# 36.4\. 一個完整的觸發器例子 這里是一個用C寫的非常簡單的觸發器例子。（用程序語言寫的觸發器例子可以在程序語言文檔中找到）。 函數`trigf`報告`ttest`表的行數量， 并且如果命令試圖把NULL插入到字段`x`里 (也就是它做為一個非空約束但不退出事務)時略過操作。 首先，表定義： ``` CREATE TABLE ttest ( x integer ); ``` 這里是觸發器函數的源代碼： ``` #include "postgres.h" #include "executor/spi.h" /* 你用 SPI 的時候要用的頭文件 */ #include "commands/trigger.h" /* ... 觸發器 ... */ #include "utils/rel.h" /* ... 和關系 */ #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; /* 確信自己是作為觸發器調用的 */ if (!CALLED_AS_TRIGGER(fcinfo)) elog(ERROR, "trigf: not called by trigger manager"); /* 返回給執行者的行 */ if (TRIGGER_FIRED_BY_UPDATE(trigdata->tg_event)) rettuple = trigdata->tg_newtuple; else rettuple = trigdata->tg_trigtuple; /* 檢查 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; /* 與 SPI 管理器連接 */ if ((ret = SPI_connect()) < 0) elog(ERROR, "trigf (fired %s): SPI_connect returned %d", when, ret); /* 獲取表中的行數量 */ 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 ，所以要小心轉換 */ 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); } ``` 編譯完源代碼后(參見[Section 35.9.6](#calibre_link-926))，聲明函數并創建觸發器： ``` 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(); ``` 現在你可以測試觸發器的操作： ``` => INSERT INTO ttest VALUES (NULL); INFO: trigf (fired before): there are 0 rows in ttest INSERT 0 0 -- 插入被忽略，AFTER 觸發器沒有觸發 => 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 ^^^^^^^^ 還記得講過的關于可視性的原則嗎？ 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 ^^^^^^ 還記得講過的關于可視性的原則嗎？ 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 ^^^^^^ 還記得講過的關于可視性的原則嗎？ DELETE 2 => SELECT * FROM ttest; x --- (0 rows) ``` 在`src/test/regress/regress.c`和 [spi](#calibre_link-399)里還有更復雜的例子。