c - 如何访问Postgresql c语言函数中的大对象？

Question

我正在写一个 c-language postgresql 9.3.5 的用户定义函数。我没有使用 pl/pgsql 或 pl/python，因为其中一部分需要用 C 编写以提高速度。

该函数获取一个现有的 OID 作为参数。如何从 C 语言函数中访问和修改 OID 数据？我找不到任何文档!

到目前为止，函数内部只有读取参数的部分:

#include "postgres.h"
#include "fmgr.h"
PG_MODULE_MAGIC;

Datum tsdb_write_lob(PG_FUNCTION_ARGS) ;

PG_FUNCTION_INFO_V1(tsdb_write_lob);



Datum tsdb_write_lob(PG_FUNCTION_ARGS) {
    int16 arg_dataformat = PG_GETARG_INT16(0);
    int16 arg_granularity_days = PG_GETARG_INT16(1);
    Oid arg_dataoid = PG_GETARG_OID(2);

   ereport( INFO, (errcode( ERRCODE_SUCCESSFUL_COMPLETION ),
                errmsg("Arguments were: %d, %d, %ld\n",arg_dataformat,arg_granularity_days,arg_dataoid)));


   /*TODO:
    * open oid (lo_open ?)
    * read data
    * decompress data
    * parse data
    * add new data
    * compress data
    * save to same oid
    */



    PG_RETURN_VOID();
}

更新:这似乎是一个例子:

https://github.com/postgres/postgres/blob/master/src/backend/libpq/be-fsstubs.c

Answer 1

这是一个想法

#include <postgres.h>
#include <utils/rel.h>
#include <utils/array.h>
#include <commands/trigger.h>
#include <executor/spi.h>

PG_MODULE_MAGIC;

Datum tsdb_write_lob(PG_FUNCTION_ARGS) ;

extern SPITupleTable *SPI_tuptable;

PG_FUNCTION_INFO_V1(tsdb_write_lob);
Datum tsdb_write_lob(PG_FUNCTION_ARGS) {
{
    int16 arg_dataformat       = PG_GETARG_INT16(0);
    int16 arg_granularity_days = PG_GETARG_INT16(1);
    Oid   arg_dataoid          = PG_GETARG_OID(2);
    char  command[128];

    if (SPI_ERROR_CONNECT == SPI_connect())
        PG_RETURN_VOID();
    snprintf(command, sizeof(command), "SELECT lo_open(INV_READ, %d)", arg_dataoid);
    if (SPI_OK_SELECT != SPI_execute(command, false, 0))
        PG_RETURN_VOID();
    .
    .
    .
    /* do loread here the same way by using SPI_execute("SELECT loread ...") */
    SPI_finish();

    PG_RETURN_VOID();
}

您需要阅读有关 SPI_tubtable 的内容找出如何解析查询的指针，以及整个 SPI Server Programming Interface .