Postgresql 中文操作指南

36.12. Large Objects #

ECPG 直接不支持大型对象，但是 ECPG 应用程序可以通过 libpq 大型对象函数来操作大型对象，通过调用 ECPGget_PGconn() 函数来获取必要的 PGconn 对象。（然而，使用 ECPGget_PGconn() 函数和直接接触 PGconn 对象的操作应当非常小心，最好不要和其他 ECPG 数据库访问调用混合使用。）

Large objects are not directly supported by ECPG, but ECPG application can manipulate large objects through the libpq large object functions, obtaining the necessary PGconn object by calling the ECPGget_PGconn() function. (However, use of the ECPGget_PGconn() function and touching PGconn objects directly should be done very carefully and ideally not mixed with other ECPG database access calls.)

有关 ECPGget_PGconn() 的更多详细信息，请参阅 Section 36.11。有关大型对象函数接口的信息，请参阅 Chapter 35。

For more details about the ECPGget_PGconn(), see Section 36.11. For information about the large object function interface, see Chapter 35.

大型对象函数必须在一个事务块中调用，因此当自动提交处于关闭状态时，必须明确发出 BEGIN 命令。

Large object functions have to be called in a transaction block, so when autocommit is off, BEGIN commands have to be issued explicitly.

Example 36.2 显示了一个示例程序，它展示了如何在 ECPG 应用程序中创建、写入和读取一个大型对象。

Example 36.2 shows an example program that illustrates how to create, write, and read a large object in an ECPG application.

Example 36.2. ECPG Program Accessing Large Objects

#include <stdio.h>
#include <stdlib.h>
#include <libpq-fe.h>
#include <libpq/libpq-fs.h>

EXEC SQL WHENEVER SQLERROR STOP;

int
main(void)
{
    PGconn     *conn;
    Oid         loid;
    int         fd;
    char        buf[256];
    int         buflen = 256;
    char        buf2[256];
    int         rc;

    memset(buf, 1, buflen);

    EXEC SQL CONNECT TO testdb AS con1;
    EXEC SQL SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT;

    conn = ECPGget_PGconn("con1");
    printf("conn = %p\n", conn);

    /* create */
    loid = lo_create(conn, 0);
    if (loid &lt; 0)
        printf("lo_create() failed: %s", PQerrorMessage(conn));

    printf("loid = %d\n", loid);

    /* write test */
    fd = lo_open(conn, loid, INV_READ|INV_WRITE);
    if (fd &lt; 0)
        printf("lo_open() failed: %s", PQerrorMessage(conn));

    printf("fd = %d\n", fd);

    rc = lo_write(conn, fd, buf, buflen);
    if (rc &lt; 0)
        printf("lo_write() failed\n");

    rc = lo_close(conn, fd);
    if (rc &lt; 0)
        printf("lo_close() failed: %s", PQerrorMessage(conn));

    /* read test */
    fd = lo_open(conn, loid, INV_READ);
    if (fd &lt; 0)
        printf("lo_open() failed: %s", PQerrorMessage(conn));

    printf("fd = %d\n", fd);

    rc = lo_read(conn, fd, buf2, buflen);
    if (rc &lt; 0)
        printf("lo_read() failed\n");

    rc = lo_close(conn, fd);
    if (rc &lt; 0)
        printf("lo_close() failed: %s", PQerrorMessage(conn));

    /* check */
    rc = memcmp(buf, buf2, buflen);
    printf("memcmp() = %d\n", rc);

    /* cleanup */
    rc = lo_unlink(conn, loid);
    if (rc &lt; 0)
        printf("lo_unlink() failed: %s", PQerrorMessage(conn));

    EXEC SQL COMMIT;
    EXEC SQL DISCONNECT ALL;
    return 0;
}