17.5. 例子

这个 SPI 使用的样例演示了可视性规则． 在 src/test/regress/regress.c 和 contrib/spi 里有更复杂的例子．

这是一个非常简单的 SPI 使用的例子． 过程 execq 在其第一个参数里接收一个 SQL 查询， 第二个参数接收一个tcount（译注：元组个数）， 用 SPI_exec 执行这个查询并且返回查询执行过的记录个数：

#include "executor/spi.h"   /* this is what you need to work with SPI */

int execq(TEXT *sql, int cnt);

int
execq(TEXT *sql, int cnt)
{
    char *query;
    int ret;
    int proc;

    /* Convert given TEXT object to a C string */
    query = DatumGetCString(DirectFunctionCall1(TEXTout,
                                                PointerGetDatum(sql)));

    SPI_connect();
    
    ret = SPI_exec(query, cnt);
    
    proc = SPI_processed;
    /*
     * 如果这是 SELECT 并且取出了一些元组 - 
     * 通过 elog (INFO) 向调用者 RETURNS 元组
     */
    if ( ret == SPI_OK_SELECT && SPI_processed > 0 )
    {
        TupleDesc tupdesc = SPI_tuptable->tupdesc;
        SPITupleTable *tuptable = SPI_tuptable;
        char buf[8192];
        int i,j;
        
        for (j = 0; j < proc; j++)
        {
            HeapTuple tuple = tuptable->vals[j];
            
            for (i = 1, buf[0] = 0; i <= tupdesc->natts; i++)
                spnrintf(buf + strlen (buf), sizeof(buf) - strlen(buf), " %s%s",
                        SPI_getvalue(tuple, tupdesc, i),
                        (i == tupdesc->natts) ? " " : " |");
            elog (INFO, "EXECQ: %s", buf);
        }
    }

    SPI_finish();

    pfree(query);

    return (proc);
}

然后，编译并创建函数：

CREATE FUNCTION execq (TEXT, INTEGER) RETURNS INTEGER
	AS '...path_to_so'
	LANGUAGE C;

vac=> SELECT execq('CREATE TABLE a (x INTEGER)', 0);
execq
-----
    0
(1 row)

vac=> INSERT INTO a VALUES (execq('INSERT INTO a VALUES (0)',0));
INSERT 167631 1
vac=> SELECT execq('SELECT * FROM a',0);
INFO:EXECQ:  0 <<< inserted by execq

INFO:EXECQ:  1 <<< value returned by execq and inserted by upper INSERT

execq
-----
    2
(1 row)

vac=> SELECT execq('INSERT INTO a SELECT x + 2 FROM a',1);
execq
-----
    1
(1 row)

vac=> SELECT execq('SELECT * FROM a', 10);
INFO:EXECQ:  0 

INFO:EXECQ:  1 

INFO:EXECQ:  2 <<< 0 + 2, only one tuple inserted - as specified

execq
-----
    3            <<< 10 是最大值, 3 是元组实际数量
(1 row)

vac=> delete FROM a;
DELETE 3
vac=> INSERT INTO a VALUES (execq('SELECT * FROM a', 0) + 1);
INSERT 167712 1
vac=> SELECT * FROM a;
x
-
1                <<< no tuples in a (0) + 1
(1 row)

vac=> INSERT INTO a VALUES (execq('SELECT * FROM a', 0) + 1);
INFO:EXECQ:  0 
INSERT 167713 1
vac=> SELECT * FROM a;
x
-
1
2                <<< there was single tuple in a + 1
(2 rows)

--   This demonstrates data changes visibility rule:

vac=> INSERT INTO a SELECT execq('SELECT * FROM a', 0) * x FROM a;
INFO:EXECQ:  1 
INFO:EXECQ:  2 
INFO:EXECQ:  1 
INFO:EXECQ:  2 
INFO:EXECQ:  2 
INSERT 0 2
vac=> SELECT * FROM a;
x
-
1
2
2                <<< 2 tuples * 1 (x in first tuple)
6                <<< 3 tuples (2 + 1 just inserted) * 2 (x in second tuple)
(4 rows)             ^^^^^^^^ 
                     tuples visible to execq() in different invocations