itemptr.h

Go to the documentation of this file.

/*-------------------------------------------------------------------------
 *
 * itemptr.h
 *    POSTGRES disk item pointer definitions.
 *
 *
 * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/include/storage/itemptr.h
 *
 *-------------------------------------------------------------------------
 */
#ifndef ITEMPTR_H
#define ITEMPTR_H

#include "storage/block.h"
#include "storage/off.h"

/*
 * ItemPointer:
 *
 * This is a pointer to an item within a disk page of a known file
 * (for example, a cross-link from an index to its parent table).
 * ip_blkid tells us which block, ip_posid tells us which entry in
 * the linp (ItemIdData) array we want.
 *
 * Note: because there is an item pointer in each tuple header and index
 * tuple header on disk, it's very important not to waste space with
 * structure padding bytes.  The struct is designed to be six bytes long
 * (it contains three int16 fields) but a few compilers will pad it to
 * eight bytes unless coerced.  We apply appropriate persuasion where
 * possible.  If your compiler can't be made to play along, you'll waste
 * lots of space.
 */
typedef struct ItemPointerData
{
    BlockIdData ip_blkid;
    OffsetNumber ip_posid;
}

/* If compiler understands packed and aligned pragmas, use those */
#if defined(pg_attribute_packed) && defined(pg_attribute_aligned)
            pg_attribute_packed()
            pg_attribute_aligned(2)
#endif
ItemPointerData;

typedef ItemPointerData *ItemPointer;

/* ----------------
 *      special values used in heap tuples (t_ctid)
 * ----------------
 */

/*
 * If a heap tuple holds a speculative insertion token rather than a real
 * TID, ip_posid is set to SpecTokenOffsetNumber, and the token is stored in
 * ip_blkid. SpecTokenOffsetNumber must be higher than MaxOffsetNumber, so
 * that it can be distinguished from a valid offset number in a regular item
 * pointer.
 */
#define SpecTokenOffsetNumber       0xfffe

/*
 * When a tuple is moved to a different partition by UPDATE, the t_ctid of
 * the old tuple version is set to this magic value.
 */
#define MovedPartitionsOffsetNumber 0xfffd
#define MovedPartitionsBlockNumber  InvalidBlockNumber


/* ----------------
 *      support macros
 * ----------------
 */

/*
 * ItemPointerIsValid
 *      True iff the disk item pointer is not NULL.
 */
#define ItemPointerIsValid(pointer) \
    ((bool) (PointerIsValid(pointer) && ((pointer)->ip_posid != 0)))

/*
 * ItemPointerGetBlockNumberNoCheck
 *      Returns the block number of a disk item pointer.
 */
#define ItemPointerGetBlockNumberNoCheck(pointer) \
( \
    BlockIdGetBlockNumber(&(pointer)->ip_blkid) \
)

/*
 * ItemPointerGetBlockNumber
 *      As above, but verifies that the item pointer looks valid.
 */
#define ItemPointerGetBlockNumber(pointer) \
( \
    AssertMacro(ItemPointerIsValid(pointer)), \
    ItemPointerGetBlockNumberNoCheck(pointer) \
)

/*
 * ItemPointerGetOffsetNumberNoCheck
 *      Returns the offset number of a disk item pointer.
 */
#define ItemPointerGetOffsetNumberNoCheck(pointer) \
( \
    (pointer)->ip_posid \
)

/*
 * ItemPointerGetOffsetNumber
 *      As above, but verifies that the item pointer looks valid.
 */
#define ItemPointerGetOffsetNumber(pointer) \
( \
    AssertMacro(ItemPointerIsValid(pointer)), \
    ItemPointerGetOffsetNumberNoCheck(pointer) \
)

/*
 * ItemPointerSet
 *      Sets a disk item pointer to the specified block and offset.
 */
#define ItemPointerSet(pointer, blockNumber, offNum) \
( \
    AssertMacro(PointerIsValid(pointer)), \
    BlockIdSet(&((pointer)->ip_blkid), blockNumber), \
    (pointer)->ip_posid = offNum \
)

/*
 * ItemPointerSetBlockNumber
 *      Sets a disk item pointer to the specified block.
 */
#define ItemPointerSetBlockNumber(pointer, blockNumber) \
( \
    AssertMacro(PointerIsValid(pointer)), \
    BlockIdSet(&((pointer)->ip_blkid), blockNumber) \
)

/*
 * ItemPointerSetOffsetNumber
 *      Sets a disk item pointer to the specified offset.
 */
#define ItemPointerSetOffsetNumber(pointer, offsetNumber) \
( \
    AssertMacro(PointerIsValid(pointer)), \
    (pointer)->ip_posid = (offsetNumber) \
)

/*
 * ItemPointerCopy
 *      Copies the contents of one disk item pointer to another.
 *
 * Should there ever be padding in an ItemPointer this would need to be handled
 * differently as it's used as hash key.
 */
#define ItemPointerCopy(fromPointer, toPointer) \
( \
    AssertMacro(PointerIsValid(toPointer)), \
    AssertMacro(PointerIsValid(fromPointer)), \
    *(toPointer) = *(fromPointer) \
)

/*
 * ItemPointerSetInvalid
 *      Sets a disk item pointer to be invalid.
 */
#define ItemPointerSetInvalid(pointer) \
( \
    AssertMacro(PointerIsValid(pointer)), \
    BlockIdSet(&((pointer)->ip_blkid), InvalidBlockNumber), \
    (pointer)->ip_posid = InvalidOffsetNumber \
)

/*
 * ItemPointerIndicatesMovedPartitions
 *      True iff the block number indicates the tuple has moved to another
 *      partition.
 */
#define ItemPointerIndicatesMovedPartitions(pointer) \
( \
    ItemPointerGetOffsetNumber(pointer) == MovedPartitionsOffsetNumber && \
    ItemPointerGetBlockNumberNoCheck(pointer) == MovedPartitionsBlockNumber \
)

/*
 * ItemPointerSetMovedPartitions
 *      Indicate that the item referenced by the itempointer has moved into a
 *      different partition.
 */
#define ItemPointerSetMovedPartitions(pointer) \
    ItemPointerSet((pointer), MovedPartitionsBlockNumber, MovedPartitionsOffsetNumber)

/* ----------------
 *      externs
 * ----------------
 */

extern bool ItemPointerEquals(ItemPointer pointer1, ItemPointer pointer2);
extern int32 ItemPointerCompare(ItemPointer arg1, ItemPointer arg2);

#endif                          /* ITEMPTR_H */