diff options
Diffstat (limited to 'doc/src/sgml/html/sql-createtrigger.html')
| -rw-r--r-- | doc/src/sgml/html/sql-createtrigger.html | 461 |
1 files changed, 461 insertions, 0 deletions
diff --git a/doc/src/sgml/html/sql-createtrigger.html b/doc/src/sgml/html/sql-createtrigger.html new file mode 100644 index 0000000..386a538 --- /dev/null +++ b/doc/src/sgml/html/sql-createtrigger.html @@ -0,0 +1,461 @@ +<?xml version="1.0" encoding="UTF-8" standalone="no"?> +<!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 Vsnapshot" /><link rel="prev" href="sql-createtransform.html" title="CREATE TRANSFORM" /><link rel="next" href="sql-createtype.html" title="CREATE TYPE" /></head><body id="docContent" class="container-fluid col-10"><div 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 16.2 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 /></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"> +CREATE [ OR REPLACE ] [ CONSTRAINT ] TRIGGER <em class="replaceable"><code>name</code></em> { BEFORE | AFTER | INSTEAD OF } { <em class="replaceable"><code>event</code></em> [ OR ... ] } + ON <em class="replaceable"><code>table_name</code></em> + [ FROM <em class="replaceable"><code>referenced_table_name</code></em> ] + [ NOT DEFERRABLE | [ DEFERRABLE ] [ INITIALLY IMMEDIATE | INITIALLY DEFERRED ] ] + [ REFERENCING { { OLD | NEW } TABLE [ AS ] <em class="replaceable"><code>transition_relation_name</code></em> } [ ... ] ] + [ FOR [ EACH ] { ROW | STATEMENT } ] + [ WHEN ( <em class="replaceable"><code>condition</code></em> ) ] + EXECUTE { FUNCTION | PROCEDURE } <em class="replaceable"><code>function_name</code></em> ( <em class="replaceable"><code>arguments</code></em> ) + +<span class="phrase">where <em class="replaceable"><code>event</code></em> can be one of:</span> + + INSERT + UPDATE [ OF <em class="replaceable"><code>column_name</code></em> [, ... ] ] + DELETE + TRUNCATE +</pre></div><div class="refsect1" id="id-1.9.3.93.6"><h2>Description</h2><p> + <code class="command">CREATE TRIGGER</code> creates a new trigger. + <code class="command">CREATE OR REPLACE TRIGGER</code> will either create a + new trigger, or replace an existing trigger. The + trigger will be associated with the specified table, view, or foreign table + and will execute the specified + function <em class="replaceable"><code>function_name</code></em> when + certain operations are performed on that table. + </p><p> + To replace the current definition of an existing trigger, use + <code class="command">CREATE OR REPLACE TRIGGER</code>, specifying the existing + trigger's name and parent table. All other properties are replaced. + </p><p> + The trigger can be specified to fire before the + operation is attempted on a row (before constraints are checked and + the <code class="command">INSERT</code>, <code class="command">UPDATE</code>, or + <code class="command">DELETE</code> is attempted); or after the operation has + completed (after constraints are checked and the + <code class="command">INSERT</code>, <code class="command">UPDATE</code>, or + <code class="command">DELETE</code> has completed); or instead of the operation + (in the case of inserts, updates or deletes on a view). + If the trigger fires before or instead of the event, the trigger can skip + the operation for the current row, or change the row being inserted (for + <code class="command">INSERT</code> and <code class="command">UPDATE</code> operations + only). If the trigger fires after the event, all changes, including + the effects of other triggers, are <span class="quote">“<span class="quote">visible</span>”</span> + to the trigger. + </p><p> + A trigger that is marked <code class="literal">FOR EACH ROW</code> is called + once for every row that the operation modifies. For example, a + <code class="command">DELETE</code> that affects 10 rows will cause any + <code class="literal">ON DELETE</code> triggers on the target relation to be + called 10 separate times, once for each deleted row. In contrast, a + trigger that is marked <code class="literal">FOR EACH STATEMENT</code> only + executes once for any given operation, regardless of how many rows + it modifies (in particular, an operation that modifies zero rows + will still result in the execution of any applicable <code class="literal">FOR + EACH STATEMENT</code> triggers). + </p><p> + Triggers that are specified to fire <code class="literal">INSTEAD OF</code> the trigger + event must be marked <code class="literal">FOR EACH ROW</code>, and can only be defined + on views. <code class="literal">BEFORE</code> and <code class="literal">AFTER</code> triggers on a view + must be marked as <code class="literal">FOR EACH STATEMENT</code>. + </p><p> + In addition, triggers may be defined to fire for + <code class="command">TRUNCATE</code>, though only + <code class="literal">FOR EACH STATEMENT</code>. + </p><p> + The following table summarizes which types of triggers may be used on + tables, views, and foreign tables: + </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 and foreign 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 and foreign 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> + Also, a trigger definition can specify a Boolean <code class="literal">WHEN</code> + condition, which will be tested to see whether the trigger should + be fired. In row-level triggers the <code class="literal">WHEN</code> condition can + examine the old and/or new values of columns of the row. Statement-level + triggers can also have <code class="literal">WHEN</code> conditions, although the feature + is not so useful for them since the condition cannot refer to any values + in the table. + </p><p> + If multiple triggers of the same kind are defined for the same event, + they will be fired in alphabetical order by name. + </p><p> + When the <code class="literal">CONSTRAINT</code> option is specified, this command creates a + <em class="firstterm">constraint trigger</em>.<a id="id-1.9.3.93.6.12.3" class="indexterm"></a> + This is the same as a regular trigger + except that the timing of the trigger firing can be adjusted using + <a class="link" href="sql-set-constraints.html" title="SET CONSTRAINTS"><code class="command">SET CONSTRAINTS</code></a>. + Constraint triggers must be <code class="literal">AFTER ROW</code> triggers on plain + tables (not foreign tables). They + can be fired either at the end of the statement causing the triggering + event, or at the end of the containing transaction; in the latter case they + are said to be <em class="firstterm">deferred</em>. A pending deferred-trigger firing + can also be forced to happen immediately by using <code class="command">SET + CONSTRAINTS</code>. Constraint triggers are expected to raise an exception + when the constraints they implement are violated. + </p><p> + The <code class="literal">REFERENCING</code> option enables collection + of <em class="firstterm">transition relations</em>, which are row sets that include all + of the rows inserted, deleted, or modified by the current SQL statement. + This feature lets the trigger see a global view of what the statement did, + not just one row at a time. This option is only allowed for + an <code class="literal">AFTER</code> trigger that is not a constraint trigger; also, if + the trigger is an <code class="literal">UPDATE</code> trigger, it must not specify + a <em class="replaceable"><code>column_name</code></em> list. + <code class="literal">OLD TABLE</code> may only be specified once, and only for a trigger + that can fire on <code class="literal">UPDATE</code> or <code class="literal">DELETE</code>; it creates a + transition relation containing the <em class="firstterm">before-images</em> of all rows + updated or deleted by the statement. + Similarly, <code class="literal">NEW TABLE</code> may only be specified once, and only for + a trigger that can fire on <code class="literal">UPDATE</code> or <code class="literal">INSERT</code>; + it creates a transition relation containing the <em class="firstterm">after-images</em> + of all rows updated or inserted by the statement. + </p><p> + <code class="command">SELECT</code> does not modify any rows so you cannot + create <code class="command">SELECT</code> triggers. Rules and views may provide + workable solutions to problems that seem to need <code class="command">SELECT</code> + triggers. + </p><p> + Refer to <a class="xref" href="triggers.html" title="Chapter 39. Triggers">Chapter 39</a> for more information about triggers. + </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> + The name to give the new trigger. This must be distinct from + the name of any other trigger for the same table. + The name cannot be schema-qualified — the trigger inherits the + schema of its table. For a constraint trigger, this is also the name to + use when modifying the trigger's behavior using + <code class="command">SET CONSTRAINTS</code>. + </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> + Determines whether the function is called before, after, or instead of + the event. A constraint trigger can only be specified as + <code class="literal">AFTER</code>. + </p></dd><dt><span class="term"><em class="replaceable"><code>event</code></em></span></dt><dd><p> + One of <code class="literal">INSERT</code>, <code class="literal">UPDATE</code>, + <code class="literal">DELETE</code>, or <code class="literal">TRUNCATE</code>; + this specifies the event that will fire the trigger. Multiple + events can be specified using <code class="literal">OR</code>, except when + transition relations are requested. + </p><p> + For <code class="literal">UPDATE</code> events, it is possible to + specify a list of columns using this syntax: +</p><pre class="synopsis"> +UPDATE OF <em class="replaceable"><code>column_name1</code></em> [, <em class="replaceable"><code>column_name2</code></em> ... ] +</pre><p> + The trigger will only fire if at least one of the listed columns + is mentioned as a target of the <code class="command">UPDATE</code> command + or if one of the listed columns is a generated column that depends on a + column that is the target of the <code class="command">UPDATE</code>. + </p><p> + <code class="literal">INSTEAD OF UPDATE</code> events do not allow a list of columns. + A column list cannot be specified when requesting transition relations, + either. + </p></dd><dt><span class="term"><em class="replaceable"><code>table_name</code></em></span></dt><dd><p> + The name (optionally schema-qualified) of the table, view, or foreign + table the trigger is for. + </p></dd><dt><span class="term"><em class="replaceable"><code>referenced_table_name</code></em></span></dt><dd><p> + The (possibly schema-qualified) name of another table referenced by the + constraint. This option is used for foreign-key constraints and is not + recommended for general use. This can only be specified for + constraint triggers. + </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> + The default timing of the trigger. + See the <a class="xref" href="sql-createtable.html" title="CREATE TABLE"><span class="refentrytitle">CREATE TABLE</span></a> documentation for details of + these constraint options. This can only be specified for constraint + triggers. + </p></dd><dt><span class="term"><code class="literal">REFERENCING</code></span></dt><dd><p> + This keyword immediately precedes the declaration of one or two + relation names that provide access to the transition relations of the + triggering statement. + </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> + This clause indicates whether the following relation name is for the + before-image transition relation or the after-image transition + relation. + </p></dd><dt><span class="term"><em class="replaceable"><code>transition_relation_name</code></em></span></dt><dd><p> + The (unqualified) name to be used within the trigger for this + transition relation. + </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> + This specifies whether the trigger function should be fired + once for every row affected by the trigger event, or just once + per SQL statement. If neither is specified, <code class="literal">FOR EACH + STATEMENT</code> is the default. Constraint triggers can only + be specified <code class="literal">FOR EACH ROW</code>. + </p></dd><dt><span class="term"><em class="replaceable"><code>condition</code></em></span></dt><dd><p> + A Boolean expression that determines whether the trigger function + will actually be executed. If <code class="literal">WHEN</code> is specified, the + function will only be called if the <em class="replaceable"><code>condition</code></em> returns <code class="literal">true</code>. + In <code class="literal">FOR EACH ROW</code> triggers, the <code class="literal">WHEN</code> + condition can refer to columns of the old and/or new row values + by writing <code class="literal">OLD.<em class="replaceable"><code>column_name</code></em></code> or + <code class="literal">NEW.<em class="replaceable"><code>column_name</code></em></code> respectively. + Of course, <code class="literal">INSERT</code> triggers cannot refer to <code class="literal">OLD</code> + and <code class="literal">DELETE</code> triggers cannot refer to <code class="literal">NEW</code>. + </p><p><code class="literal">INSTEAD OF</code> triggers do not support <code class="literal">WHEN</code> + conditions. + </p><p> + Currently, <code class="literal">WHEN</code> expressions cannot contain + subqueries. + </p><p> + Note that for constraint triggers, evaluation of the <code class="literal">WHEN</code> + condition is not deferred, but occurs immediately after the row update + operation is performed. If the condition does not evaluate to true then + the trigger is not queued for deferred execution. + </p></dd><dt><span class="term"><em class="replaceable"><code>function_name</code></em></span></dt><dd><p> + A user-supplied function that is declared as taking no arguments + and returning type <code class="literal">trigger</code>, which is executed when + the trigger fires. + </p><p> + In the syntax of <code class="literal">CREATE TRIGGER</code>, the keywords + <code class="literal">FUNCTION</code> and <code class="literal">PROCEDURE</code> are + equivalent, but the referenced function must in any case be a function, + not a procedure. The use of the keyword <code class="literal">PROCEDURE</code> + here is historical and deprecated. + </p></dd><dt><span class="term"><em class="replaceable"><code>arguments</code></em></span></dt><dd><p> + An optional comma-separated list of arguments to be provided to + the function when the trigger is executed. The arguments are + literal string constants. Simple names and numeric constants + can be written here, too, but they will all be converted to + strings. Please check the description of the implementation + language of the trigger function to find out how these arguments + can be accessed within the function; it might be different from + normal function arguments. + </p></dd></dl></div></div><div class="refsect1" id="SQL-CREATETRIGGER-NOTES"><h2>Notes</h2><p> + To create or replace a trigger on a table, the user must have the + <code class="literal">TRIGGER</code> privilege on the table. The user must + also have <code class="literal">EXECUTE</code> privilege on the trigger function. + </p><p> + Use <a class="link" href="sql-droptrigger.html" title="DROP TRIGGER"><code class="command">DROP TRIGGER</code></a> to remove a trigger. + </p><p> + Creating a row-level trigger on a partitioned table will cause an + identical <span class="quote">“<span class="quote">clone</span>”</span> trigger to be created on each of its + existing partitions; and any partitions created or attached later will have + an identical trigger, too. If there is a conflictingly-named trigger on a + child partition already, an error occurs unless <code class="command">CREATE OR REPLACE + TRIGGER</code> is used, in which case that trigger is replaced with a + clone trigger. When a partition is detached from its parent, its clone + triggers are removed. + </p><p> + A column-specific trigger (one defined using the <code class="literal">UPDATE OF + <em class="replaceable"><code>column_name</code></em></code> syntax) will fire when any + of its columns are listed as targets in the <code class="command">UPDATE</code> + command's <code class="literal">SET</code> list. It is possible for a column's value + to change even when the trigger is not fired, because changes made to the + row's contents by <code class="literal">BEFORE UPDATE</code> triggers are not considered. + Conversely, a command such as <code class="literal">UPDATE ... SET x = x ...</code> + will fire a trigger on column <code class="literal">x</code>, even though the column's + value did not change. + </p><p> + In a <code class="literal">BEFORE</code> trigger, the <code class="literal">WHEN</code> condition is + evaluated just before the function is or would be executed, so using + <code class="literal">WHEN</code> is not materially different from testing the same + condition at the beginning of the trigger function. Note in particular + that the <code class="literal">NEW</code> row seen by the condition is the current value, + as possibly modified by earlier triggers. Also, a <code class="literal">BEFORE</code> + trigger's <code class="literal">WHEN</code> condition is not allowed to examine the + system columns of the <code class="literal">NEW</code> row (such as <code class="literal">ctid</code>), + because those won't have been set yet. + </p><p> + In an <code class="literal">AFTER</code> trigger, the <code class="literal">WHEN</code> condition is + evaluated just after the row update occurs, and it determines whether an + event is queued to fire the trigger at the end of statement. So when an + <code class="literal">AFTER</code> trigger's <code class="literal">WHEN</code> condition does not return + true, it is not necessary to queue an event nor to re-fetch the row at end + of statement. This can result in significant speedups in statements that + modify many rows, if the trigger only needs to be fired for a few of the + rows. + </p><p> + In some cases it is possible for a single SQL command to fire more than + one kind of trigger. For instance an <code class="command">INSERT</code> with + an <code class="literal">ON CONFLICT DO UPDATE</code> clause may cause both insert and + update operations, so it will fire both kinds of triggers as needed. + The transition relations supplied to triggers are + specific to their event type; thus an <code class="command">INSERT</code> trigger + will see only the inserted rows, while an <code class="command">UPDATE</code> + trigger will see only the updated rows. + </p><p> + Row updates or deletions caused by foreign-key enforcement actions, such + as <code class="literal">ON UPDATE CASCADE</code> or <code class="literal">ON DELETE SET NULL</code>, are + treated as part of the SQL command that caused them (note that such + actions are never deferred). Relevant triggers on the affected table will + be fired, so that this provides another way in which an SQL command might + fire triggers not directly matching its type. In simple cases, triggers + that request transition relations will see all changes caused in their + table by a single original SQL command as a single transition relation. + However, there are cases in which the presence of an <code class="literal">AFTER ROW</code> + trigger that requests transition relations will cause the foreign-key + enforcement actions triggered by a single SQL command to be split into + multiple steps, each with its own transition relation(s). In such cases, + any statement-level triggers that are present will be fired once per + creation of a transition relation set, ensuring that the triggers see + each affected row in a transition relation once and only once. + </p><p> + Statement-level triggers on a view are fired only if the action on the + view is handled by a row-level <code class="literal">INSTEAD OF</code> trigger. + If the action is handled by an <code class="literal">INSTEAD</code> rule, then + whatever statements are emitted by the rule are executed in place of the + original statement naming the view, so that the triggers that will be + fired are those on tables named in the replacement statements. + Similarly, if the view is automatically updatable, then the action is + handled by automatically rewriting the statement into an action on the + view's base table, so that the base table's statement-level triggers are + the ones that are fired. + </p><p> + Modifying a partitioned table or a table with inheritance children fires + statement-level triggers attached to the explicitly named table, but not + statement-level triggers for its partitions or child tables. In contrast, + row-level triggers are fired on the rows in affected partitions or + child tables, even if they are not explicitly named in the query. + If a statement-level trigger has been defined with transition relations + named by a <code class="literal">REFERENCING</code> clause, then before and after + images of rows are visible from all affected partitions or child tables. + In the case of inheritance children, the row images include only columns + that are present in the table that the trigger is attached to. + </p><p> + Currently, row-level triggers with transition relations cannot be defined + on partitions or inheritance child tables. Also, triggers on partitioned + tables may not be <code class="literal">INSTEAD OF</code>. + </p><p> + Currently, the <code class="literal">OR REPLACE</code> option is not supported for + constraint triggers. + </p><p> + Replacing an existing trigger within a transaction that has already + performed updating actions on the trigger's table is not recommended. + Trigger firing decisions, or portions of firing decisions, that have + already been made will not be reconsidered, so the effects could be + surprising. + </p><p> + There are a few built-in trigger functions that can be used to + solve common problems without having to write your own trigger code; + see <a class="xref" href="functions-trigger.html" title="9.28. Trigger Functions">Section 9.28</a>. + </p></div><div class="refsect1" id="SQL-CREATETRIGGER-EXAMPLES"><h2>Examples</h2><p> + Execute the function <code class="function">check_account_update</code> whenever + a row of the table <code class="literal">accounts</code> is about to be updated: + +</p><pre class="programlisting"> +CREATE TRIGGER check_update + BEFORE UPDATE ON accounts + FOR EACH ROW + EXECUTE FUNCTION check_account_update(); +</pre><p> + + Modify that trigger definition to only execute the function if + column <code class="literal">balance</code> is specified as a target in + the <code class="command">UPDATE</code> command: + +</p><pre class="programlisting"> +CREATE OR REPLACE TRIGGER check_update + BEFORE UPDATE OF balance ON accounts + FOR EACH ROW + EXECUTE FUNCTION check_account_update(); +</pre><p> + + This form only executes the function if column <code class="literal">balance</code> + has in fact changed value: + +</p><pre class="programlisting"> +CREATE TRIGGER check_update + BEFORE UPDATE ON accounts + FOR EACH ROW + WHEN (OLD.balance IS DISTINCT FROM NEW.balance) + EXECUTE FUNCTION check_account_update(); +</pre><p> + + Call a function to log updates of <code class="literal">accounts</code>, but only if + something changed: + +</p><pre class="programlisting"> +CREATE TRIGGER log_update + AFTER UPDATE ON accounts + FOR EACH ROW + WHEN (OLD.* IS DISTINCT FROM NEW.*) + EXECUTE FUNCTION log_account_update(); +</pre><p> + + Execute the function <code class="function">view_insert_row</code> for each row to insert + rows into the tables underlying a view: + +</p><pre class="programlisting"> +CREATE TRIGGER view_insert + INSTEAD OF INSERT ON my_view + FOR EACH ROW + EXECUTE FUNCTION view_insert_row(); +</pre><p> + + Execute the function <code class="function">check_transfer_balances_to_zero</code> for each + statement to confirm that the <code class="literal">transfer</code> rows offset to a net of + zero: + +</p><pre class="programlisting"> +CREATE TRIGGER transfer_insert + AFTER INSERT ON transfer + REFERENCING NEW TABLE AS inserted + FOR EACH STATEMENT + EXECUTE FUNCTION check_transfer_balances_to_zero(); +</pre><p> + + Execute the function <code class="function">check_matching_pairs</code> for each row to + confirm that changes are made to matching pairs at the same time (by the + same statement): + +</p><pre class="programlisting"> +CREATE TRIGGER paired_items_update + AFTER UPDATE ON paired_items + REFERENCING NEW TABLE AS newtab OLD TABLE AS oldtab + FOR EACH ROW + EXECUTE FUNCTION check_matching_pairs(); +</pre><p> + </p><p> + <a class="xref" href="trigger-example.html" title="39.4. A Complete Trigger Example">Section 39.4</a> contains a complete example of a trigger + function written in C. + </p></div><div class="refsect1" id="SQL-CREATETRIGGER-COMPATIBILITY"><h2>Compatibility</h2><p> + The <code class="command">CREATE TRIGGER</code> statement in + <span class="productname">PostgreSQL</span> implements a subset of the + <acronym class="acronym">SQL</acronym> standard. The following functionalities are currently + missing: + + </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p> + While transition table names for <code class="literal">AFTER</code> triggers are + specified using the <code class="literal">REFERENCING</code> clause in the standard way, + the row variables used in <code class="literal">FOR EACH ROW</code> triggers may not be + specified in a <code class="literal">REFERENCING</code> clause. They are available in a + manner that is dependent on the language in which the trigger function + is written, but is fixed for any one language. Some languages + effectively behave as though there is a <code class="literal">REFERENCING</code> clause + containing <code class="literal">OLD ROW AS OLD NEW ROW AS NEW</code>. + </p></li><li class="listitem"><p> + The standard allows transition tables to be used with + column-specific <code class="literal">UPDATE</code> triggers, but then the set of rows + that should be visible in the transition tables depends on the + trigger's column list. This is not currently implemented by + <span class="productname">PostgreSQL</span>. + </p></li><li class="listitem"><p> + <span class="productname">PostgreSQL</span> only allows the execution + of a user-defined function for the triggered action. The standard + allows the execution of a number of other SQL commands, such as + <code class="command">CREATE TABLE</code>, as the triggered action. This + limitation is not hard to work around by creating a user-defined + function that executes the desired commands. + </p></li></ul></div><p> + </p><p> + SQL specifies that multiple triggers should be fired in + time-of-creation order. <span class="productname">PostgreSQL</span> uses + name order, which was judged to be more convenient. + </p><p> + SQL specifies that <code class="literal">BEFORE DELETE</code> triggers on cascaded + deletes fire <span class="emphasis"><em>after</em></span> the cascaded <code class="literal">DELETE</code> completes. + The <span class="productname">PostgreSQL</span> behavior is for <code class="literal">BEFORE + DELETE</code> to always fire before the delete action, even a cascading + one. This is considered more consistent. There is also nonstandard + behavior if <code class="literal">BEFORE</code> triggers modify rows or prevent + updates during an update that is caused by a referential action. This can + lead to constraint violations or stored data that does not honor the + referential constraint. + </p><p> + The ability to specify multiple actions for a single trigger using + <code class="literal">OR</code> is a <span class="productname">PostgreSQL</span> extension of + the SQL standard. + </p><p> + The ability to fire triggers for <code class="command">TRUNCATE</code> is a + <span class="productname">PostgreSQL</span> extension of the SQL standard, as is the + ability to define statement-level triggers on views. + </p><p> + <code class="command">CREATE CONSTRAINT TRIGGER</code> is a + <span class="productname">PostgreSQL</span> extension of the <acronym class="acronym">SQL</acronym> + standard. + So is the <code class="literal">OR REPLACE</code> option. + </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" title="CREATE TRANSFORM">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="sql-commands.html" title="SQL Commands">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="sql-createtype.html" title="CREATE TYPE">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" title="PostgreSQL 16.2 Documentation">Home</a></td><td width="40%" align="right" valign="top"> CREATE TYPE</td></tr></table></div></body></html>
\ No newline at end of file |