itemptr.c

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/*-------------------------------------------------------------------------
 *
 * itemptr.c
 *    POSTGRES disk item pointer code.
 *
 * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/storage/page/itemptr.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "storage/itemptr.h"


/*
 * ItemPointerEquals
 *  Returns true if both item pointers point to the same item,
 *   otherwise returns false.
 *
 * Note:
 *  Asserts that the disk item pointers are both valid!
 */
bool
ItemPointerEquals(ItemPointer pointer1, ItemPointer pointer2)
{
    /*
     * We really want ItemPointerData to be exactly 6 bytes.  This is rather a
     * random place to check, but there is no better place.
     */
    StaticAssertStmt(sizeof(ItemPointerData) == 3 * sizeof(uint16),
                     "ItemPointerData struct is improperly padded");

    if (ItemPointerGetBlockNumber(pointer1) ==
        ItemPointerGetBlockNumber(pointer2) &&
        ItemPointerGetOffsetNumber(pointer1) ==
        ItemPointerGetOffsetNumber(pointer2))
        return true;
    else
        return false;
}

/*
 * ItemPointerCompare
 *      Generic btree-style comparison for item pointers.
 */
int32
ItemPointerCompare(ItemPointer arg1, ItemPointer arg2)
{
    /*
     * Use ItemPointerGet{Offset,Block}NumberNoCheck to avoid asserting
     * ip_posid != 0, which may not be true for a user-supplied TID.
     */
    BlockNumber b1 = ItemPointerGetBlockNumberNoCheck(arg1);
    BlockNumber b2 = ItemPointerGetBlockNumberNoCheck(arg2);

    if (b1 < b2)
        return -1;
    else if (b1 > b2)
        return 1;
    else if (ItemPointerGetOffsetNumberNoCheck(arg1) <
             ItemPointerGetOffsetNumberNoCheck(arg2))
        return -1;
    else if (ItemPointerGetOffsetNumberNoCheck(arg1) >
             ItemPointerGetOffsetNumberNoCheck(arg2))
        return 1;
    else
        return 0;
}

/*
 * ItemPointerInc
 *      Increment 'pointer' by 1 only paying attention to the ItemPointer's
 *      type's range limits and not MaxOffsetNumber and FirstOffsetNumber.
 *      This may result in 'pointer' becoming !OffsetNumberIsValid.
 *
 * If the pointer is already the maximum possible values permitted by the
 * range of the ItemPointer's types, then do nothing.
 */
void
ItemPointerInc(ItemPointer pointer)
{
    BlockNumber blk = ItemPointerGetBlockNumberNoCheck(pointer);
    OffsetNumber off = ItemPointerGetOffsetNumberNoCheck(pointer);

    if (off == PG_UINT16_MAX)
    {
        if (blk != InvalidBlockNumber)
        {
            off = 0;
            blk++;
        }
    }
    else
        off++;

    ItemPointerSet(pointer, blk, off);
}

/*
 * ItemPointerDec
 *      Decrement 'pointer' by 1 only paying attention to the ItemPointer's
 *      type's range limits and not MaxOffsetNumber and FirstOffsetNumber.
 *      This may result in 'pointer' becoming !OffsetNumberIsValid.
 *
 * If the pointer is already the minimum possible values permitted by the
 * range of the ItemPointer's types, then do nothing.  This does rely on
 * FirstOffsetNumber being 1 rather than 0.
 */
void
ItemPointerDec(ItemPointer pointer)
{
    BlockNumber blk = ItemPointerGetBlockNumberNoCheck(pointer);
    OffsetNumber off = ItemPointerGetOffsetNumberNoCheck(pointer);

    if (off == 0)
    {
        if (blk != 0)
        {
            off = PG_UINT16_MAX;
            blk--;
        }
    }
    else
        off--;

    ItemPointerSet(pointer, blk, off);
}