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  1. <?xml version="1.0" encoding="UTF-8" standalone="no"?>

  2. <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>CREATE TRIGGER</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets V1.79.1" /><link rel="prev" href="sql-createtransform.html" title="CREATE TRANSFORM" /><link rel="next" href="sql-createtype.html" title="CREATE TYPE" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">CREATE TRIGGER</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="sql-createtransform.html" title="CREATE TRANSFORM">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="sql-commands.html" title="SQL Commands">Up</a></td><th width="60%" align="center">SQL Commands</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 12.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="sql-createtype.html" title="CREATE TYPE">Next</a></td></tr></table><hr></hr></div><div class="refentry" id="SQL-CREATETRIGGER"><div class="titlepage"></div><a id="id-1.9.3.93.1" class="indexterm"></a><a id="id-1.9.3.93.2" class="indexterm"></a><div class="refnamediv"><h2><span class="refentrytitle">CREATE TRIGGER</span></h2><p>CREATE TRIGGER — define a new trigger</p></div><div class="refsynopsisdiv"><h2>Synopsis</h2><pre class="synopsis">

  3. CREATE [ CONSTRAINT ] TRIGGER <em class="replaceable"><code>name</code></em> { BEFORE | AFTER | INSTEAD OF } { <em class="replaceable"><code>event</code></em> [ OR ... ] }

  4.     ON <em class="replaceable"><code>table_name</code></em>

  5.     [ FROM <em class="replaceable"><code>referenced_table_name</code></em> ]

  6.     [ NOT DEFERRABLE | [ DEFERRABLE ] [ INITIALLY IMMEDIATE | INITIALLY DEFERRED ] ]

  7.     [ REFERENCING { { OLD | NEW } TABLE [ AS ] <em class="replaceable"><code>transition_relation_name</code></em> } [ ... ] ]

  8.     [ FOR [ EACH ] { ROW | STATEMENT } ]

  9.     [ WHEN ( <em class="replaceable"><code>condition</code></em> ) ]

  10.     EXECUTE { FUNCTION | PROCEDURE } <em class="replaceable"><code>function_name</code></em> ( <em class="replaceable"><code>arguments</code></em> )

  11. 

  12. <span class="phrase">where <em class="replaceable"><code>event</code></em> can be one of:</span>

  13. 

  14.     INSERT

  15.     UPDATE [ OF <em class="replaceable"><code>column_name</code></em> [, ... ] ]

  16.     DELETE

  17.     TRUNCATE

  18. </pre></div><div class="refsect1" id="id-1.9.3.93.6"><h2>Description</h2><p>

  19.    <code class="command">CREATE TRIGGER</code> creates a new trigger.  The

  20.    trigger will be associated with the specified table, view, or foreign table

  21.    and will execute the specified

  22.    function <em class="replaceable"><code>function_name</code></em> when

  23.    certain operations are performed on that table.

  24.   </p><p>

  25.    The trigger can be specified to fire before the

  26.    operation is attempted on a row (before constraints are checked and

  27.    the <code class="command">INSERT</code>, <code class="command">UPDATE</code>, or

  28.    <code class="command">DELETE</code> is attempted); or after the operation has

  29.    completed (after constraints are checked and the

  30.    <code class="command">INSERT</code>, <code class="command">UPDATE</code>, or

  31.    <code class="command">DELETE</code> has completed); or instead of the operation

  32.    (in the case of inserts, updates or deletes on a view).

  33.    If the trigger fires before or instead of the event, the trigger can skip

  34.    the operation for the current row, or change the row being inserted (for

  35.    <code class="command">INSERT</code> and <code class="command">UPDATE</code> operations

  36.    only). If the trigger fires after the event, all changes, including

  37.    the effects of other triggers, are <span class="quote">“<span class="quote">visible</span>”</span>

  38.    to the trigger.

  39.   </p><p>

  40.    A trigger that is marked <code class="literal">FOR EACH ROW</code> is called

  41.    once for every row that the operation modifies. For example, a

  42.    <code class="command">DELETE</code> that affects 10 rows will cause any

  43.    <code class="literal">ON DELETE</code> triggers on the target relation to be

  44.    called 10 separate times, once for each deleted row. In contrast, a

  45.    trigger that is marked <code class="literal">FOR EACH STATEMENT</code> only

  46.    executes once for any given operation, regardless of how many rows

  47.    it modifies (in particular, an operation that modifies zero rows

  48.    will still result in the execution of any applicable <code class="literal">FOR

  49.    EACH STATEMENT</code> triggers).

  50.   </p><p>

  51.    Triggers that are specified to fire <code class="literal">INSTEAD OF</code> the trigger

  52.    event must be marked <code class="literal">FOR EACH ROW</code>, and can only be defined

  53.    on views. <code class="literal">BEFORE</code> and <code class="literal">AFTER</code> triggers on a view

  54.    must be marked as <code class="literal">FOR EACH STATEMENT</code>.

  55.   </p><p>

  56.    In addition, triggers may be defined to fire for

  57.    <code class="command">TRUNCATE</code>, though only

  58.    <code class="literal">FOR EACH STATEMENT</code>.

  59.   </p><p>

  60.    The following table summarizes which types of triggers may be used on

  61.    tables, views, and foreign tables:

  62.   </p><div class="informaltable" id="SUPPORTED-TRIGGER-TYPES"><table class="informaltable" border="1"><colgroup><col /><col /><col /><col /></colgroup><thead><tr><th>When</th><th>Event</th><th>Row-level</th><th>Statement-level</th></tr></thead><tbody><tr><td rowspan="2" align="center"><code class="literal">BEFORE</code></td><td align="center"><code class="command">INSERT</code>/<code class="command">UPDATE</code>/<code class="command">DELETE</code></td><td align="center">Tables and foreign tables</td><td align="center">Tables, views, and foreign tables</td></tr><tr><td align="center"><code class="command">TRUNCATE</code></td><td align="center">—</td><td align="center">Tables</td></tr><tr><td rowspan="2" align="center"><code class="literal">AFTER</code></td><td align="center"><code class="command">INSERT</code>/<code class="command">UPDATE</code>/<code class="command">DELETE</code></td><td align="center">Tables and foreign tables</td><td align="center">Tables, views, and foreign tables</td></tr><tr><td align="center"><code class="command">TRUNCATE</code></td><td align="center">—</td><td align="center">Tables</td></tr><tr><td rowspan="2" align="center"><code class="literal">INSTEAD OF</code></td><td align="center"><code class="command">INSERT</code>/<code class="command">UPDATE</code>/<code class="command">DELETE</code></td><td align="center">Views</td><td align="center">—</td></tr><tr><td align="center"><code class="command">TRUNCATE</code></td><td align="center">—</td><td align="center">—</td></tr></tbody></table></div><p>

  63.    Also, a trigger definition can specify a Boolean <code class="literal">WHEN</code>

  64.    condition, which will be tested to see whether the trigger should

  65.    be fired.  In row-level triggers the <code class="literal">WHEN</code> condition can

  66.    examine the old and/or new values of columns of the row.  Statement-level

  67.    triggers can also have <code class="literal">WHEN</code> conditions, although the feature

  68.    is not so useful for them since the condition cannot refer to any values

  69.    in the table.

  70.   </p><p>

  71.    If multiple triggers of the same kind are defined for the same event,

  72.    they will be fired in alphabetical order by name.

  73.   </p><p>

  74.    When the <code class="literal">CONSTRAINT</code> option is specified, this command creates a

  75.    <em class="firstterm">constraint trigger</em>.  This is the same as a regular trigger

  76.    except that the timing of the trigger firing can be adjusted using

  77.    <a class="xref" href="sql-set-constraints.html" title="SET CONSTRAINTS"><span class="refentrytitle">SET CONSTRAINTS</span></a>.

  78.    Constraint triggers must be <code class="literal">AFTER ROW</code> triggers on plain

  79.    tables (not foreign tables).  They

  80.    can be fired either at the end of the statement causing the triggering

  81.    event, or at the end of the containing transaction; in the latter case they

  82.    are said to be <em class="firstterm">deferred</em>.  A pending deferred-trigger firing

  83.    can also be forced to happen immediately by using <code class="command">SET

  84.    CONSTRAINTS</code>.  Constraint triggers are expected to raise an exception

  85.    when the constraints they implement are violated.

  86.   </p><p>

  87.    The <code class="literal">REFERENCING</code> option enables collection

  88.    of <em class="firstterm">transition relations</em>, which are row sets that include all

  89.    of the rows inserted, deleted, or modified by the current SQL statement.

  90.    This feature lets the trigger see a global view of what the statement did,

  91.    not just one row at a time.  This option is only allowed for

  92.    an <code class="literal">AFTER</code> trigger that is not a constraint trigger; also, if

  93.    the trigger is an <code class="literal">UPDATE</code> trigger, it must not specify

  94.    a <em class="replaceable"><code>column_name</code></em> list.

  95.    <code class="literal">OLD TABLE</code> may only be specified once, and only for a trigger

  96.    that can fire on <code class="literal">UPDATE</code> or <code class="literal">DELETE</code>; it creates a

  97.    transition relation containing the <em class="firstterm">before-images</em> of all rows

  98.    updated or deleted by the statement.

  99.    Similarly, <code class="literal">NEW TABLE</code> may only be specified once, and only for

  100.    a trigger that can fire on <code class="literal">UPDATE</code> or <code class="literal">INSERT</code>;

  101.    it creates a transition relation containing the <em class="firstterm">after-images</em>

  102.    of all rows updated or inserted by the statement.

  103.   </p><p>

  104.    <code class="command">SELECT</code> does not modify any rows so you cannot

  105.    create <code class="command">SELECT</code> triggers.  Rules and views may provide

  106.    workable solutions to problems that seem to need <code class="command">SELECT</code>

  107.    triggers.

  108.   </p><p>

  109.    Refer to <a class="xref" href="triggers.html" title="Chapter 38. Triggers">Chapter 38</a> for more information about triggers.

  110.   </p></div><div class="refsect1" id="id-1.9.3.93.7"><h2>Parameters</h2><div class="variablelist"><dl class="variablelist"><dt><span class="term"><em class="replaceable"><code>name</code></em></span></dt><dd><p>

  111.       The name to give the new trigger.  This must be distinct from

  112.       the name of any other trigger for the same table.

  113.       The name cannot be schema-qualified — the trigger inherits the

  114.       schema of its table.  For a constraint trigger, this is also the name to

  115.       use when modifying the trigger's behavior using

  116.       <code class="command">SET CONSTRAINTS</code>.

  117.      </p></dd><dt><span class="term"><code class="literal">BEFORE</code><br /></span><span class="term"><code class="literal">AFTER</code><br /></span><span class="term"><code class="literal">INSTEAD OF</code></span></dt><dd><p>

  118.       Determines whether the function is called before, after, or instead of

  119.       the event.  A constraint trigger can only be specified as

  120.       <code class="literal">AFTER</code>.

  121.      </p></dd><dt><span class="term"><em class="replaceable"><code>event</code></em></span></dt><dd><p>

  122.       One of <code class="literal">INSERT</code>, <code class="literal">UPDATE</code>,

  123.       <code class="literal">DELETE</code>, or <code class="literal">TRUNCATE</code>;

  124.       this specifies the event that will fire the trigger. Multiple

  125.       events can be specified using <code class="literal">OR</code>, except when

  126.       transition relations are requested.

  127.      </p><p>

  128.       For <code class="literal">UPDATE</code> events, it is possible to

  129.       specify a list of columns using this syntax:

  130. </p><pre class="synopsis">

  131. UPDATE OF <em class="replaceable"><code>column_name1</code></em> [, <em class="replaceable"><code>column_name2</code></em> ... ]

  132. </pre><p>

  133.       The trigger will only fire if at least one of the listed columns

  134.       is mentioned as a target of the <code class="command">UPDATE</code> command

  135.       or if one of the listed columns is a generated column that depends on a

  136.       column that is the target of the <code class="command">UPDATE</code>.

  137.      </p><p>

  138.       <code class="literal">INSTEAD OF UPDATE</code> events do not allow a list of columns.

  139.       A column list cannot be specified when requesting transition relations,

  140.       either.

  141.      </p></dd><dt><span class="term"><em class="replaceable"><code>table_name</code></em></span></dt><dd><p>

  142.       The name (optionally schema-qualified) of the table, view, or foreign

  143.       table the trigger is for.

  144.      </p></dd><dt><span class="term"><em class="replaceable"><code>referenced_table_name</code></em></span></dt><dd><p>

  145.       The (possibly schema-qualified) name of another table referenced by the

  146.       constraint.  This option is used for foreign-key constraints and is not

  147.       recommended for general use.  This can only be specified for

  148.       constraint triggers.

  149.      </p></dd><dt><span class="term"><code class="literal">DEFERRABLE</code><br /></span><span class="term"><code class="literal">NOT DEFERRABLE</code><br /></span><span class="term"><code class="literal">INITIALLY IMMEDIATE</code><br /></span><span class="term"><code class="literal">INITIALLY DEFERRED</code></span></dt><dd><p>

  150.       The default timing of the trigger.

  151.       See the <a class="xref" href="sql-createtable.html" title="CREATE TABLE"><span class="refentrytitle">CREATE TABLE</span></a> documentation for details of

  152.       these constraint options.  This can only be specified for constraint

  153.       triggers.

  154.      </p></dd><dt><span class="term"><code class="literal">REFERENCING</code></span></dt><dd><p>

  155.       This keyword immediately precedes the declaration of one or two

  156.       relation names that provide access to the transition relations of the

  157.       triggering statement.

  158.      </p></dd><dt><span class="term"><code class="literal">OLD TABLE</code><br /></span><span class="term"><code class="literal">NEW TABLE</code></span></dt><dd><p>

  159.       This clause indicates whether the following relation name is for the

  160.       before-image transition relation or the after-image transition

  161.       relation.

  162.      </p></dd><dt><span class="term"><em class="replaceable"><code>transition_relation_name</code></em></span></dt><dd><p>

  163.       The (unqualified) name to be used within the trigger for this

  164.       transition relation.

  165.      </p></dd><dt><span class="term"><code class="literal">FOR EACH ROW</code><br /></span><span class="term"><code class="literal">FOR EACH STATEMENT</code></span></dt><dd><p>

  166.       This specifies whether the trigger function should be fired

  167.       once for every row affected by the trigger event, or just once

  168.       per SQL statement. If neither is specified, <code class="literal">FOR EACH

  169.       STATEMENT</code> is the default.  Constraint triggers can only

  170.       be specified <code class="literal">FOR EACH ROW</code>.

  171.      </p></dd><dt><span class="term"><em class="replaceable"><code>condition</code></em></span></dt><dd><p>

  172.       A Boolean expression that determines whether the trigger function

  173.       will actually be executed.  If <code class="literal">WHEN</code> is specified, the

  174.       function will only be called if the <em class="replaceable"><code>condition</code></em> returns <code class="literal">true</code>.

  175.       In <code class="literal">FOR EACH ROW</code> triggers, the <code class="literal">WHEN</code>

  176.       condition can refer to columns of the old and/or new row values

  177.       by writing <code class="literal">OLD.<em class="replaceable"><code>column_name</code></em></code> or

  178.       <code class="literal">NEW.<em class="replaceable"><code>column_name</code></em></code> respectively.

  179.       Of course, <code class="literal">INSERT</code> triggers cannot refer to <code class="literal">OLD</code>

  180.       and <code class="literal">DELETE</code> triggers cannot refer to <code class="literal">NEW</code>.

  181.      </p><p><code class="literal">INSTEAD OF</code> triggers do not support <code class="literal">WHEN</code>

  182.       conditions.

  183.      </p><p>

  184.       Currently, <code class="literal">WHEN</code> expressions cannot contain

  185.       subqueries.

  186.      </p><p>

  187.       Note that for constraint triggers, evaluation of the <code class="literal">WHEN</code>

  188.       condition is not deferred, but occurs immediately after the row update

  189.       operation is performed. If the condition does not evaluate to true then

  190.       the trigger is not queued for deferred execution.

  191.      </p></dd><dt><span class="term"><em class="replaceable"><code>function_name</code></em></span></dt><dd><p>

  192.       A user-supplied function that is declared as taking no arguments

  193.       and returning type <code class="literal">trigger</code>, which is executed when

  194.       the trigger fires.

  195.      </p><p>

  196.       In the syntax of <code class="literal">CREATE TRIGGER</code>, the keywords

  197.       <code class="literal">FUNCTION</code> and <code class="literal">PROCEDURE</code> are

  198.       equivalent, but the referenced function must in any case be a function,

  199.       not a procedure.  The use of the keyword <code class="literal">PROCEDURE</code>

  200.       here is historical and deprecated.

  201.      </p></dd><dt><span class="term"><em class="replaceable"><code>arguments</code></em></span></dt><dd><p>

  202.       An optional comma-separated list of arguments to be provided to

  203.       the function when the trigger is executed.  The arguments are

  204.       literal string constants.  Simple names and numeric constants

  205.       can be written here, too, but they will all be converted to

  206.       strings.  Please check the description of the implementation

  207.       language of the trigger function to find out how these arguments

  208.       can be accessed within the function; it might be different from

  209.       normal function arguments.

  210.      </p></dd></dl></div></div><div class="refsect1" id="SQL-CREATETRIGGER-NOTES"><h2>Notes</h2><p>

  211.    To create a trigger on a table, the user must have the

  212.    <code class="literal">TRIGGER</code> privilege on the table.  The user must

  213.    also have <code class="literal">EXECUTE</code> privilege on the trigger function.

  214.   </p><p>

  215.    Use <a class="xref" href="sql-droptrigger.html" title="DROP TRIGGER"><span class="refentrytitle">DROP TRIGGER</span></a> to remove a trigger.

  216.   </p><p>

  217.    A column-specific trigger (one defined using the <code class="literal">UPDATE OF

  218.    <em class="replaceable"><code>column_name</code></em></code> syntax) will fire when any

  219.    of its columns are listed as targets in the <code class="command">UPDATE</code>

  220.    command's <code class="literal">SET</code> list.  It is possible for a column's value

  221.    to change even when the trigger is not fired, because changes made to the

  222.    row's contents by <code class="literal">BEFORE UPDATE</code> triggers are not considered.

  223.    Conversely, a command such as <code class="literal">UPDATE ... SET x = x ...</code>

  224.    will fire a trigger on column <code class="literal">x</code>, even though the column's

  225.    value did not change.

  226.   </p><p>

  227.    There are a few built-in trigger functions that can be used to

  228.    solve common problems without having to write your own trigger code;

  229.    see <a class="xref" href="functions-trigger.html" title="9.27. Trigger Functions">Section 9.27</a>.

  230.   </p><p>

  231.    In a <code class="literal">BEFORE</code> trigger, the <code class="literal">WHEN</code> condition is

  232.    evaluated just before the function is or would be executed, so using

  233.    <code class="literal">WHEN</code> is not materially different from testing the same

  234.    condition at the beginning of the trigger function.  Note in particular

  235.    that the <code class="literal">NEW</code> row seen by the condition is the current value,

  236.    as possibly modified by earlier triggers.  Also, a <code class="literal">BEFORE</code>

  237.    trigger's <code class="literal">WHEN</code> condition is not allowed to examine the

  238.    system columns of the <code class="literal">NEW</code> row (such as <code class="literal">ctid</code>),

  239.    because those won't have been set yet.

  240.   </p><p>

  241.    In an <code class="literal">AFTER</code> trigger, the <code class="literal">WHEN</code> condition is

  242.    evaluated just after the row update occurs, and it determines whether an

  243.    event is queued to fire the trigger at the end of statement.  So when an

  244.    <code class="literal">AFTER</code> trigger's <code class="literal">WHEN</code> condition does not return

  245.    true, it is not necessary to queue an event nor to re-fetch the row at end

  246.    of statement.  This can result in significant speedups in statements that

  247.    modify many rows, if the trigger only needs to be fired for a few of the

  248.    rows.

  249.   </p><p>

  250.    In some cases it is possible for a single SQL command to fire more than

  251.    one kind of trigger.  For instance an <code class="command">INSERT</code> with

  252.    an <code class="literal">ON CONFLICT DO UPDATE</code> clause may cause both insert and

  253.    update operations, so it will fire both kinds of triggers as needed.

  254.    The transition relations supplied to triggers are

  255.    specific to their event type; thus an <code class="command">INSERT</code> trigger

  256.    will see only the inserted rows, while an <code class="command">UPDATE</code>

  257.    trigger will see only the updated rows.

  258.   </p><p>

  259.    Row updates or deletions caused by foreign-key enforcement actions, such

  260.    as <code class="literal">ON UPDATE CASCADE</code> or <code class="literal">ON DELETE SET NULL</code>, are

  261.    treated as part of the SQL command that caused them (note that such

  262.    actions are never deferred).  Relevant triggers on the affected table will

  263.    be fired, so that this provides another way in which a SQL command might

  264.    fire triggers not directly matching its type.  In simple cases, triggers

  265.    that request transition relations will see all changes caused in their

  266.    table by a single original SQL command as a single transition relation.

  267.    However, there are cases in which the presence of an <code class="literal">AFTER ROW</code>

  268.    trigger that requests transition relations will cause the foreign-key

  269.    enforcement actions triggered by a single SQL command to be split into

  270.    multiple steps, each with its own transition relation(s).  In such cases,

  271.    any statement-level triggers that are present will be fired once per

  272.    creation of a transition relation set, ensuring that the triggers see

  273.    each affected row in a transition relation once and only once.

  274.   </p><p>

  275.    Statement-level triggers on a view are fired only if the action on the

  276.    view is handled by a row-level <code class="literal">INSTEAD OF</code> trigger.

  277.    If the action is handled by an <code class="literal">INSTEAD</code> rule, then

  278.    whatever statements are emitted by the rule are executed in place of the

  279.    original statement naming the view, so that the triggers that will be

  280.    fired are those on tables named in the replacement statements.

  281.    Similarly, if the view is automatically updatable, then the action is

  282.    handled by automatically rewriting the statement into an action on the

  283.    view's base table, so that the base table's statement-level triggers are

  284.    the ones that are fired.

  285.   </p><p>

  286.    Creating a row-level trigger on a partitioned table will cause identical

  287.    triggers to be created in all its existing partitions; and any partitions

  288.    created or attached later will contain an identical trigger, too.

  289.    If the partition is detached from its parent, the trigger is removed.

  290.    Triggers on partitioned tables may only be <code class="literal">AFTER</code>.

  291.   </p><p>

  292.    Modifying a partitioned table or a table with inheritance children fires

  293.    statement-level triggers attached to the explicitly named table, but not

  294.    statement-level triggers for its partitions or child tables.  In contrast,

  295.    row-level triggers are fired on the rows in affected partitions or

  296.    child tables, even if they are not explicitly named in the query.

  297.    If a statement-level trigger has been defined with transition relations

  298.    named by a <code class="literal">REFERENCING</code> clause, then before and after

  299.    images of rows are visible from all affected partitions or child tables.

  300.    In the case of inheritance children, the row images include only columns

  301.    that are present in the table that the trigger is attached to.  Currently,

  302.    row-level triggers with transition relations cannot be defined on

  303.    partitions or inheritance child tables.

  304.   </p><p>

  305.    In <span class="productname">PostgreSQL</span> versions before 7.3, it was

  306.    necessary to declare trigger functions as returning the placeholder

  307.    type <code class="type">opaque</code>, rather than <code class="type">trigger</code>.  To support loading

  308.    of old dump files, <code class="command">CREATE TRIGGER</code> will accept a function

  309.    declared as returning <code class="type">opaque</code>, but it will issue a notice and

  310.    change the function's declared return type to <code class="type">trigger</code>.

  311.   </p></div><div class="refsect1" id="SQL-CREATETRIGGER-EXAMPLES"><h2>Examples</h2><p>

  312.    Execute the function <code class="function">check_account_update</code> whenever

  313.    a row of the table <code class="literal">accounts</code> is about to be updated:

  314. 

  315. </p><pre class="programlisting">

  316. CREATE TRIGGER check_update

  317.     BEFORE UPDATE ON accounts

  318.     FOR EACH ROW

  319.     EXECUTE FUNCTION check_account_update();

  320. </pre><p>

  321. 

  322.    The same, but only execute the function if column <code class="literal">balance</code>

  323.    is specified as a target in the <code class="command">UPDATE</code> command:

  324. 

  325. </p><pre class="programlisting">

  326. CREATE TRIGGER check_update

  327.     BEFORE UPDATE OF balance ON accounts

  328.     FOR EACH ROW

  329.     EXECUTE FUNCTION check_account_update();

  330. </pre><p>

  331. 

  332.    This form only executes the function if column <code class="literal">balance</code>

  333.    has in fact changed value:

  334. 

  335. </p><pre class="programlisting">

  336. CREATE TRIGGER check_update

  337.     BEFORE UPDATE ON accounts

  338.     FOR EACH ROW

  339.     WHEN (OLD.balance IS DISTINCT FROM NEW.balance)

  340.     EXECUTE FUNCTION check_account_update();

  341. </pre><p>

  342. 

  343.    Call a function to log updates of <code class="literal">accounts</code>, but only if

  344.    something changed:

  345. 

  346. </p><pre class="programlisting">

  347. CREATE TRIGGER log_update

  348.     AFTER UPDATE ON accounts

  349.     FOR EACH ROW

  350.     WHEN (OLD.* IS DISTINCT FROM NEW.*)

  351.     EXECUTE FUNCTION log_account_update();

  352. </pre><p>

  353. 

  354.    Execute the function <code class="function">view_insert_row</code> for each row to insert

  355.    rows into the tables underlying a view:

  356. 

  357. </p><pre class="programlisting">

  358. CREATE TRIGGER view_insert

  359.     INSTEAD OF INSERT ON my_view

  360.     FOR EACH ROW

  361.     EXECUTE FUNCTION view_insert_row();

  362. </pre><p>

  363. 

  364.    Execute the function <code class="function">check_transfer_balances_to_zero</code> for each

  365.    statement to confirm that the <code class="literal">transfer</code> rows offset to a net of

  366.    zero:

  367. 

  368. </p><pre class="programlisting">

  369. CREATE TRIGGER transfer_insert

  370.     AFTER INSERT ON transfer

  371.     REFERENCING NEW TABLE AS inserted

  372.     FOR EACH STATEMENT

  373.     EXECUTE FUNCTION check_transfer_balances_to_zero();

  374. </pre><p>

  375. 

  376.    Execute the function <code class="function">check_matching_pairs</code> for each row to

  377.    confirm that changes are made to matching pairs at the same time (by the

  378.    same statement):

  379. 

  380. </p><pre class="programlisting">

  381. CREATE TRIGGER paired_items_update

  382.     AFTER UPDATE ON paired_items

  383.     REFERENCING NEW TABLE AS newtab OLD TABLE AS oldtab

  384.     FOR EACH ROW

  385.     EXECUTE FUNCTION check_matching_pairs();

  386. </pre><p>

  387.   </p><p>

  388.    <a class="xref" href="trigger-example.html" title="38.4. A Complete Trigger Example">Section 38.4</a> contains a complete example of a trigger

  389.    function written in C.

  390.   </p></div><div class="refsect1" id="SQL-CREATETRIGGER-COMPATIBILITY"><h2>Compatibility</h2><p>

  391.    The <code class="command">CREATE TRIGGER</code> statement in

  392.    <span class="productname">PostgreSQL</span> implements a subset of the

  393.    <acronym class="acronym">SQL</acronym> standard. The following functionalities are currently

  394.    missing:

  395. 

  396.    </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>

  397.       While transition table names for <code class="literal">AFTER</code> triggers are

  398.       specified using the <code class="literal">REFERENCING</code> clause in the standard way,

  399.       the row variables used in <code class="literal">FOR EACH ROW</code> triggers may not be

  400.       specified in a <code class="literal">REFERENCING</code> clause.  They are available in a

  401.       manner that is dependent on the language in which the trigger function

  402.       is written, but is fixed for any one language.  Some languages

  403.       effectively behave as though there is a <code class="literal">REFERENCING</code> clause

  404.       containing <code class="literal">OLD ROW AS OLD NEW ROW AS NEW</code>.

  405.      </p></li><li class="listitem"><p>

  406.       The standard allows transition tables to be used with

  407.       column-specific <code class="literal">UPDATE</code> triggers, but then the set of rows

  408.       that should be visible in the transition tables depends on the

  409.       trigger's column list.  This is not currently implemented by

  410.       <span class="productname">PostgreSQL</span>.

  411.      </p></li><li class="listitem"><p>

  412.       <span class="productname">PostgreSQL</span> only allows the execution

  413.       of a user-defined function for the triggered action.  The standard

  414.       allows the execution of a number of other SQL commands, such as

  415.       <code class="command">CREATE TABLE</code>, as the triggered action.  This

  416.       limitation is not hard to work around by creating a user-defined

  417.       function that executes the desired commands.

  418.      </p></li></ul></div><p>

  419.   </p><p>

  420.    SQL specifies that multiple triggers should be fired in

  421.    time-of-creation order.  <span class="productname">PostgreSQL</span> uses

  422.    name order, which was judged to be more convenient.

  423.   </p><p>

  424.    SQL specifies that <code class="literal">BEFORE DELETE</code> triggers on cascaded

  425.    deletes fire <span class="emphasis"><em>after</em></span> the cascaded <code class="literal">DELETE</code> completes.

  426.    The <span class="productname">PostgreSQL</span> behavior is for <code class="literal">BEFORE

  427.    DELETE</code> to always fire before the delete action, even a cascading

  428.    one.  This is considered more consistent.  There is also nonstandard

  429.    behavior if <code class="literal">BEFORE</code> triggers modify rows or prevent

  430.    updates during an update that is caused by a referential action.  This can

  431.    lead to constraint violations or stored data that does not honor the

  432.    referential constraint.

  433.   </p><p>

  434.    The ability to specify multiple actions for a single trigger using

  435.    <code class="literal">OR</code> is a <span class="productname">PostgreSQL</span> extension of

  436.    the SQL standard.

  437.   </p><p>

  438.    The ability to fire triggers for <code class="command">TRUNCATE</code> is a

  439.    <span class="productname">PostgreSQL</span> extension of the SQL standard, as is the

  440.    ability to define statement-level triggers on views.

  441.   </p><p>

  442.    <code class="command">CREATE CONSTRAINT TRIGGER</code> is a

  443.    <span class="productname">PostgreSQL</span> extension of the <acronym class="acronym">SQL</acronym>

  444.    standard.

  445.   </p></div><div class="refsect1" id="id-1.9.3.93.11"><h2>See Also</h2><span class="simplelist"><a class="xref" href="sql-altertrigger.html" title="ALTER TRIGGER"><span class="refentrytitle">ALTER TRIGGER</span></a>, <a class="xref" href="sql-droptrigger.html" title="DROP TRIGGER"><span class="refentrytitle">DROP TRIGGER</span></a>, <a class="xref" href="sql-createfunction.html" title="CREATE FUNCTION"><span class="refentrytitle">CREATE FUNCTION</span></a>, <a class="xref" href="sql-set-constraints.html" title="SET CONSTRAINTS"><span class="refentrytitle">SET CONSTRAINTS</span></a></span></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="sql-createtransform.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="sql-commands.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="sql-createtype.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">CREATE TRANSFORM </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> CREATE TYPE</td></tr></table></div></body></html>
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