL).  Consult the <command>CREATE
     FUNCTION</command> reference for details.
    </para>

    <para>
     To make use of these optimization attributes, you have to use the
     <literal>WITH</literal> modifier in your <command>CREATE
     FUNCTION</command> statement.  Something like:

<programlisting>
CREATE FUNCTION foo(...) RETURNS INTEGER AS '
...
' LANGUAGE 'plpgsql'
WITH (isstrict, iscachable);
</programlisting>
    </para>
   </sect3>
  </sect2>

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

   <sect3>
    <title>
     Code for my <function>instr</function> functions
    </title>

    <comment>
     This function should probably be integrated into the core.
    </comment>

<programlisting>
--
-- instr functions that mimic Oracle's counterpart
-- Syntax: instr(string1,string2,[n],[m]) where [] denotes optional params.
-- 
-- 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).
--
-- by Roberto Mello (rmello@fslc.usu.edu)
-- modified by Robert Gaszewski (graszew@poland.com)
-- Licensed under the GPL v2 or later.
--

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';

--
-- Written by Robert Gaszewski (graszew@poland.com)
-- Licensed under the GPL v2 or later.
--
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>
   </sect3>
  </sect2>
  
 </sect1>

</chapter>

<!-- Keep this comment at the end of the file
Local variables:
mode:sgml
sgml-omittag:nil
sgml-shorttag:t
sgml-minimize-attributes:nil
sgml-always-quote-attributes:t
sgml-indent-step:1
sgml-indent-data:t
sgml-parent-document:nil
sgml-default-dtd-file:"./reference.ced"
sgml-exposed-tags:nil
sgml-local-catalogs:("/usr/lib/sgml/catalog")
sgml-local-ecat-files:nil
End:
-->
