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Since procedures are now a different thing from functions, change the CREATE TRIGGER and CREATE EVENT TRIGGER syntax to use FUNCTION in the clause that specifies the function. PROCEDURE is still accepted for compatibility. pg_dump and ruleutils.c output is not changed yet, because that would require a change in information_schema.sql and thus a catversion change. Reported-by: Peter Geoghegan <pg@bowt.ie> Reviewed-by: Jonathan S. Katz <jonathan.katz@excoventures.com>
957 lines
37 KiB
Plaintext
957 lines
37 KiB
Plaintext
<!-- doc/src/sgml/trigger.sgml -->
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<chapter id="triggers">
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<title>Triggers</title>
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<indexterm zone="triggers">
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<primary>trigger</primary>
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</indexterm>
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<para>
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This chapter provides general information about writing trigger functions.
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Trigger functions can be written in most of the available procedural
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languages, including
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<application>PL/pgSQL</application> (<xref linkend="plpgsql"/>),
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<application>PL/Tcl</application> (<xref linkend="pltcl"/>),
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<application>PL/Perl</application> (<xref linkend="plperl"/>), and
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<application>PL/Python</application> (<xref linkend="plpython"/>).
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After reading this chapter, you should consult the chapter for
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your favorite procedural language to find out the language-specific
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details of writing a trigger in it.
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</para>
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<para>
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It is also possible to write a trigger function in C, although
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most people find it easier to use one of the procedural languages.
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It is not currently possible to write a trigger function in the
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plain SQL function language.
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</para>
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<sect1 id="trigger-definition">
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<title>Overview of Trigger Behavior</title>
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<para>
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A trigger is a specification that the database should automatically
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execute a particular function whenever a certain type of operation is
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performed. Triggers can be attached to tables (partitioned or not),
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views, and foreign tables.
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</para>
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<para>
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On tables and foreign tables, triggers can be defined to execute either
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before or after any <command>INSERT</command>, <command>UPDATE</command>,
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or <command>DELETE</command> operation, either once per modified row,
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or once per <acronym>SQL</acronym> statement.
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<command>UPDATE</command> triggers can moreover be set to fire only if
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certain columns are mentioned in the <literal>SET</literal> clause of
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the <command>UPDATE</command> statement. Triggers can also fire
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for <command>TRUNCATE</command> statements. If a trigger event occurs,
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the trigger's function is called at the appropriate time to handle the
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event.
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</para>
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<para>
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On views, triggers can be defined to execute instead of
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<command>INSERT</command>, <command>UPDATE</command>, or
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<command>DELETE</command> operations.
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Such <literal>INSTEAD OF</literal> triggers
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are fired once for each row that needs to be modified in the view.
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It is the responsibility of the
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trigger's function to perform the necessary modifications to the view's
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underlying base table(s) and, where appropriate, return the modified
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row as it will appear in the view. Triggers on views can also be defined
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to execute once per <acronym>SQL</acronym> statement, before or after
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<command>INSERT</command>, <command>UPDATE</command>, or
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<command>DELETE</command> operations.
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However, such triggers are fired only if there is also
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an <literal>INSTEAD OF</literal> trigger on the view. Otherwise,
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any statement targeting the view must be rewritten into a statement
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affecting its underlying base table(s), and then the triggers
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that will be fired are the ones attached to the base table(s).
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</para>
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<para>
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The trigger function must be defined before the trigger itself can be
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created. The trigger function must be declared as a
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function taking no arguments and returning type <literal>trigger</literal>.
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(The trigger function receives its input through a specially-passed
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<structname>TriggerData</structname> structure, not in the form of ordinary function
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arguments.)
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</para>
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<para>
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Once a suitable trigger function has been created, the trigger is
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established with
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<xref linkend="sql-createtrigger"/>.
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The same trigger function can be used for multiple triggers.
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</para>
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<para>
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<productname>PostgreSQL</productname> offers both <firstterm>per-row</firstterm>
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triggers and <firstterm>per-statement</firstterm> triggers. With a per-row
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trigger, the trigger function
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is invoked once for each row that is affected by the statement
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that fired the trigger. In contrast, a per-statement trigger is
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invoked only once when an appropriate statement is executed,
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regardless of the number of rows affected by that statement. In
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particular, a statement that affects zero rows will still result
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in the execution of any applicable per-statement triggers. These
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two types of triggers are sometimes called <firstterm>row-level</firstterm>
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triggers and <firstterm>statement-level</firstterm> triggers,
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respectively. Triggers on <command>TRUNCATE</command> may only be
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defined at statement level, not per-row.
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</para>
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<para>
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Triggers are also classified according to whether they fire
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<firstterm>before</firstterm>, <firstterm>after</firstterm>, or
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<firstterm>instead of</firstterm> the operation. These are referred to
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as <literal>BEFORE</literal> triggers, <literal>AFTER</literal> triggers, and
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<literal>INSTEAD OF</literal> triggers respectively.
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Statement-level <literal>BEFORE</literal> triggers naturally fire before the
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statement starts to do anything, while statement-level <literal>AFTER</literal>
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triggers fire at the very end of the statement. These types of
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triggers may be defined on tables, views, or foreign tables. Row-level
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<literal>BEFORE</literal> triggers fire immediately before a particular row is
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operated on, while row-level <literal>AFTER</literal> triggers fire at the end of
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the statement (but before any statement-level <literal>AFTER</literal> triggers).
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These types of triggers may only be defined on non-partitioned tables and
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foreign tables, not views. <literal>INSTEAD OF</literal> triggers may only be
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defined on views, and only at row level; they fire immediately as each
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row in the view is identified as needing to be operated on.
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</para>
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<para>
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A statement that targets a parent table in an inheritance or partitioning
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hierarchy does not cause the statement-level triggers of affected child
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tables to be fired; only the parent table's statement-level triggers are
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fired. However, row-level triggers of any affected child tables will be
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fired.
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</para>
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<para>
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If an <command>INSERT</command> contains an <literal>ON CONFLICT
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DO UPDATE</literal> clause, it is possible that the effects of
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row-level <literal>BEFORE</literal> <command>INSERT</command> triggers and
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row-level <literal>BEFORE</literal> <command>UPDATE</command> triggers can
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both be applied in a way that is apparent from the final state of
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the updated row, if an <varname>EXCLUDED</varname> column is referenced.
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There need not be an <varname>EXCLUDED</varname> column reference for
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both sets of row-level <literal>BEFORE</literal> triggers to execute,
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though. The
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possibility of surprising outcomes should be considered when there
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are both <literal>BEFORE</literal> <command>INSERT</command> and
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<literal>BEFORE</literal> <command>UPDATE</command> row-level triggers
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that change a row being inserted/updated (this can be
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problematic even if the modifications are more or less equivalent, if
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they're not also idempotent). Note that statement-level
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<command>UPDATE</command> triggers are executed when <literal>ON
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CONFLICT DO UPDATE</literal> is specified, regardless of whether or not
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any rows were affected by the <command>UPDATE</command> (and
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regardless of whether the alternative <command>UPDATE</command>
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path was ever taken). An <command>INSERT</command> with an
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<literal>ON CONFLICT DO UPDATE</literal> clause will execute
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statement-level <literal>BEFORE</literal> <command>INSERT</command>
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triggers first, then statement-level <literal>BEFORE</literal>
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<command>UPDATE</command> triggers, followed by statement-level
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<literal>AFTER</literal> <command>UPDATE</command> triggers and finally
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statement-level <literal>AFTER</literal> <command>INSERT</command>
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triggers.
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</para>
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<para>
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If an <command>UPDATE</command> on a partitioned table causes a row to move
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to another partition, it will be performed as a <command>DELETE</command>
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from the original partition followed by an <command>INSERT</command> into
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the new partition. In this case, all row-level <literal>BEFORE</literal>
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<command>UPDATE</command> triggers and all row-level
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<literal>BEFORE</literal> <command>DELETE</command> triggers are fired on
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the original partition. Then all row-level <literal>BEFORE</literal>
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<command>INSERT</command> triggers are fired on the destination partition.
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The possibility of surprising outcomes should be considered when all these
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triggers affect the row being moved. As far as <literal>AFTER ROW</literal>
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triggers are concerned, <literal>AFTER</literal> <command>DELETE</command>
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and <literal>AFTER</literal> <command>INSERT</command> triggers are
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applied; but <literal>AFTER</literal> <command>UPDATE</command> triggers
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are not applied because the <command>UPDATE</command> has been converted to
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a <command>DELETE</command> and an <command>INSERT</command>. As far as
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statement-level triggers are concerned, none of the
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<command>DELETE</command> or <command>INSERT</command> triggers are fired,
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even if row movement occurs; only the <command>UPDATE</command> triggers
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defined on the target table used in the <command>UPDATE</command> statement
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will be fired.
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</para>
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<para>
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Trigger functions invoked by per-statement triggers should always
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return <symbol>NULL</symbol>. Trigger functions invoked by per-row
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triggers can return a table row (a value of
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type <structname>HeapTuple</structname>) to the calling executor,
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if they choose. A row-level trigger fired before an operation has
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the following choices:
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<itemizedlist>
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<listitem>
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<para>
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It can return <symbol>NULL</symbol> to skip the operation for the
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current row. This instructs the executor to not perform the
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row-level operation that invoked the trigger (the insertion,
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modification, or deletion of a particular table row).
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</para>
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</listitem>
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<listitem>
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<para>
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For row-level <command>INSERT</command>
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and <command>UPDATE</command> triggers only, the returned row
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becomes the row that will be inserted or will replace the row
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being updated. This allows the trigger function to modify the
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row being inserted or updated.
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</para>
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</listitem>
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</itemizedlist>
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A row-level <literal>BEFORE</literal> trigger that does not intend to cause
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either of these behaviors must be careful to return as its result the same
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row that was passed in (that is, the <varname>NEW</varname> row
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for <command>INSERT</command> and <command>UPDATE</command>
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triggers, the <varname>OLD</varname> row for
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<command>DELETE</command> triggers).
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</para>
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<para>
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A row-level <literal>INSTEAD OF</literal> trigger should either return
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<symbol>NULL</symbol> to indicate that it did not modify any data from
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the view's underlying base tables, or it should return the view
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row that was passed in (the <varname>NEW</varname> row
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for <command>INSERT</command> and <command>UPDATE</command>
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operations, or the <varname>OLD</varname> row for
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<command>DELETE</command> operations). A nonnull return value is
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used to signal that the trigger performed the necessary data
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modifications in the view. This will cause the count of the number
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of rows affected by the command to be incremented. For
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<command>INSERT</command> and <command>UPDATE</command> operations, the trigger
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may modify the <varname>NEW</varname> row before returning it. This will
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change the data returned by
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<command>INSERT RETURNING</command> or <command>UPDATE RETURNING</command>,
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and is useful when the view will not show exactly the same data
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that was provided.
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</para>
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<para>
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The return value is ignored for row-level triggers fired after an
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operation, and so they can return <symbol>NULL</symbol>.
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</para>
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<para>
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If more than one trigger is defined for the same event on the same
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relation, the triggers will be fired in alphabetical order by
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trigger name. In the case of <literal>BEFORE</literal> and
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<literal>INSTEAD OF</literal> triggers, the possibly-modified row returned by
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each trigger becomes the input to the next trigger. If any
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<literal>BEFORE</literal> or <literal>INSTEAD OF</literal> trigger returns
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<symbol>NULL</symbol>, the operation is abandoned for that row and subsequent
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triggers are not fired (for that row).
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</para>
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<para>
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A trigger definition can also specify a Boolean <literal>WHEN</literal>
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condition, which will be tested to see whether the trigger should
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be fired. In row-level triggers the <literal>WHEN</literal> condition can
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examine the old and/or new values of columns of the row. (Statement-level
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triggers can also have <literal>WHEN</literal> conditions, although the feature
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is not so useful for them.) In a <literal>BEFORE</literal> trigger, the
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<literal>WHEN</literal>
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condition is evaluated just before the function is or would be executed,
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so using <literal>WHEN</literal> is not materially different from testing the
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same condition at the beginning of the trigger function. However, in
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an <literal>AFTER</literal> trigger, the <literal>WHEN</literal> condition is evaluated
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just after the row update occurs, and it determines whether an event is
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queued to fire the trigger at the end of statement. So when an
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<literal>AFTER</literal> trigger's
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<literal>WHEN</literal> condition does not return true, it is not necessary
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to queue an event nor to re-fetch the row at end of statement. This
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can result in significant speedups in statements that modify many
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rows, if the trigger only needs to be fired for a few of the rows.
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<literal>INSTEAD OF</literal> triggers do not support
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<literal>WHEN</literal> conditions.
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</para>
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<para>
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Typically, row-level <literal>BEFORE</literal> triggers are used for checking or
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modifying the data that will be inserted or updated. For example,
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a <literal>BEFORE</literal> trigger might be used to insert the current time into a
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<type>timestamp</type> column, or to check that two elements of the row are
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consistent. Row-level <literal>AFTER</literal> triggers are most sensibly
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used to propagate the updates to other tables, or make consistency
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checks against other tables. The reason for this division of labor is
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that an <literal>AFTER</literal> trigger can be certain it is seeing the final
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value of the row, while a <literal>BEFORE</literal> trigger cannot; there might
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be other <literal>BEFORE</literal> triggers firing after it. If you have no
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specific reason to make a trigger <literal>BEFORE</literal> or
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<literal>AFTER</literal>, the <literal>BEFORE</literal> case is more efficient, since
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the information about
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the operation doesn't have to be saved until end of statement.
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</para>
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<para>
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If a trigger function executes SQL commands then these
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commands might fire triggers again. This is known as cascading
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triggers. There is no direct limitation on the number of cascade
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levels. It is possible for cascades to cause a recursive invocation
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of the same trigger; for example, an <command>INSERT</command>
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trigger might execute a command that inserts an additional row
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into the same table, causing the <command>INSERT</command> trigger
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to be fired again. It is the trigger programmer's responsibility
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to avoid infinite recursion in such scenarios.
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</para>
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<para>
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<indexterm>
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<primary>trigger</primary>
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<secondary>arguments for trigger functions</secondary>
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</indexterm>
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When a trigger is being defined, arguments can be specified for
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it. The purpose of including arguments in the
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trigger definition is to allow different triggers with similar
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requirements to call the same function. As an example, there
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could be a generalized trigger function that takes as its
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arguments two column names and puts the current user in one and
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the current time stamp in the other. Properly written, this
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trigger function would be independent of the specific table it is
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triggering on. So the same function could be used for
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<command>INSERT</command> events on any table with suitable
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columns, to automatically track creation of records in a
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transaction table for example. It could also be used to track
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last-update events if defined as an <command>UPDATE</command>
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trigger.
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</para>
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<para>
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Each programming language that supports triggers has its own method
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for making the trigger input data available to the trigger function.
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This input data includes the type of trigger event (e.g.,
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<command>INSERT</command> or <command>UPDATE</command>) as well as any
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arguments that were listed in <command>CREATE TRIGGER</command>.
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For a row-level trigger, the input data also includes the
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<varname>NEW</varname> row for <command>INSERT</command> and
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<command>UPDATE</command> triggers, and/or the <varname>OLD</varname> row
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for <command>UPDATE</command> and <command>DELETE</command> triggers.
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</para>
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<para>
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By default, statement-level triggers do not have any way to examine the
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individual row(s) modified by the statement. But an <literal>AFTER
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STATEMENT</literal> trigger can request that <firstterm>transition tables</firstterm>
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be created to make the sets of affected rows available to the trigger.
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<literal>AFTER ROW</literal> triggers can also request transition tables, so
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that they can see the total changes in the table as well as the change in
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the individual row they are currently being fired for. The method for
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examining the transition tables again depends on the programming language
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that is being used, but the typical approach is to make the transition
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tables act like read-only temporary tables that can be accessed by SQL
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commands issued within the trigger function.
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</para>
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</sect1>
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<sect1 id="trigger-datachanges">
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<title>Visibility of Data Changes</title>
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<para>
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If you execute SQL commands in your trigger function, and these
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commands access the table that the trigger is for, then
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you need to be aware of the data visibility rules, because they determine
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whether these SQL commands will see the data change that the trigger
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is fired for. Briefly:
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<itemizedlist>
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<listitem>
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<para>
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Statement-level triggers follow simple visibility rules: none of
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the changes made by a statement are visible to statement-level
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<literal>BEFORE</literal> triggers, whereas all
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modifications are visible to statement-level <literal>AFTER</literal>
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triggers.
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</para>
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</listitem>
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<listitem>
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<para>
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The data change (insertion, update, or deletion) causing the
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trigger to fire is naturally <emphasis>not</emphasis> visible
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to SQL commands executed in a row-level <literal>BEFORE</literal> trigger,
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because it hasn't happened yet.
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</para>
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</listitem>
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<listitem>
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<para>
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However, SQL commands executed in a row-level <literal>BEFORE</literal>
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trigger <emphasis>will</emphasis> see the effects of data
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changes for rows previously processed in the same outer
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command. This requires caution, since the ordering of these
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change events is not in general predictable; a SQL command that
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affects multiple rows can visit the rows in any order.
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</para>
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</listitem>
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<listitem>
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<para>
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Similarly, a row-level <literal>INSTEAD OF</literal> trigger will see the
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effects of data changes made by previous firings of <literal>INSTEAD
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OF</literal> triggers in the same outer command.
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</para>
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</listitem>
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<listitem>
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<para>
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When a row-level <literal>AFTER</literal> trigger is fired, all data
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changes made
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by the outer command are already complete, and are visible to
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the invoked trigger function.
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</para>
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</listitem>
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</itemizedlist>
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</para>
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<para>
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If your trigger function is written in any of the standard procedural
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languages, then the above statements apply only if the function is
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declared <literal>VOLATILE</literal>. Functions that are declared
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<literal>STABLE</literal> or <literal>IMMUTABLE</literal> will not see changes made by
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the calling command in any case.
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</para>
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<para>
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Further information about data visibility rules can be found in
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<xref linkend="spi-visibility"/>. The example in <xref
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linkend="trigger-example"/> contains a demonstration of these rules.
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</para>
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</sect1>
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<sect1 id="trigger-interface">
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<title>Writing Trigger Functions in C</title>
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<indexterm zone="trigger-interface">
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<primary>trigger</primary>
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<secondary>in C</secondary>
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</indexterm>
|
|
|
|
<indexterm>
|
|
<primary>transition tables</primary>
|
|
<secondary>referencing from C trigger</secondary>
|
|
</indexterm>
|
|
|
|
<para>
|
|
This section describes the low-level details of the interface to a
|
|
trigger function. This information is only needed when writing
|
|
trigger functions in C. If you are using a higher-level language then
|
|
these details are handled for you. In most cases you should consider
|
|
using a procedural language before writing your triggers in C. The
|
|
documentation of each procedural language explains how to write a
|
|
trigger in that language.
|
|
</para>
|
|
|
|
<para>
|
|
Trigger functions must use the <quote>version 1</quote> function manager
|
|
interface.
|
|
</para>
|
|
|
|
<para>
|
|
When a function is called by the trigger manager, it is not passed
|
|
any normal arguments, but it is passed a <quote>context</quote>
|
|
pointer pointing to a <structname>TriggerData</structname> structure. C
|
|
functions can check whether they were called from the trigger
|
|
manager or not by executing the macro:
|
|
<programlisting>
|
|
CALLED_AS_TRIGGER(fcinfo)
|
|
</programlisting>
|
|
which expands to:
|
|
<programlisting>
|
|
((fcinfo)->context != NULL && IsA((fcinfo)->context, TriggerData))
|
|
</programlisting>
|
|
If this returns true, then it is safe to cast
|
|
<literal>fcinfo->context</literal> to type <literal>TriggerData
|
|
*</literal> and make use of the pointed-to
|
|
<structname>TriggerData</structname> structure. The function must
|
|
<emphasis>not</emphasis> alter the <structname>TriggerData</structname>
|
|
structure or any of the data it points to.
|
|
</para>
|
|
|
|
<para>
|
|
<structname>struct TriggerData</structname> is defined in
|
|
<filename>commands/trigger.h</filename>:
|
|
|
|
<programlisting>
|
|
typedef struct TriggerData
|
|
{
|
|
NodeTag type;
|
|
TriggerEvent tg_event;
|
|
Relation tg_relation;
|
|
HeapTuple tg_trigtuple;
|
|
HeapTuple tg_newtuple;
|
|
Trigger *tg_trigger;
|
|
Buffer tg_trigtuplebuf;
|
|
Buffer tg_newtuplebuf;
|
|
Tuplestorestate *tg_oldtable;
|
|
Tuplestorestate *tg_newtable;
|
|
} TriggerData;
|
|
</programlisting>
|
|
|
|
where the members are defined as follows:
|
|
|
|
<variablelist>
|
|
<varlistentry>
|
|
<term><structfield>type</structfield></term>
|
|
<listitem>
|
|
<para>
|
|
Always <literal>T_TriggerData</literal>.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><structfield>tg_event</structfield></term>
|
|
<listitem>
|
|
<para>
|
|
Describes the event for which the function is called. You can use the
|
|
following macros to examine <literal>tg_event</literal>:
|
|
|
|
<variablelist>
|
|
<varlistentry>
|
|
<term><literal>TRIGGER_FIRED_BEFORE(tg_event)</literal></term>
|
|
<listitem>
|
|
<para>
|
|
Returns true if the trigger fired before the operation.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><literal>TRIGGER_FIRED_AFTER(tg_event)</literal></term>
|
|
<listitem>
|
|
<para>
|
|
Returns true if the trigger fired after the operation.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><literal>TRIGGER_FIRED_INSTEAD(tg_event)</literal></term>
|
|
<listitem>
|
|
<para>
|
|
Returns true if the trigger fired instead of the operation.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><literal>TRIGGER_FIRED_FOR_ROW(tg_event)</literal></term>
|
|
<listitem>
|
|
<para>
|
|
Returns true if the trigger fired for a row-level event.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><literal>TRIGGER_FIRED_FOR_STATEMENT(tg_event)</literal></term>
|
|
<listitem>
|
|
<para>
|
|
Returns true if the trigger fired for a statement-level event.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><literal>TRIGGER_FIRED_BY_INSERT(tg_event)</literal></term>
|
|
<listitem>
|
|
<para>
|
|
Returns true if the trigger was fired by an <command>INSERT</command> command.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><literal>TRIGGER_FIRED_BY_UPDATE(tg_event)</literal></term>
|
|
<listitem>
|
|
<para>
|
|
Returns true if the trigger was fired by an <command>UPDATE</command> command.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><literal>TRIGGER_FIRED_BY_DELETE(tg_event)</literal></term>
|
|
<listitem>
|
|
<para>
|
|
Returns true if the trigger was fired by a <command>DELETE</command> command.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><literal>TRIGGER_FIRED_BY_TRUNCATE(tg_event)</literal></term>
|
|
<listitem>
|
|
<para>
|
|
Returns true if the trigger was fired by a <command>TRUNCATE</command> command.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
</variablelist>
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><structfield>tg_relation</structfield></term>
|
|
<listitem>
|
|
<para>
|
|
A pointer to a structure describing the relation that the trigger fired for.
|
|
Look at <filename>utils/rel.h</filename> for details about
|
|
this structure. The most interesting things are
|
|
<literal>tg_relation->rd_att</literal> (descriptor of the relation
|
|
tuples) and <literal>tg_relation->rd_rel->relname</literal>
|
|
(relation name; the type is not <type>char*</type> but
|
|
<type>NameData</type>; use
|
|
<literal>SPI_getrelname(tg_relation)</literal> to get a <type>char*</type> if you
|
|
need a copy of the name).
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><structfield>tg_trigtuple</structfield></term>
|
|
<listitem>
|
|
<para>
|
|
A pointer to the row for which the trigger was fired. This is
|
|
the row being inserted, updated, or deleted. If this trigger
|
|
was fired for an <command>INSERT</command> or
|
|
<command>DELETE</command> then this is what you should return
|
|
from the function if you don't want to replace the row with
|
|
a different one (in the case of <command>INSERT</command>) or
|
|
skip the operation. For triggers on foreign tables, values of system
|
|
columns herein are unspecified.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><structfield>tg_newtuple</structfield></term>
|
|
<listitem>
|
|
<para>
|
|
A pointer to the new version of the row, if the trigger was
|
|
fired for an <command>UPDATE</command>, and <symbol>NULL</symbol> if
|
|
it is for an <command>INSERT</command> or a
|
|
<command>DELETE</command>. This is what you have to return
|
|
from the function if the event is an <command>UPDATE</command>
|
|
and you don't want to replace this row by a different one or
|
|
skip the operation. For triggers on foreign tables, values of system
|
|
columns herein are unspecified.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><structfield>tg_trigger</structfield></term>
|
|
<listitem>
|
|
<para>
|
|
A pointer to a structure of type <structname>Trigger</structname>,
|
|
defined in <filename>utils/reltrigger.h</filename>:
|
|
|
|
<programlisting>
|
|
typedef struct Trigger
|
|
{
|
|
Oid tgoid;
|
|
char *tgname;
|
|
Oid tgfoid;
|
|
int16 tgtype;
|
|
char tgenabled;
|
|
bool tgisinternal;
|
|
Oid tgconstrrelid;
|
|
Oid tgconstrindid;
|
|
Oid tgconstraint;
|
|
bool tgdeferrable;
|
|
bool tginitdeferred;
|
|
int16 tgnargs;
|
|
int16 tgnattr;
|
|
int16 *tgattr;
|
|
char **tgargs;
|
|
char *tgqual;
|
|
char *tgoldtable;
|
|
char *tgnewtable;
|
|
} Trigger;
|
|
</programlisting>
|
|
|
|
where <structfield>tgname</structfield> is the trigger's name,
|
|
<structfield>tgnargs</structfield> is the number of arguments in
|
|
<structfield>tgargs</structfield>, and <structfield>tgargs</structfield> is an array of
|
|
pointers to the arguments specified in the <command>CREATE
|
|
TRIGGER</command> statement. The other members are for internal use
|
|
only.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><structfield>tg_trigtuplebuf</structfield></term>
|
|
<listitem>
|
|
<para>
|
|
The buffer containing <structfield>tg_trigtuple</structfield>, or <symbol>InvalidBuffer</symbol> if there
|
|
is no such tuple or it is not stored in a disk buffer.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><structfield>tg_newtuplebuf</structfield></term>
|
|
<listitem>
|
|
<para>
|
|
The buffer containing <structfield>tg_newtuple</structfield>, or <symbol>InvalidBuffer</symbol> if there
|
|
is no such tuple or it is not stored in a disk buffer.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><structfield>tg_oldtable</structfield></term>
|
|
<listitem>
|
|
<para>
|
|
A pointer to a structure of type <structname>Tuplestorestate</structname>
|
|
containing zero or more rows in the format specified by
|
|
<structfield>tg_relation</structfield>, or a <symbol>NULL</symbol> pointer
|
|
if there is no <literal>OLD TABLE</literal> transition relation.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><structfield>tg_newtable</structfield></term>
|
|
<listitem>
|
|
<para>
|
|
A pointer to a structure of type <structname>Tuplestorestate</structname>
|
|
containing zero or more rows in the format specified by
|
|
<structfield>tg_relation</structfield>, or a <symbol>NULL</symbol> pointer
|
|
if there is no <literal>NEW TABLE</literal> transition relation.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
</variablelist>
|
|
</para>
|
|
|
|
<para>
|
|
To allow queries issued through SPI to reference transition tables, see
|
|
<xref linkend="spi-spi-register-trigger-data"/>.
|
|
</para>
|
|
|
|
<para>
|
|
A trigger function must return either a
|
|
<structname>HeapTuple</structname> pointer or a <symbol>NULL</symbol> pointer
|
|
(<emphasis>not</emphasis> an SQL null value, that is, do not set <parameter>isNull</parameter> true).
|
|
Be careful to return either
|
|
<structfield>tg_trigtuple</structfield> or <structfield>tg_newtuple</structfield>,
|
|
as appropriate, if you don't want to modify the row being operated on.
|
|
</para>
|
|
</sect1>
|
|
|
|
<sect1 id="trigger-example">
|
|
<title>A Complete Trigger Example</title>
|
|
|
|
<para>
|
|
Here is a very simple example of a trigger function written in C.
|
|
(Examples of triggers written in procedural languages can be found
|
|
in the documentation of the procedural languages.)
|
|
</para>
|
|
|
|
<para>
|
|
The function <function>trigf</function> reports the number of rows in the
|
|
table <structname>ttest</structname> and skips the actual operation if the
|
|
command attempts to insert a null value into the column
|
|
<structfield>x</structfield>. (So the trigger acts as a not-null constraint but
|
|
doesn't abort the transaction.)
|
|
</para>
|
|
|
|
<para>
|
|
First, the table definition:
|
|
<programlisting>
|
|
CREATE TABLE ttest (
|
|
x integer
|
|
);
|
|
</programlisting>
|
|
</para>
|
|
|
|
<para>
|
|
This is the source code of the trigger function:
|
|
<programlisting><![CDATA[
|
|
#include "postgres.h"
|
|
#include "fmgr.h"
|
|
#include "executor/spi.h" /* this is what you need to work with SPI */
|
|
#include "commands/trigger.h" /* ... triggers ... */
|
|
#include "utils/rel.h" /* ... and relations */
|
|
|
|
PG_MODULE_MAGIC;
|
|
|
|
PG_FUNCTION_INFO_V1(trigf);
|
|
|
|
Datum
|
|
trigf(PG_FUNCTION_ARGS)
|
|
{
|
|
TriggerData *trigdata = (TriggerData *) fcinfo->context;
|
|
TupleDesc tupdesc;
|
|
HeapTuple rettuple;
|
|
char *when;
|
|
bool checknull = false;
|
|
bool isnull;
|
|
int ret, i;
|
|
|
|
/* make sure it's called as a trigger at all */
|
|
if (!CALLED_AS_TRIGGER(fcinfo))
|
|
elog(ERROR, "trigf: not called by trigger manager");
|
|
|
|
/* tuple to return to executor */
|
|
if (TRIGGER_FIRED_BY_UPDATE(trigdata->tg_event))
|
|
rettuple = trigdata->tg_newtuple;
|
|
else
|
|
rettuple = trigdata->tg_trigtuple;
|
|
|
|
/* check for null values */
|
|
if (!TRIGGER_FIRED_BY_DELETE(trigdata->tg_event)
|
|
&& TRIGGER_FIRED_BEFORE(trigdata->tg_event))
|
|
checknull = true;
|
|
|
|
if (TRIGGER_FIRED_BEFORE(trigdata->tg_event))
|
|
when = "before";
|
|
else
|
|
when = "after ";
|
|
|
|
tupdesc = trigdata->tg_relation->rd_att;
|
|
|
|
/* connect to SPI manager */
|
|
if ((ret = SPI_connect()) < 0)
|
|
elog(ERROR, "trigf (fired %s): SPI_connect returned %d", when, ret);
|
|
|
|
/* get number of rows in table */
|
|
ret = SPI_exec("SELECT count(*) FROM ttest", 0);
|
|
|
|
if (ret < 0)
|
|
elog(ERROR, "trigf (fired %s): SPI_exec returned %d", when, ret);
|
|
|
|
/* count(*) returns int8, so be careful to convert */
|
|
i = DatumGetInt64(SPI_getbinval(SPI_tuptable->vals[0],
|
|
SPI_tuptable->tupdesc,
|
|
1,
|
|
&isnull));
|
|
|
|
elog (INFO, "trigf (fired %s): there are %d rows in ttest", when, i);
|
|
|
|
SPI_finish();
|
|
|
|
if (checknull)
|
|
{
|
|
SPI_getbinval(rettuple, tupdesc, 1, &isnull);
|
|
if (isnull)
|
|
rettuple = NULL;
|
|
}
|
|
|
|
return PointerGetDatum(rettuple);
|
|
}
|
|
]]>
|
|
</programlisting>
|
|
</para>
|
|
|
|
<para>
|
|
After you have compiled the source code (see <xref
|
|
linkend="dfunc"/>), declare the function and the triggers:
|
|
<programlisting>
|
|
CREATE FUNCTION trigf() RETURNS trigger
|
|
AS '<replaceable>filename</replaceable>'
|
|
LANGUAGE C;
|
|
|
|
CREATE TRIGGER tbefore BEFORE INSERT OR UPDATE OR DELETE ON ttest
|
|
FOR EACH ROW EXECUTE FUNCTION trigf();
|
|
|
|
CREATE TRIGGER tafter AFTER INSERT OR UPDATE OR DELETE ON ttest
|
|
FOR EACH ROW EXECUTE FUNCTION trigf();
|
|
</programlisting>
|
|
</para>
|
|
|
|
<para>
|
|
Now you can test the operation of the trigger:
|
|
<screen>
|
|
=> INSERT INTO ttest VALUES (NULL);
|
|
INFO: trigf (fired before): there are 0 rows in ttest
|
|
INSERT 0 0
|
|
|
|
-- Insertion skipped and AFTER trigger is not fired
|
|
|
|
=> SELECT * FROM ttest;
|
|
x
|
|
---
|
|
(0 rows)
|
|
|
|
=> INSERT INTO ttest VALUES (1);
|
|
INFO: trigf (fired before): there are 0 rows in ttest
|
|
INFO: trigf (fired after ): there are 1 rows in ttest
|
|
^^^^^^^^
|
|
remember what we said about visibility.
|
|
INSERT 167793 1
|
|
vac=> SELECT * FROM ttest;
|
|
x
|
|
---
|
|
1
|
|
(1 row)
|
|
|
|
=> INSERT INTO ttest SELECT x * 2 FROM ttest;
|
|
INFO: trigf (fired before): there are 1 rows in ttest
|
|
INFO: trigf (fired after ): there are 2 rows in ttest
|
|
^^^^^^
|
|
remember what we said about visibility.
|
|
INSERT 167794 1
|
|
=> SELECT * FROM ttest;
|
|
x
|
|
---
|
|
1
|
|
2
|
|
(2 rows)
|
|
|
|
=> UPDATE ttest SET x = NULL WHERE x = 2;
|
|
INFO: trigf (fired before): there are 2 rows in ttest
|
|
UPDATE 0
|
|
=> UPDATE ttest SET x = 4 WHERE x = 2;
|
|
INFO: trigf (fired before): there are 2 rows in ttest
|
|
INFO: trigf (fired after ): there are 2 rows in ttest
|
|
UPDATE 1
|
|
vac=> SELECT * FROM ttest;
|
|
x
|
|
---
|
|
1
|
|
4
|
|
(2 rows)
|
|
|
|
=> DELETE FROM ttest;
|
|
INFO: trigf (fired before): there are 2 rows in ttest
|
|
INFO: trigf (fired before): there are 1 rows in ttest
|
|
INFO: trigf (fired after ): there are 0 rows in ttest
|
|
INFO: trigf (fired after ): there are 0 rows in ttest
|
|
^^^^^^
|
|
remember what we said about visibility.
|
|
DELETE 2
|
|
=> SELECT * FROM ttest;
|
|
x
|
|
---
|
|
(0 rows)
|
|
</screen>
|
|
|
|
</para>
|
|
|
|
<para>
|
|
There are more complex examples in
|
|
<filename>src/test/regress/regress.c</filename> and
|
|
in <xref linkend="contrib-spi"/>.
|
|
</para>
|
|
</sect1>
|
|
</chapter>
|