How to access large objects in Postgresql c-language functions?

Question

I'm writing a c-language user defined function for postgresql 9.3.5. I didn't use pl/pgsql or pl/python because part of it needs to be written in C for speed.

The function gets an existing OID as an argument. How can I access & modify the OID data from within the c-language function? I can't find any docs for that!

What I have so far inside the function is just the part that reads the arguments:

#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();
}

UPDATE: This seems to be an example:

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

Answer 1

This is an Idea

#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();
}

You need to read about SPI_tubtable pointer to find out how to parse the queries, as well as the whole SPI Server Programming Interface .

Answer 2

I found a solution, thanks to the pgsql mailing list:

int inv_fd;
Oid arg_dataoid = PG_GETARG_OID(0);
...

//open        
inv_fd = DatumGetInt32(DirectFunctionCall2(lo_open, arg_dataoid, Int32GetDatum(INV_READ|INV_WRITE)));


if (inv_fd < 0) {
    elog(ERROR, "lo_open: lookup failed for oid:%ld", DatumGetInt64(arg_dataoid));
    PG_RETURN_INT32(-1);
}

//write
if ( lo_write(inv_fd, cstr, nbytes ) != nbytes)
        elog(ERROR, "error while writing large object");

//close LO
DirectFunctionCall1(lo_close, Int32GetDatum(inv_fd));

Lots of examples for other lo* functions can be found here and here