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diff --git a/doc/src/sgml/html/trigger-example.html b/doc/src/sgml/html/trigger-example.html new file mode 100644 index 0000000..742b3ed --- /dev/null +++ b/doc/src/sgml/html/trigger-example.html @@ -0,0 +1,180 @@ +<?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>39.4. A Complete Trigger Example</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-interface.html" title="39.3. Writing Trigger Functions in C" /><link rel="next" href="event-triggers.html" title="Chapter 40. Event Triggers" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">39.4. A Complete Trigger Example</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="trigger-interface.html" title="39.3. Writing Trigger Functions in C">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 16.2 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="event-triggers.html" title="Chapter 40. Event Triggers">Next</a></td></tr></table><hr /></div><div class="sect1" id="TRIGGER-EXAMPLE"><div class="titlepage"><div><div><h2 class="title" style="clear: both">39.4. A Complete Trigger Example <a href="#TRIGGER-EXAMPLE" class="id_link">#</a></h2></div></div></div><p> + 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.) + </p><p> + The function <code class="function">trigf</code> reports the number of rows in the + table <code class="structname">ttest</code> and skips the actual operation if the + command attempts to insert a null value into the column + <code class="structfield">x</code>. (So the trigger acts as a not-null constraint but + doesn't abort the transaction.) + </p><p> + First, the table definition: +</p><pre class="programlisting"> +CREATE TABLE ttest ( + x integer +); +</pre><p> + </p><p> + This is the source code of the trigger function: +</p><pre class="programlisting"> +#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); +} + +</pre><p> + </p><p> + After you have compiled the source code (see <a class="xref" href="xfunc-c.html#DFUNC" title="38.10.5. Compiling and Linking Dynamically-Loaded Functions">Section 38.10.5</a>), declare the function and the triggers: +</p><pre class="programlisting"> +CREATE FUNCTION trigf() RETURNS trigger + AS '<em class="replaceable"><code>filename</code></em>' + 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(); +</pre><p> + </p><p> + Now you can test the operation of the trigger: +</p><pre class="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) +</pre><p> + + </p><p> + There are more complex examples in + <code class="filename">src/test/regress/regress.c</code> and + in <a class="xref" href="contrib-spi.html" title="F.41. spi — Server Programming Interface features/examples">spi</a>. + </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="trigger-interface.html" title="39.3. Writing Trigger Functions in C">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="event-triggers.html" title="Chapter 40. Event Triggers">Next</a></td></tr><tr><td width="40%" align="left" valign="top">39.3. Writing Trigger Functions in C </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"> Chapter 40. Event Triggers</td></tr></table></div></body></html>
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