sortsupport.c

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/*-------------------------------------------------------------------------
 *
 * sortsupport.c
 *    Support routines for accelerated sorting.
 *
 *
 * Portions Copyright (c) 1996-2013, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/backend/utils/sort/sortsupport.c
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

/* See sortsupport.h */
#define SORTSUPPORT_INCLUDE_DEFINITIONS

#include "fmgr.h"
#include "utils/lsyscache.h"
#include "utils/sortsupport.h"


/* Info needed to use an old-style comparison function as a sort comparator */
typedef struct
{
    FunctionCallInfoData fcinfo;    /* reusable callinfo structure */
    FmgrInfo    flinfo;         /* lookup data for comparison function */
} SortShimExtra;


/*
 * Shim function for calling an old-style comparator
 *
 * This is essentially an inlined version of FunctionCall2Coll(), except
 * we assume that the FunctionCallInfoData was already mostly set up by
 * PrepareSortSupportComparisonShim.
 */
static int
comparison_shim(Datum x, Datum y, SortSupport ssup)
{
    SortShimExtra *extra = (SortShimExtra *) ssup->ssup_extra;
    Datum       result;

    extra->fcinfo.arg[0] = x;
    extra->fcinfo.arg[1] = y;

    /* just for paranoia's sake, we reset isnull each time */
    extra->fcinfo.isnull = false;

    result = FunctionCallInvoke(&extra->fcinfo);

    /* Check for null result, since caller is clearly not expecting one */
    if (extra->fcinfo.isnull)
        elog(ERROR, "function %u returned NULL", extra->flinfo.fn_oid);

    return result;
}

/*
 * Set up a shim function to allow use of an old-style btree comparison
 * function as if it were a sort support comparator.
 */
void
PrepareSortSupportComparisonShim(Oid cmpFunc, SortSupport ssup)
{
    SortShimExtra *extra;

    extra = (SortShimExtra *) MemoryContextAlloc(ssup->ssup_cxt,
                                                 sizeof(SortShimExtra));

    /* Lookup the comparison function */
    fmgr_info_cxt(cmpFunc, &extra->flinfo, ssup->ssup_cxt);

    /* We can initialize the callinfo just once and re-use it */
    InitFunctionCallInfoData(extra->fcinfo, &extra->flinfo, 2,
                             ssup->ssup_collation, NULL, NULL);
    extra->fcinfo.argnull[0] = false;
    extra->fcinfo.argnull[1] = false;

    ssup->ssup_extra = extra;
    ssup->comparator = comparison_shim;
}

/*
 * Fill in SortSupport given an ordering operator (btree "<" or ">" operator).
 *
 * Caller must previously have zeroed the SortSupportData structure and then
 * filled in ssup_cxt, ssup_collation, and ssup_nulls_first.  This will fill
 * in ssup_reverse as well as the comparator function pointer.
 */
void
PrepareSortSupportFromOrderingOp(Oid orderingOp, SortSupport ssup)
{
    Oid         sortFunction;
    bool        issupport;

    if (!get_sort_function_for_ordering_op(orderingOp,
                                           &sortFunction,
                                           &issupport,
                                           &ssup->ssup_reverse))
        elog(ERROR, "operator %u is not a valid ordering operator",
             orderingOp);

    if (issupport)
    {
        /* The sort support function should provide a comparator */
        OidFunctionCall1(sortFunction, PointerGetDatum(ssup));
        Assert(ssup->comparator != NULL);
    }
    else
    {
        /* We'll use a shim to call the old-style btree comparator */
        PrepareSortSupportComparisonShim(sortFunction, ssup);
    }
}