# 36.4\. 一个完整的触发器例子 这里是一个用C写的非常简单的触发器例子。（用程序语言写的触发器例子可以在程序语言文档中找到）。 函数`trigf`报告`ttest`表的行数量， 并且如果命令试图把NULL插入到字段`x`里 (也就是它做为一个非空约束但不退出事务)时略过操作。 首先，表定义： ``` CREATE TABLE ttest ( x integer ); ``` 这里是触发器函数的源代码： ``` #include "postgres.h" #include "executor/spi.h" /* 你用 SPI 的时候要用的头文件 */ #include "commands/trigger.h" /* ... 触发器 ... */ #include "utils/rel.h" /* ... 和关系 */ #ifdef PG_MODULE_MAGIC PG_MODULE_MAGIC; #endif 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; /* 确信自己是作为触发器调用的 */ if (!CALLED_AS_TRIGGER(fcinfo)) elog(ERROR, "trigf: not called by trigger manager"); /* 返回给执行者的行 */ if (TRIGGER_FIRED_BY_UPDATE(trigdata->tg_event)) rettuple = trigdata->tg_newtuple; else rettuple = trigdata->tg_trigtuple; /* 检查 NULL 值 */ 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; /* 与 SPI 管理器连接 */ if ((ret = SPI_connect()) < 0) elog(ERROR, "trigf (fired %s): SPI_connect returned %d", when, ret); /* 获取表中的行数量 */ ret = SPI_exec("SELECT count(*) FROM ttest", 0); if (ret < 0) elog(ERROR, "trigf (fired %s): SPI_exec returned %d", when, ret); /* count(*) 返回 int8 ，所以要小心转换 */ 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); } ``` 编译完源代码后(参见[Section 35.9.6](#calibre_link-926))，声明函数并创建触发器： ``` CREATE FUNCTION trigf() RETURNS trigger AS '_filename_' LANGUAGE C; CREATE TRIGGER tbefore BEFORE INSERT OR UPDATE OR DELETE ON ttest FOR EACH ROW EXECUTE PROCEDURE trigf(); CREATE TRIGGER tafter AFTER INSERT OR UPDATE OR DELETE ON ttest FOR EACH ROW EXECUTE PROCEDURE trigf(); ``` 现在你可以测试触发器的操作： ``` => INSERT INTO ttest VALUES (NULL); INFO: trigf (fired before): there are 0 rows in ttest INSERT 0 0 -- 插入被忽略，AFTER 触发器没有触发 => 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 ^^^^^^^^ 还记得讲过的关于可视性的原则吗？ 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 ^^^^^^ 还记得讲过的关于可视性的原则吗？ 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 ^^^^^^ 还记得讲过的关于可视性的原则吗？ DELETE 2 => SELECT * FROM ttest; x --- (0 rows) ``` 在`src/test/regress/regress.c`和 [spi](#calibre_link-399)里还有更复杂的例子。