diff options
Diffstat (limited to 'doc/src/sgml/html/trigger-interface.html')
| -rw-r--r-- | doc/src/sgml/html/trigger-interface.html | 184 |
1 files changed, 184 insertions, 0 deletions
diff --git a/doc/src/sgml/html/trigger-interface.html b/doc/src/sgml/html/trigger-interface.html new file mode 100644 index 0000000..ed345ef --- /dev/null +++ b/doc/src/sgml/html/trigger-interface.html @@ -0,0 +1,184 @@ +<?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>39.3. Writing Trigger Functions in C</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="trigger-datachanges.html" title="39.2. Visibility of Data Changes" /><link rel="next" href="trigger-example.html" title="39.4. A Complete Trigger Example" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">39.3. Writing Trigger Functions in C</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="trigger-datachanges.html" title="39.2. Visibility of Data Changes">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="triggers.html" title="Chapter 39. Triggers">Up</a></td><th width="60%" align="center">Chapter 39. Triggers</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="trigger-example.html" title="39.4. A Complete Trigger Example">Next</a></td></tr></table><hr /></div><div class="sect1" id="TRIGGER-INTERFACE"><div class="titlepage"><div><div><h2 class="title" style="clear: both">39.3. Writing Trigger Functions in C</h2></div></div></div><a id="id-1.8.4.7.2" class="indexterm"></a><a id="id-1.8.4.7.3" class="indexterm"></a><p> + 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. + </p><p> + Trigger functions must use the <span class="quote">“<span class="quote">version 1</span>”</span> function manager + interface. + </p><p> + When a function is called by the trigger manager, it is not passed + any normal arguments, but it is passed a <span class="quote">“<span class="quote">context</span>”</span> + pointer pointing to a <code class="structname">TriggerData</code> structure. C + functions can check whether they were called from the trigger + manager or not by executing the macro: +</p><pre class="programlisting"> +CALLED_AS_TRIGGER(fcinfo) +</pre><p> + which expands to: +</p><pre class="programlisting"> +((fcinfo)->context != NULL && IsA((fcinfo)->context, TriggerData)) +</pre><p> + If this returns true, then it is safe to cast + <code class="literal">fcinfo->context</code> to type <code class="literal">TriggerData + *</code> and make use of the pointed-to + <code class="structname">TriggerData</code> structure. The function must + <span class="emphasis"><em>not</em></span> alter the <code class="structname">TriggerData</code> + structure or any of the data it points to. + </p><p> + <code class="structname">struct TriggerData</code> is defined in + <code class="filename">commands/trigger.h</code>: + +</p><pre class="programlisting"> +typedef struct TriggerData +{ + NodeTag type; + TriggerEvent tg_event; + Relation tg_relation; + HeapTuple tg_trigtuple; + HeapTuple tg_newtuple; + Trigger *tg_trigger; + TupleTableSlot *tg_trigslot; + TupleTableSlot *tg_newslot; + Tuplestorestate *tg_oldtable; + Tuplestorestate *tg_newtable; + const Bitmapset *tg_updatedcols; +} TriggerData; +</pre><p> + + where the members are defined as follows: + + </p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="structfield">type</code></span></dt><dd><p> + Always <code class="literal">T_TriggerData</code>. + </p></dd><dt><span class="term"><code class="structfield">tg_event</code></span></dt><dd><p> + Describes the event for which the function is called. You can use the + following macros to examine <code class="literal">tg_event</code>: + + </p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="literal">TRIGGER_FIRED_BEFORE(tg_event)</code></span></dt><dd><p> + Returns true if the trigger fired before the operation. + </p></dd><dt><span class="term"><code class="literal">TRIGGER_FIRED_AFTER(tg_event)</code></span></dt><dd><p> + Returns true if the trigger fired after the operation. + </p></dd><dt><span class="term"><code class="literal">TRIGGER_FIRED_INSTEAD(tg_event)</code></span></dt><dd><p> + Returns true if the trigger fired instead of the operation. + </p></dd><dt><span class="term"><code class="literal">TRIGGER_FIRED_FOR_ROW(tg_event)</code></span></dt><dd><p> + Returns true if the trigger fired for a row-level event. + </p></dd><dt><span class="term"><code class="literal">TRIGGER_FIRED_FOR_STATEMENT(tg_event)</code></span></dt><dd><p> + Returns true if the trigger fired for a statement-level event. + </p></dd><dt><span class="term"><code class="literal">TRIGGER_FIRED_BY_INSERT(tg_event)</code></span></dt><dd><p> + Returns true if the trigger was fired by an <code class="command">INSERT</code> command. + </p></dd><dt><span class="term"><code class="literal">TRIGGER_FIRED_BY_UPDATE(tg_event)</code></span></dt><dd><p> + Returns true if the trigger was fired by an <code class="command">UPDATE</code> command. + </p></dd><dt><span class="term"><code class="literal">TRIGGER_FIRED_BY_DELETE(tg_event)</code></span></dt><dd><p> + Returns true if the trigger was fired by a <code class="command">DELETE</code> command. + </p></dd><dt><span class="term"><code class="literal">TRIGGER_FIRED_BY_TRUNCATE(tg_event)</code></span></dt><dd><p> + Returns true if the trigger was fired by a <code class="command">TRUNCATE</code> command. + </p></dd></dl></div><p> + </p></dd><dt><span class="term"><code class="structfield">tg_relation</code></span></dt><dd><p> + A pointer to a structure describing the relation that the trigger fired for. + Look at <code class="filename">utils/rel.h</code> for details about + this structure. The most interesting things are + <code class="literal">tg_relation->rd_att</code> (descriptor of the relation + tuples) and <code class="literal">tg_relation->rd_rel->relname</code> + (relation name; the type is not <code class="type">char*</code> but + <code class="type">NameData</code>; use + <code class="literal">SPI_getrelname(tg_relation)</code> to get a <code class="type">char*</code> if you + need a copy of the name). + </p></dd><dt><span class="term"><code class="structfield">tg_trigtuple</code></span></dt><dd><p> + 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 <code class="command">INSERT</code> or + <code class="command">DELETE</code> 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 <code class="command">INSERT</code>) or + skip the operation. For triggers on foreign tables, values of system + columns herein are unspecified. + </p></dd><dt><span class="term"><code class="structfield">tg_newtuple</code></span></dt><dd><p> + A pointer to the new version of the row, if the trigger was + fired for an <code class="command">UPDATE</code>, and <code class="symbol">NULL</code> if + it is for an <code class="command">INSERT</code> or a + <code class="command">DELETE</code>. This is what you have to return + from the function if the event is an <code class="command">UPDATE</code> + 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. + </p></dd><dt><span class="term"><code class="structfield">tg_trigger</code></span></dt><dd><p> + A pointer to a structure of type <code class="structname">Trigger</code>, + defined in <code class="filename">utils/reltrigger.h</code>: + +</p><pre class="programlisting"> +typedef struct Trigger +{ + Oid tgoid; + char *tgname; + Oid tgfoid; + int16 tgtype; + char tgenabled; + bool tgisinternal; + bool tgisclone; + 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; +</pre><p> + + where <code class="structfield">tgname</code> is the trigger's name, + <code class="structfield">tgnargs</code> is the number of arguments in + <code class="structfield">tgargs</code>, and <code class="structfield">tgargs</code> is an array of + pointers to the arguments specified in the <code class="command">CREATE + TRIGGER</code> statement. The other members are for internal use + only. + </p></dd><dt><span class="term"><code class="structfield">tg_trigslot</code></span></dt><dd><p> + The slot containing <code class="structfield">tg_trigtuple</code>, + or a <code class="symbol">NULL</code> pointer if there is no such tuple. + </p></dd><dt><span class="term"><code class="structfield">tg_newslot</code></span></dt><dd><p> + The slot containing <code class="structfield">tg_newtuple</code>, + or a <code class="symbol">NULL</code> pointer if there is no such tuple. + </p></dd><dt><span class="term"><code class="structfield">tg_oldtable</code></span></dt><dd><p> + A pointer to a structure of type <code class="structname">Tuplestorestate</code> + containing zero or more rows in the format specified by + <code class="structfield">tg_relation</code>, or a <code class="symbol">NULL</code> pointer + if there is no <code class="literal">OLD TABLE</code> transition relation. + </p></dd><dt><span class="term"><code class="structfield">tg_newtable</code></span></dt><dd><p> + A pointer to a structure of type <code class="structname">Tuplestorestate</code> + containing zero or more rows in the format specified by + <code class="structfield">tg_relation</code>, or a <code class="symbol">NULL</code> pointer + if there is no <code class="literal">NEW TABLE</code> transition relation. + </p></dd><dt><span class="term"><code class="structfield">tg_updatedcols</code></span></dt><dd><p> + For <code class="literal">UPDATE</code> triggers, a bitmap set indicating the + columns that were updated by the triggering command. Generic trigger + functions can use this to optimize actions by not having to deal with + columns that were not changed. + </p><p> + As an example, to determine whether a column with attribute number + <code class="varname">attnum</code> (1-based) is a member of this bitmap set, + call <code class="literal">bms_is_member(attnum - + FirstLowInvalidHeapAttributeNumber, + trigdata->tg_updatedcols))</code>. + </p><p> + For triggers other than <code class="literal">UPDATE</code> triggers, this will + be <code class="symbol">NULL</code>. + </p></dd></dl></div><p> + </p><p> + To allow queries issued through SPI to reference transition tables, see + <a class="xref" href="spi-spi-register-trigger-data.html" title="SPI_register_trigger_data"><span class="refentrytitle">SPI_register_trigger_data</span></a>. + </p><p> + A trigger function must return either a + <code class="structname">HeapTuple</code> pointer or a <code class="symbol">NULL</code> pointer + (<span class="emphasis"><em>not</em></span> an SQL null value, that is, do not set <em class="parameter"><code>isNull</code></em> true). + Be careful to return either + <code class="structfield">tg_trigtuple</code> or <code class="structfield">tg_newtuple</code>, + as appropriate, if you don't want to modify the row being operated on. + </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="trigger-datachanges.html" title="39.2. Visibility of Data Changes">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="triggers.html" title="Chapter 39. Triggers">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="trigger-example.html" title="39.4. A Complete Trigger Example">Next</a></td></tr><tr><td width="40%" align="left" valign="top">39.2. Visibility of Data Changes </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 39.4. A Complete Trigger Example</td></tr></table></div></body></html>
\ No newline at end of file |