在 src/test/regress/regress.c 和 contrib/spi 里有更复杂的例子.
这里是一个非常简单的触发器使用的例子. 函数 trigf 报告在被触发的关系 ttest 中元组数量, 并且如果查询试图把NULL 插入到 x 里 (也就是说 -它做为一个 NOT NULL 约束但不退出事务的约束)时略过操作.
#include "executor/spi.h" /* 你用SPI的时候要用的头文件 */ #include "commands/trigger.h" /* -"- 用触发器要用的头 */ extern Datum trigf(PG_FUNCTION_ARGS); 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 trigdata is pointing at what I expect */ if (!CALLED_AS_TRIGGER(fcinfo)) elog(ERROR, "trigf: not fired 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 NULLs ? */ 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(NOTICE, "trigf (fired %s): SPI_connect returned %d", when, ret); /* Get number of tuples in relation */ ret = SPI_exec("select count(*) from ttest", 0); if (ret < 0) elog(NOTICE, "trigf (fired %s): SPI_exec returned %d", when, ret); i = SPI_getbinval(SPI_tuptable->vals[0], SPI_tuptable->tupdesc, 1, &isnull); elog (NOTICE, "trigf (fired %s): there are %d tuples in ttest", when, i); SPI_finish(); if (checknull) { i = SPI_getbinval(rettuple, tupdesc, 1, &isnull); if (isnull) rettuple = NULL; } return PointerGetDatum(rettuple); }
然后,编译和创建触发器函数:
create function trigf () returns opaque as '...path_to_so' language 'C'; create table ttest (x int4);
vac=> create trigger tbefore before insert or update or delete on ttest for each row execute procedure trigf(); CREATE vac=> create trigger tafter after insert or update or delete on ttest for each row execute procedure trigf(); CREATE vac=> insert into ttest values (null); NOTICE:trigf (fired before): there are 0 tuples in ttest INSERT 0 0 -- Insertion skipped and AFTER trigger is not fired vac=> select * from ttest; x - (0 rows) vac=> insert into ttest values (1); NOTICE:trigf (fired before): there are 0 tuples in ttest NOTICE:trigf (fired after ): there are 1 tuples in ttest ^^^^^^^^ remember what we said about visibility. INSERT 167793 1 vac=> select * from ttest; x - 1 (1 row) vac=> insert into ttest select x * 2 from ttest; NOTICE:trigf (fired before): there are 1 tuples in ttest NOTICE:trigf (fired after ): there are 2 tuples in ttest ^^^^^^^^ remember what we said about visibility. INSERT 167794 1 vac=> select * from ttest; x - 1 2 (2 rows) vac=> update ttest set x = null where x = 2; NOTICE:trigf (fired before): there are 2 tuples in ttest UPDATE 0 vac=> update ttest set x = 4 where x = 2; NOTICE:trigf (fired before): there are 2 tuples in ttest NOTICE:trigf (fired after ): there are 2 tuples in ttest UPDATE 1 vac=> select * from ttest; x - 1 4 (2 rows) vac=> delete from ttest; NOTICE:trigf (fired before): there are 2 tuples in ttest NOTICE:trigf (fired after ): there are 1 tuples in ttest NOTICE:trigf (fired before): there are 1 tuples in ttest NOTICE:trigf (fired after ): there are 0 tuples in ttest ^^^^^^^^ remember what we said about visibility. DELETE 2 vac=> select * from ttest; x - (0 rows)