itup.h

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/*-------------------------------------------------------------------------
 *
 * itup.h
 *    POSTGRES index tuple definitions.
 *
 *
 * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/include/access/itup.h
 *
 *-------------------------------------------------------------------------
 */
#ifndef ITUP_H
#define ITUP_H

#include "access/tupdesc.h"
#include "access/tupmacs.h"
#include "storage/bufpage.h"
#include "storage/itemptr.h"

/*
 * Index tuple header structure
 *
 * All index tuples start with IndexTupleData.  If the HasNulls bit is set,
 * this is followed by an IndexAttributeBitMapData.  The index attribute
 * values follow, beginning at a MAXALIGN boundary.
 *
 * Note that the space allocated for the bitmap does not vary with the number
 * of attributes; that is because we don't have room to store the number of
 * attributes in the header.  Given the MAXALIGN constraint there's no space
 * savings to be had anyway, for usual values of INDEX_MAX_KEYS.
 */

typedef struct IndexTupleData
{
    ItemPointerData t_tid;      /* reference TID to heap tuple */

    /* ---------------
     * t_info is laid out in the following fashion:
     *
     * 15th (high) bit: has nulls
     * 14th bit: has var-width attributes
     * 13th bit: AM-defined meaning
     * 12-0 bit: size of tuple
     * ---------------
     */

    unsigned short t_info;      /* various info about tuple */

} IndexTupleData;               /* MORE DATA FOLLOWS AT END OF STRUCT */

typedef IndexTupleData *IndexTuple;

typedef struct IndexAttributeBitMapData
{
    bits8       bits[(INDEX_MAX_KEYS + 8 - 1) / 8];
}           IndexAttributeBitMapData;

typedef IndexAttributeBitMapData * IndexAttributeBitMap;

/*
 * t_info manipulation macros
 */
#define INDEX_SIZE_MASK 0x1FFF
#define INDEX_AM_RESERVED_BIT 0x2000    /* reserved for index-AM specific
                                         * usage */
#define INDEX_VAR_MASK  0x4000
#define INDEX_NULL_MASK 0x8000

#define IndexTupleSize(itup)        ((Size) ((itup)->t_info & INDEX_SIZE_MASK))
#define IndexTupleHasNulls(itup)    ((((IndexTuple) (itup))->t_info & INDEX_NULL_MASK))
#define IndexTupleHasVarwidths(itup) ((((IndexTuple) (itup))->t_info & INDEX_VAR_MASK))


/*
 * Takes an infomask as argument (primarily because this needs to be usable
 * at index_form_tuple time so enough space is allocated).
 */
#define IndexInfoFindDataOffset(t_info) \
( \
    (!((t_info) & INDEX_NULL_MASK)) ? \
    ( \
        (Size)MAXALIGN(sizeof(IndexTupleData)) \
    ) \
    : \
    ( \
        (Size)MAXALIGN(sizeof(IndexTupleData) + sizeof(IndexAttributeBitMapData)) \
    ) \
)

/* ----------------
 *      index_getattr
 *
 *      This gets called many times, so we macro the cacheable and NULL
 *      lookups, and call nocache_index_getattr() for the rest.
 *
 * ----------------
 */
#define index_getattr(tup, attnum, tupleDesc, isnull) \
( \
    AssertMacro(PointerIsValid(isnull) && (attnum) > 0), \
    *(isnull) = false, \
    !IndexTupleHasNulls(tup) ? \
    ( \
        TupleDescAttr((tupleDesc), (attnum)-1)->attcacheoff >= 0 ? \
        ( \
            fetchatt(TupleDescAttr((tupleDesc), (attnum)-1), \
            (char *) (tup) + IndexInfoFindDataOffset((tup)->t_info) \
            + TupleDescAttr((tupleDesc), (attnum)-1)->attcacheoff) \
        ) \
        : \
            nocache_index_getattr((tup), (attnum), (tupleDesc)) \
    ) \
    : \
    ( \
        (att_isnull((attnum)-1, (char *)(tup) + sizeof(IndexTupleData))) ? \
        ( \
            *(isnull) = true, \
            (Datum)NULL \
        ) \
        : \
        ( \
            nocache_index_getattr((tup), (attnum), (tupleDesc)) \
        ) \
    ) \
)

/*
 * MaxIndexTuplesPerPage is an upper bound on the number of tuples that can
 * fit on one index page.  An index tuple must have either data or a null
 * bitmap, so we can safely assume it's at least 1 byte bigger than a bare
 * IndexTupleData struct.  We arrive at the divisor because each tuple
 * must be maxaligned, and it must have an associated line pointer.
 *
 * To be index-type-independent, this does not account for any special space
 * on the page, and is thus conservative.
 *
 * Note: in btree non-leaf pages, the first tuple has no key (it's implicitly
 * minus infinity), thus breaking the "at least 1 byte bigger" assumption.
 * On such a page, N tuples could take one MAXALIGN quantum less space than
 * estimated here, seemingly allowing one more tuple than estimated here.
 * But such a page always has at least MAXALIGN special space, so we're safe.
 */
#define MaxIndexTuplesPerPage   \
    ((int) ((BLCKSZ - SizeOfPageHeaderData) / \
            (MAXALIGN(sizeof(IndexTupleData) + 1) + sizeof(ItemIdData))))


/* routines in indextuple.c */
extern IndexTuple index_form_tuple(TupleDesc tupleDescriptor,
                                   Datum *values, bool *isnull);
extern Datum nocache_index_getattr(IndexTuple tup, int attnum,
                                   TupleDesc tupleDesc);
extern void index_deform_tuple(IndexTuple tup, TupleDesc tupleDescriptor,
                               Datum *values, bool *isnull);
extern IndexTuple CopyIndexTuple(IndexTuple source);
extern IndexTuple index_truncate_tuple(TupleDesc sourceDescriptor,
                                       IndexTuple source, int leavenatts);

#endif                          /* ITUP_H */