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

  <para>
   PL/pgSQL is a loadable procedural language for the
   <productname>Postgres</productname> database system.
  </para>

  <para>
   This package was originally written by Jan Wieck.
  </para>

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

   <para>
    The design goals of 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 PL/pgSQL call handler parses the function's source text and
    produces an internal binary instruction tree the first time the
    function is called. The produced bytecode is identified
    in the call handler by the object ID of the function. This ensures
    that changing a function by a DROP/CREATE sequence will take effect
    without establishing a new database connection. 
   </para>
   <para>
    For all expressions and <acronym>SQL</acronym> statements used in
    the function, the PL/pgSQL bytecode interpreter creates a
    prepared execution plan using the <acronym>SPI</acronym> manager's <function>SPI_prepare()</function> and
    <function>SPI_saveplan()</function> functions. This is done the first time the individual
    statement is processed in the PL/pgSQL function. Thus, a function with
    conditional code that contains many statements for which execution
    plans would be required, will only prepare and save those plans
    that are really used during the lifetime of the database
    connection.
   </para>
   <para>
    Because PL/pgSQL saves execution plans in this way, queries that appear
    directly in a PL/pgSQL 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 PL/pgSQL
    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 PL/pgSQL. It is possible to
    create complex conditional computation functions and later use
    them to define operators or use them in functional indices.
   </para>
  </sect1>

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

  <sect1 id="plpgsql-description">
   <title>Description</title>

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

   <sect2>
    <title>Structure of PL/pgSQL</title>

    <para>
     The PL/pgSQL language is case insensitive. All keywords and
     identifiers can be used in mixed upper- and lower-case.
    </para>
    <para>
     PL/pgSQL is a block oriented language. 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>
     There can be any number of sub-blocks in the statement section
     of a block. Sub-blocks can be used to hide variables from outside a
     block of statements. 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.
    </para>

    <para>
     It is important not to confuse the use of BEGIN/END for
     grouping statements in PL/pgSQL with the database commands for
     transaction control.  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>Postgres</productname> does not have nested transactions.
    </para>
   </sect2>

   <sect2>
    <title>Comments</title>

    <para>
     There are two types of comments in 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>

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

   <sect2>
    <title>Declarations</title>

    <para>
     All variables, rows and records used in a block or its
     sub-blocks must be declared in the declarations section of a block,
     except for the loop variable of a FOR-loop iterating over a range
     of integer values. Parameters given to a PL/pgSQL function are
     automatically declared with the usual identifiers <literal>$1</literal>, <literal>$2</literal>, etc.
     The declarations have the following syntax:
    </para>
    <variablelist>

     <varlistentry>
      <term>
<replaceable>name</replaceable> <optional> CONSTANT </optional>
<replaceable>type</replaceable> <optional> NOT NULL </optional> <optional> DEFAULT | :=
	<replaceable>value</replaceable> </optional>;
      </term>
      <listitem>
       <para>
	Declares a variable of the specified base type. If the variable
	is declared as CONSTANT, the value cannot be changed. If NOT NULL
	is specified, an assignment of a NULL value results in a runtime
	error. Since the default value of all variables is the
	<acronym>SQL</acronym> NULL value, all variables declared as NOT NULL
	must also have a default value specified.
       </para>
       <para>
	The default value is evaluated every time the block is entered. So
	assigning <literal>'now'</literal> to a variable of type
	<type>timestamp</type> causes the variable to have the
	time of the actual function call, not when the function was
	precompiled into its bytecode.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>
<replaceable>name</replaceable> <replaceable>table</replaceable>%ROWTYPE;
      </term>
      <listitem>
       <para>
	Declares a row with the structure of the given table. <replaceable>table</replaceable> must be
	an existing table or view name of the database. The fields of the row
	are accessed in the dot notation. Parameters to a function can
	be composite types (complete table rows). In that case, the
	corresponding identifier $n will be a rowtype, but it
	must be aliased using the ALIAS command described below. Only the user
	attributes of a table row are accessible in the row, no Oid or other
	system attributes (because the row could be from a view and view rows
	don't have useful system attributes).
       </para>
       <para>
	The fields of the rowtype inherit the table's field sizes 
	or precision for <type>char()</type> etc. data types.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>
<replaceable>name</replaceable> RECORD;
      </term>
      <listitem>
       <para>
	Records are similar to rowtypes, but they have no predefined structure.
	They are used in selections and FOR loops to hold one actual
	database row from a SELECT operation. One and the same record can be
	used in different selections. Accessing a record or an attempt to assign
	a value to a record field when there is no actual row in it results
	in a runtime error.
       </para>
       <para>
	The NEW and OLD rows in a trigger are given to the procedure as
	records. This is necessary because in <productname>Postgres</productname>
	one and the same trigger procedure can handle trigger events for
	different tables.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>
<replaceable>name</replaceable> ALIAS FOR $n;
      </term>
      <listitem>
       <para>
	For better readability of the code it is possible to define an alias
	for a positional parameter to a function.
       </para>
       <para>
	This aliasing is required for composite types given as arguments to
	a function. The dot notation $1.salary as in <acronym>SQL</acronym> functions is not
	allowed in PL/pgSQL.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>
RENAME <replaceable>oldname</replaceable> TO <replaceable>newname</replaceable>;
      </term>
      <listitem>
       <para>
	Change the name of a variable, record or row. This is useful
	if NEW or OLD should be referenced by another name inside a 
	trigger procedure.
       </para>
      </listitem>
     </varlistentry>

    </variablelist>
   </sect2>

   <!-- **** PL/pgSQL data types **** -->

   <sect2>
    <title>Data Types</title>

    <para>
     The type of a variable can be any of the existing base types of
     the database. <replaceable>type</replaceable> in the declarations
     section above is defined as:
    </para>
    <para>
     <itemizedlist>
      <listitem>
       <para>
	<productname>Postgres</productname>-basetype
       </para>
      </listitem>
      <listitem>
       <para>
	<replaceable>variable</replaceable>%TYPE
       </para>
      </listitem>
      <listitem>
       <para>
	<replaceable>table.field</replaceable>%TYPE
       </para>
      </listitem>
     </itemizedlist>
    </para>
    <para>
     <replaceable>variable</replaceable> is the name of a variable,
     previously declared in the 
     same function, that is visible at this point.
    </para>
    <para>
     <replaceable>table</replaceable> is the name of an existing table
     or view where <replaceable>field</replaceable> is the name of
     an attribute.
    </para>
    <para>
     Using the <replaceable>table.field</replaceable>%TYPE
     causes PL/pgSQL to look up the attributes definitions at the
     first call to the function during the lifetime of a backend.
     Suppose we have a table with a <type>char(20)</type> attribute and some PL/pgSQL functions
     that deal with its content in local variables. Now someone
     decides that <type>char(20)</type> is not enough, dumps the table, drops it,
     recreates it now with the attribute in question defined as
     <type>char(40)</type> and restores the data. Hah - he forgot about the
     functions. The computations inside them will truncate the values
     to 20 characters. But if they are defined using the
     <replaceable>table.field</replaceable>%TYPE
     declarations, they will automagically handle the size change or
     if the new table schema defines the attribute as <type>text</type> type.
    </para>
   </sect2>

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

   <sect2>
    <title>Expressions</title>

    <para>
     All expressions used in PL/pgSQL statements are processed using
     the backend's executor. Expressions that appear to contain
     constants may in fact require runtime evaluation (e.g., <literal>'now'</literal> for the
     <type>timestamp</type> type) so
     it is impossible for the PL/pgSQL 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 variable
     identifiers are substituted by parameters and the actual values from
     the variables are passed to the executor in the parameter array. All
     expressions used in a PL/pgSQL function are only prepared and
     saved once.  The only exception to this rule is an EXECUTE statement
     if parsing of a query is needed each time it is encountered.
    </para>
    <para>
     The type checking done by the <productname>Postgres</productname>
     main parser has some side
     effects to the interpretation of constant values. In detail there
     is a difference between what the two functions

<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>

     do. In the case of <function>logfunc1()</function>, the
     <productname>Postgres</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 logtable 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>Postgres</productname> 
     main parser does not know
     what type <literal>'now'</literal> should become and therefore it returns a data type of
     <type>text</type> containing the string <literal>'now'</literal>. During the assignment
     to the local variable curtime, the PL/pgSQL interpreter casts this
     string to the timestamp type by calling the <function>text_out()</function> and <function>timestamp_in()</function>
     functions for the conversion.
    </para>
    <para>
     This type checking done by the <productname>Postgres</productname> main
     parser got implemented after PL/pgSQL was nearly done.
     It is a difference between 6.3 and 6.4 and affects all functions
     using the prepared plan feature of the <acronym>SPI</acronym> manager.
     Using a local
     variable in the above manner is currently the only way in PL/pgSQL to get
     those values interpreted correctly.
    </para>
    <para>
     If record fields are used in expressions or statements, the data types of
     fields should not change between calls of one and the same expression.
     Keep this in mind when writing trigger procedures that handle events
     for more than one table.
    </para>
   </sect2>

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

   <sect2>
    <title>Statements</title>

    <para>
     Anything not understood by the PL/pgSQL parser as specified below
     will be put into a query and sent down to the database engine
     to execute. The resulting query should not return any data.
    </para>

    <variablelist>

     <varlistentry>
      <term>Assignment</term>
      <listitem>
       <para>
	An assignment of a value to a variable or row/record field is
	written as
<synopsis>
<replaceable>identifier</replaceable> := <replaceable>expression</replaceable>;
</synopsis>
	If the expressions result data type doesn't match the variables
	data type, or the variable has a size/precision that is known
	(as for <type>char(20)</type>), the result value will be implicitly cast by
	the PL/pgSQL bytecode interpreter using the result types output- and
	the variables type input-functions. Note that this could potentially
	result in runtime errors generated by the types input functions.
       </para>
       <para>
	An assignment of a complete selection into a record or row can
	be done by
<synopsis>
SELECT  INTO <replaceable>target</replaceable> <replaceable>expressions</replaceable> FROM ...;
</synopsis>
	<replaceable>target</replaceable> can be a record, a row variable or a
	comma separated list of variables and record-/row-fields.  Note that
	this is quite different from Postgres' 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 PL/pgSQL
	function, use the equivalent syntax CREATE TABLE AS SELECT.)
       </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 runtime error
	occurs. The FROM keyword can be followed by any valid qualification,
	grouping, sorting etc. that can be given for a SELECT statement.
       </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.

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

	If the selection returns multiple rows, only the first is moved
	into the target fields. All others are silently discarded.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>Calling another function</term>
      <listitem>
       <para>
	All functions defined in a <productname>Postgres</productname>
	database return a value. Thus, the normal way to call a function
	is to execute a SELECT query or doing an assignment (resulting
	in a PL/pgSQL internal SELECT). But there are cases where someone
	is not interested in the function's result.
<synopsis>
PERFORM <replaceable>query</replaceable>
</synopsis>
	executes a <literal>SELECT <replaceable>query</replaceable></literal> over the
	<acronym>SPI</acronym> manager and discards the result. Identifiers like local
	variables are still substituted into parameters.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>Executing dynamic queries</term>
      <listitem>
       <para>
<synopsis>
EXECUTE <replaceable class="command">query-string</replaceable>
</synopsis>
       where <replaceable>query-string</replaceable> is a string of
       type <type>text</type> containing the <replaceable>query</replaceable> to be
       executed.
       </para>
	    
       <para>
        Unlike all other queries in 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 ... EXECUTE form described later.
       </para>

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

     <varlistentry>
      <term>Obtaining other results status</term>
      <listitem>
       <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 datatype 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>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>Returning from the function</term>
      <listitem>
       <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 return value of a function
	cannot be undefined. If control reaches the end of the top-level block
	of the function without hitting a RETURN statement, a runtime error
	will occur.
       </para>
       <para>
	The expressions result will be automatically casted into the
	function's return type as described for assignments.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>Aborting and messages</term>
      <listitem>
       <para>
	As indicated in the above examples there is a RAISE statement that
	can throw messages into the <productname>Postgres</productname>
	elog mechanism.
<synopsis>
RAISE <replaceable class="parameter">level</replaceable> '<replaceable class="parameter">format</replaceable>' <optional>, <replaceable class="parameter">identifier</replaceable> <optional>...</optional></optional>;
</synopsis>
	Inside the format, <literal>%</literal> is used as a placeholder for the
	subsequent comma-separated identifiers. Possible levels are
	DEBUG (silently suppressed in production running databases), NOTICE 
	(written into the database log and forwarded to the client application)
	and EXCEPTION (written into the database log and aborting the transaction).
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>Conditionals</term>
      <listitem>
       <para>
<synopsis>
IF <replaceable>expression</replaceable> THEN
    <replaceable>statements</replaceable>
<optional>ELSE
    <replaceable>statements</replaceable></optional>
END IF;
</synopsis>
	The <replaceable>expression</replaceable> must return a value that
	is of type <type>boolean</type> or can be casted to a <type>boolean</type>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>
Loops
      </term>
      <listitem>
       <para>
	There are multiple types of loops.
<synopsis>
<optional>&lt;&lt;label&gt;&gt;</optional>
LOOP
    <replaceable>statements</replaceable>
END LOOP;
</synopsis>
	An unconditional loop that must be terminated explicitly
	by an EXIT statement. The optional label can be used by
	EXIT statements of nested loops to specify which level of
	nesting should be terminated.
<synopsis>
<optional>&lt;&lt;label&gt;&gt;</optional>
WHILE <replaceable>expression</replaceable> LOOP
    <replaceable>statements</replaceable>
END LOOP;
</synopsis>
	A conditional loop that is executed as long as the evaluation
	of <replaceable>expression</replaceable> is true.
<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>
	A loop that iterates over a range of integer values. The variable
	<replaceable>name</replaceable> is automatically created as type
	integer and exists only inside the loop. The two expressions giving
	the lower and upper bound of the range are evaluated only when entering
	the loop. The iteration step is always 1.
<synopsis>
<optional>&lt;&lt;label&gt;&gt;</optional>
FOR <replaceable>record | row</replaceable> IN <replaceable>select_clause</replaceable> LOOP
    <replaceable>statements</replaceable>
END LOOP;
</synopsis>
	The record or row is assigned all the rows resulting from the select
	clause and the loop body is executed for each row. If the loop is
	terminated with an EXIT statement, the last assigned row is still
	accessible after the loop.
<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.
<synopsis>
EXIT <optional> <replaceable>label</replaceable> </optional> <optional> WHEN <replaceable>expression</replaceable> </optional>;
</synopsis>
	If no <replaceable>label</replaceable> 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 an upper level of nested loop
	blocks. Then the named loop or block is terminated and control
	continues with the statement after the loops/blocks corresponding
	END.
       </para>
      </listitem>
     </varlistentry>

    </variablelist>

   </sect2>

   <!-- **** PL/pgSQL trigger procedures **** -->

   <sect2>
    <title>Trigger Procedures</title>

    <para>
     PL/pgSQL can be used to define trigger procedures. They are created
     with the usual CREATE FUNCTION command as a function with no
     arguments and a return type of OPAQUE.
    </para>
    <para>
     There are some <productname>Postgres</productname> specific details
     in functions used as trigger procedures.
    </para>
    <para>
     First they have some special variables created automatically in the 
     top-level blocks declaration section. They are
    </para>

    <variablelist>
     <varlistentry>
      <term><varname>NEW</varname></term>
      <listitem>
       <para>
	Data type RECORD; variable holding the new database row on INSERT/UPDATE
	operations on ROW level triggers.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>OLD</varname></term>
      <listitem>
       <para>
	Data type RECORD; variable holding the old database row on UPDATE/DELETE
	operations on 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
	triggers 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
	triggers 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 actually fired.
       </para>
      </listitem>
     </varlistentry>

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

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

     <varlistentry>
      <term><varname>TG_ARGV[]</varname></term>
      <listitem>
       <para>
	Data type array of <type>text</type>; the arguments from the CREATE TRIGGER 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>
     Second they must return either NULL or a record/row containing
     exactly the structure of the table the trigger was fired for.
     Triggers fired AFTER might always return a NULL value with no
     effect. Triggers fired BEFORE signal the trigger manager
     to skip the operation for this actual row when returning NULL.
     Otherwise, the returned record/row replaces the inserted/updated
     row in the operation. It is possible to replace single values directly
     in NEW and return that or to build a complete new record/row to
     return.
    </para>
   </sect2>

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

   <sect2>
    <title>Exceptions</title>

    <para>
     <productname>Postgres</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.
     And even if, at this point the information, that the transaction
     is aborted, is already sent to the client application, so resuming
     operation does not make any sense.
    </para>
    <para>
     Thus, the only thing PL/pgSQL currently does when it encounters
     an abort during execution of a function or trigger
     procedure is to write some additional DEBUG level log messages
     telling in which function and where (line number and type of
     statement) this happened.
    </para>
   </sect2>
  </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 PL/pgSQL
    functions. For more complex examples the programmer
    might look at the regression test for PL/pgSQL.
   </para>

   <para>
    One painful detail in writing functions in PL/pgSQL is the handling
    of single quotes. The function's source text in the <command>CREATE FUNCTION</command> 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>Postgres</productname> will be
    forward compatible.
   </para>

   <example>
    <title>A Simple PL/pgSQL Functions</title>

    <para>
     The following two PL/pgSQL functions are identical to their
     counterparts from the C language function discussion.

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

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

   </example>

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

    <para>
     Again, this is the PL/pgSQL equivalent to the example from
     the C functions.

<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>
    </para>

   </example>

   <example>
    <title>A PL/pgSQL Trigger Procedure</title>

    <para>
     This 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 employees 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>
 </chapter>

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