Quelle  skipsupport.c

/*-------------------------------------------------------------------------
 *
 * skipsupport.c
 *   Support routines for B-Tree skip scan.
 *
 *
 * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *   src/backend/utils/adt/skipsupport.c
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "access/nbtree.h"
#include "utils/lsyscache.h"
#include "utils/skipsupport.h"

/*
 * Fill in SkipSupport given an operator class (opfamily + opcintype).
 *
 * On success, returns skip support struct, allocating in caller's memory
 * context.  Otherwise returns NULL, indicating that operator class has no
 * skip support function.
 */
SkipSupport
PrepareSkipSupportFromOpclass(Oid opfamily, Oid opcintype, bool reverse)
{
 Oid   skipSupportFunction;
 SkipSupport sksup;

 /* Look for a skip support function */
 skipSupportFunction = get_opfamily_proc(opfamily, opcintype, opcintype,
           BTSKIPSUPPORT_PROC);
 if (!OidIsValid(skipSupportFunction))
  return NULL;

 sksup = palloc(sizeof(SkipSupportData));
 OidFunctionCall1(skipSupportFunction, PointerGetDatum(sksup));

 if (reverse)
 {
  /*
   * DESC/reverse case: swap low_elem with high_elem, and swap decrement
   * with increment
 */
  Datum  low_elem = sksup->low_elem;
  SkipSupportIncDec decrement = sksup->decrement;

  sksup->low_elem = sksup->high_elem;
  sksup->decrement = sksup->increment;

  sksup->high_elem = low_elem;
  sksup->increment = decrement;
 }

 return sksup;
}