PostgreSQL: Fetch a sequence nextval from a C function

Question

I wrote the following to fetch the next value in a sequence. It works perfectly:

static int64 _get_md_key_next_serial()
{
    int                  ret = SPI_execute("SELECT nextval('md_key_seq')", true, 1);

    if (ret <= 0)
        return (int64)ret;

    if (SPI_processed)
    {
        SPITupleTable   *tuptable = SPI_tuptable;
        bool             fieldNull;
        Datum            datum = SPI_getbinval(tuptable->vals[0], tuptable->tupdesc, 1, &fieldNull);

        if (!fieldNull)
            return DatumGetInt64(datum);
    }

    return NULL_ZERO;
}

However, surely there is a function call I can make without having to go through SPI?

Laurenz Albe · Accepted Answer · 2016-10-05 14:14:10Z

2

Unfortunately nextval_internal is not exported, but you could try to call nextval_oid. I did not test this code, so you'll probably have to debug it:

#include "fmgr.h"
#include "commands/sequence.h"

static int64 nextval(Oid sequenceID, FunctionCallInfo fcinfo)
{
    FunctionCallInfoData locfcinfo;

    InitFunctionCallInfoData(locfcinfo, fcinfo->flinfo, 1,
                             InvalidOid, NULL, NULL);
    locfcinfo.arg[0] = ObjectIdGetDatum(sequenceID);
    locfcinfo.argnull[0] = false;

    return DatumGetInt64(nextval_oid(&locfcinfo));
}

Pass the Oid of the sequence and the fcinfo from your own SQL function.

answered Oct 5, 2016 at 14:14

Laurenz Albe

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