itup.h File Reference

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

Include dependency graph for itup.h:

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Go to the source code of this file.

Data Structures
struct	IndexTupleData
struct	IndexAttributeBitMapData

Macros
#define	INDEX_SIZE_MASK   0x1FFF
#define	INDEX_AM_RESERVED_BIT
#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))
#define	IndexInfoFindDataOffset(t_info)
#define	index_getattr(tup, attnum, tupleDesc, isnull)
#define	MaxIndexTuplesPerPage

Typedefs
typedef struct IndexTupleData	IndexTupleData
typedef IndexTupleData *	IndexTuple
typedef struct IndexAttributeBitMapData	IndexAttributeBitMapData
typedef IndexAttributeBitMapData *	IndexAttributeBitMap

Functions
IndexTuple	index_form_tuple (TupleDesc tupleDescriptor, Datum *values, bool *isnull)
Datum	nocache_index_getattr (IndexTuple tup, int attnum, TupleDesc tupleDesc)
void	index_deform_tuple (IndexTuple tup, TupleDesc tupleDescriptor, Datum *values, bool *isnull)
void	index_deform_tuple_internal (TupleDesc tupleDescriptor, Datum *values, bool *isnull, char *tp, bits8 *bp, int hasnulls)
IndexTuple	CopyIndexTuple (IndexTuple source)
IndexTuple	index_truncate_tuple (TupleDesc sourceDescriptor, IndexTuple source, int leavenatts)

Macro Definition Documentation

INDEX_AM_RESERVED_BIT

#define INDEX_AM_RESERVED_BIT

Value:

0x2000  /* reserved for index-AM specific
                                         * usage */

Definition at line 66 of file itup.h.

index_getattr

#define index_getattr	(		tup,
			attnum,
			tupleDesc,
			isnull
	)

Value:

( \
    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)) \
        ) \
    ) \
)

Definition at line 100 of file itup.h.

Referenced by _bt_check_rowcompare(), _bt_checkkeys(), _bt_compare(), _bt_keep_natts(), _bt_keep_natts_fast(), _bt_load(), _bt_mkscankey(), _hash_checkqual(), bt_normalize_tuple(), comparetup_index_btree(), gintuple_get_attrnum(), gintuple_get_key(), gistchoose(), gistDeCompressAtt(), gistFetchTuple(), gistindex_keytest(), gistMakeUnionItVec(), gistSplitByKey(), readtup_index(), and tuplesort_putindextuplevalues().

INDEX_NULL_MASK

#define INDEX_NULL_MASK   0x8000

Definition at line 69 of file itup.h.

Referenced by index_form_tuple(), and spgFormNodeTuple().

INDEX_SIZE_MASK

#define INDEX_SIZE_MASK   0x1FFF

Definition at line 65 of file itup.h.

Referenced by _bt_dedup_pass(), _bt_form_posting(), _bt_load(), _bt_setuppostingitems(), _bt_truncate(), _bt_update_posting(), bt_page_print_tuples(), BTreeTupleSetPosting(), GinFormInteriorTuple(), GinFormTuple(), index_form_tuple(), and spgFormNodeTuple().

INDEX_VAR_MASK

#define INDEX_VAR_MASK   0x4000

Definition at line 68 of file itup.h.

Referenced by index_form_tuple().

IndexInfoFindDataOffset

#define IndexInfoFindDataOffset

(

t_info

)

Value:

( \
    (!((t_info) & INDEX_NULL_MASK)) ? \
    ( \
        (Size)MAXALIGN(sizeof(IndexTupleData)) \
    ) \
    : \
    ( \
        (Size)MAXALIGN(sizeof(IndexTupleData) + sizeof(IndexAttributeBitMapData)) \
    ) \
)

Definition at line 80 of file itup.h.

Referenced by _hash_get_indextuple_hashkey(), bt_page_print_tuples(), index_deform_tuple(), index_form_tuple(), and nocache_index_getattr().

IndexTupleHasNulls

#define IndexTupleHasNulls

(

itup

)

((((IndexTuple) (itup))->t_info & INDEX_NULL_MASK))

Definition at line 72 of file itup.h.

Referenced by bt_page_print_tuples(), GinFormTuple(), index_deform_tuple(), nocache_index_getattr(), and spgExtractNodeLabels().

IndexTupleHasVarwidths

#define IndexTupleHasVarwidths

(

itup

)

((((IndexTuple) (itup))->t_info & INDEX_VAR_MASK))

Definition at line 73 of file itup.h.

Referenced by bt_normalize_tuple(), bt_page_print_tuples(), and nocache_index_getattr().

IndexTupleSize

#define IndexTupleSize

(

itup

)

((Size) ((itup)->t_info & INDEX_SIZE_MASK))

Definition at line 71 of file itup.h.

Referenced by _bt_buildadd(), _bt_check_third_page(), _bt_dedup_finish_pending(), _bt_dedup_save_htid(), _bt_dedup_start_pending(), _bt_delitems_delete(), _bt_delitems_vacuum(), _bt_doinsert(), _bt_form_posting(), _bt_insert_parent(), _bt_insertonpg(), _bt_mark_page_halfdead(), _bt_recsplitloc(), _bt_restore_page(), _bt_saveitem(), _bt_sort_dedup_finish_pending(), _bt_split(), _bt_truncate(), _hash_doinsert(), _hash_pgaddmultitup(), _hash_splitbucket(), _hash_squeezebucket(), bt_check_level_from_leftmost(), bt_page_print_tuples(), bt_pivot_tuple_identical(), bt_rootdescend(), bt_target_page_check(), bt_tuple_present_callback(), btree_xlog_insert(), btree_xlog_split(), btree_xlog_updates(), BTreeTupleGetHeapTID(), CopyIndexTuple(), decodePageSplitRecord(), entryExecPlaceToPage(), entryIsEnoughSpace(), entrySplitPage(), GetBTPageStatistics(), ginEntryFillRoot(), GinFormInteriorTuple(), GinFormTuple(), ginHeapTupleFastCollect(), ginHeapTupleFastInsert(), ginRedoInsertEntry(), ginRedoInsertListPage(), ginRedoUpdateMetapage(), ginVacuumEntryPage(), gist_indexsortbuild_pagestate_add(), gist_page_items(), gist_page_items_bytea(), gistBuildCallback(), gistfillbuffer(), gistfillitupvec(), gistfitpage(), gistGetItupFromPage(), gistnospace(), gistPlaceItupToPage(), gistplacetopage(), gistRedoPageUpdateRecord(), gistSplit(), gistXLogUpdate(), hash_xlog_move_page_contents(), hash_xlog_squeeze_page(), index_truncate_tuple(), makeSublist(), spgFormInnerTuple(), writeListPage(), and writetup_index().

MaxIndexTuplesPerPage

#define MaxIndexTuplesPerPage

Value:

((int) ((BLCKSZ - SizeOfPageHeaderData) / \
            (MAXALIGN(sizeof(IndexTupleData) + 1) + sizeof(ItemIdData))))

Definition at line 145 of file itup.h.

Referenced by _bt_delete_or_dedup_one_page(), _bt_delitems_delete(), _bt_delitems_delete_check(), _bt_delitems_vacuum(), _bt_restore_page(), _hash_load_qualified_items(), _hash_readpage(), _hash_splitbucket(), _hash_squeezebucket(), btvacuumpage(), gistgettuple(), gistprunepage(), hashgettuple(), PageIndexMultiDelete(), palloc_btree_page(), spgPageIndexMultiDelete(), storeGettuple(), vacuumLeafPage(), vacuumLeafRoot(), and vacuumRedirectAndPlaceholder().

Typedef Documentation

IndexAttributeBitMap

typedef IndexAttributeBitMapData* IndexAttributeBitMap

Definition at line 60 of file itup.h.

IndexAttributeBitMapData

typedef struct IndexAttributeBitMapData IndexAttributeBitMapData

IndexTuple

typedef IndexTupleData* IndexTuple

Definition at line 53 of file itup.h.

IndexTupleData

typedef struct IndexTupleData IndexTupleData

Function Documentation

CopyIndexTuple()

IndexTuple CopyIndexTuple

(

IndexTuple

source

)

Definition at line 528 of file indextuple.c.

References IndexTupleSize, and palloc().

Referenced by _bt_buildadd(), _bt_insert_parent(), _bt_insertonpg(), _bt_load(), _bt_newroot(), _bt_pagedel(), _bt_swap_posting(), _hash_splitbucket(), _hash_squeezebucket(), btree_xlog_insert(), btree_xlog_split(), gistformdownlink(), and index_truncate_tuple().

{
    IndexTuple  result;
    Size        size;

    size = IndexTupleSize(source);
    result = (IndexTuple) palloc(size);
    memcpy(result, source, size);
    return result;
}

index_deform_tuple()

void index_deform_tuple	(	IndexTuple	tup,
		TupleDesc	tupleDescriptor,
		Datum *	values,
		bool *	isnull
	)

Definition at line 437 of file indextuple.c.

References index_deform_tuple_internal(), IndexInfoFindDataOffset, IndexTupleHasNulls, and IndexTupleData::t_info.

Referenced by _bt_check_unique(), comparetup_index_btree(), get_actual_variable_endpoint(), gist_page_items(), index_truncate_tuple(), and StoreIndexTuple().

{
    char       *tp;             /* ptr to tuple data */
    bits8      *bp;             /* ptr to null bitmap in tuple */

    /* XXX "knows" t_bits are just after fixed tuple header! */
    bp = (bits8 *) ((char *) tup + sizeof(IndexTupleData));

    tp = (char *) tup + IndexInfoFindDataOffset(tup->t_info);

    index_deform_tuple_internal(tupleDescriptor, values, isnull,
                                tp, bp, IndexTupleHasNulls(tup));
}

index_deform_tuple_internal()

void index_deform_tuple_internal	(	TupleDesc	tupleDescriptor,
		Datum *	values,
		bool *	isnull,
		char *	tp,
		bits8 *	bp,
		int	hasnulls
	)

Definition at line 460 of file indextuple.c.

References Assert, att_addlength_pointer, att_align_nominal, att_align_pointer, att_isnull, attnum, fetchatt, INDEX_MAX_KEYS, TupleDescData::natts, and TupleDescAttr.

Referenced by index_deform_tuple(), and spgDeformLeafTuple().

{
    int         natts = tupleDescriptor->natts; /* number of atts to extract */
    int         attnum;
    int         off = 0;        /* offset in tuple data */
    bool        slow = false;   /* can we use/set attcacheoff? */

    /* Assert to protect callers who allocate fixed-size arrays */
    Assert(natts <= INDEX_MAX_KEYS);

    for (attnum = 0; attnum < natts; attnum++)
    {
        Form_pg_attribute thisatt = TupleDescAttr(tupleDescriptor, attnum);

        if (hasnulls && att_isnull(attnum, bp))
        {
            values[attnum] = (Datum) 0;
            isnull[attnum] = true;
            slow = true;        /* can't use attcacheoff anymore */
            continue;
        }

        isnull[attnum] = false;

        if (!slow && thisatt->attcacheoff >= 0)
            off = thisatt->attcacheoff;
        else if (thisatt->attlen == -1)
        {
            /*
             * We can only cache the offset for a varlena attribute if the
             * offset is already suitably aligned, so that there would be no
             * pad bytes in any case: then the offset will be valid for either
             * an aligned or unaligned value.
             */
            if (!slow &&
                off == att_align_nominal(off, thisatt->attalign))
                thisatt->attcacheoff = off;
            else
            {
                off = att_align_pointer(off, thisatt->attalign, -1,
                                        tp + off);
                slow = true;
            }
        }
        else
        {
            /* not varlena, so safe to use att_align_nominal */
            off = att_align_nominal(off, thisatt->attalign);

            if (!slow)
                thisatt->attcacheoff = off;
        }

        values[attnum] = fetchatt(thisatt, tp + off);

        off = att_addlength_pointer(off, thisatt->attlen, tp + off);

        if (thisatt->attlen <= 0)
            slow = true;        /* can't use attcacheoff anymore */
    }
}

index_form_tuple()

IndexTuple index_form_tuple	(	TupleDesc	tupleDescriptor,
		Datum *	values,
		bool *	isnull
	)

Definition at line 47 of file indextuple.c.

References Assert, DatumGetPointer, detoast_external_attr(), ereport, errcode(), errmsg(), ERROR, heap_compute_data_size(), heap_fill_tuple(), HEAP_HASEXTERNAL, HEAP_HASVARWIDTH, i, INDEX_MAX_KEYS, INDEX_NULL_MASK, INDEX_SIZE_MASK, INDEX_VAR_MASK, IndexInfoFindDataOffset, MAXALIGN, TupleDescData::natts, palloc0(), pfree(), PointerGetDatum, IndexTupleData::t_info, toast_compress_datum(), TOAST_INDEX_HACK, TOAST_INDEX_TARGET, TupleDescAttr, VARATT_IS_EXTENDED, VARATT_IS_EXTERNAL, and VARSIZE.

Referenced by bt_normalize_tuple(), bt_tuple_present_callback(), btinsert(), GinFormTuple(), gistFormTuple(), hashbuildCallback(), hashinsert(), index_truncate_tuple(), and tuplesort_putindextuplevalues().

{
    char       *tp;             /* tuple pointer */
    IndexTuple  tuple;          /* return tuple */
    Size        size,
                data_size,
                hoff;
    int         i;
    unsigned short infomask = 0;
    bool        hasnull = false;
    uint16      tupmask = 0;
    int         numberOfAttributes = tupleDescriptor->natts;

#ifdef TOAST_INDEX_HACK
    Datum       untoasted_values[INDEX_MAX_KEYS];
    bool        untoasted_free[INDEX_MAX_KEYS];
#endif

    if (numberOfAttributes > INDEX_MAX_KEYS)
        ereport(ERROR,
                (errcode(ERRCODE_TOO_MANY_COLUMNS),
                 errmsg("number of index columns (%d) exceeds limit (%d)",
                        numberOfAttributes, INDEX_MAX_KEYS)));

#ifdef TOAST_INDEX_HACK
    for (i = 0; i < numberOfAttributes; i++)
    {
        Form_pg_attribute att = TupleDescAttr(tupleDescriptor, i);

        untoasted_values[i] = values[i];
        untoasted_free[i] = false;

        /* Do nothing if value is NULL or not of varlena type */
        if (isnull[i] || att->attlen != -1)
            continue;

        /*
         * If value is stored EXTERNAL, must fetch it so we are not depending
         * on outside storage.  This should be improved someday.
         */
        if (VARATT_IS_EXTERNAL(DatumGetPointer(values[i])))
        {
            untoasted_values[i] =
                PointerGetDatum(detoast_external_attr((struct varlena *)
                                                      DatumGetPointer(values[i])));
            untoasted_free[i] = true;
        }

        /*
         * If value is above size target, and is of a compressible datatype,
         * try to compress it in-line.
         */
        if (!VARATT_IS_EXTENDED(DatumGetPointer(untoasted_values[i])) &&
            VARSIZE(DatumGetPointer(untoasted_values[i])) > TOAST_INDEX_TARGET &&
            (att->attstorage == TYPSTORAGE_EXTENDED ||
             att->attstorage == TYPSTORAGE_MAIN))
        {
            Datum       cvalue;

            cvalue = toast_compress_datum(untoasted_values[i],
                                          att->attcompression);

            if (DatumGetPointer(cvalue) != NULL)
            {
                /* successful compression */
                if (untoasted_free[i])
                    pfree(DatumGetPointer(untoasted_values[i]));
                untoasted_values[i] = cvalue;
                untoasted_free[i] = true;
            }
        }
    }
#endif

    for (i = 0; i < numberOfAttributes; i++)
    {
        if (isnull[i])
        {
            hasnull = true;
            break;
        }
    }

    if (hasnull)
        infomask |= INDEX_NULL_MASK;

    hoff = IndexInfoFindDataOffset(infomask);
#ifdef TOAST_INDEX_HACK
    data_size = heap_compute_data_size(tupleDescriptor,
                                       untoasted_values, isnull);
#else
    data_size = heap_compute_data_size(tupleDescriptor,
                                       values, isnull);
#endif
    size = hoff + data_size;
    size = MAXALIGN(size);      /* be conservative */

    tp = (char *) palloc0(size);
    tuple = (IndexTuple) tp;

    heap_fill_tuple(tupleDescriptor,
#ifdef TOAST_INDEX_HACK
                    untoasted_values,
#else
                    values,
#endif
                    isnull,
                    (char *) tp + hoff,
                    data_size,
                    &tupmask,
                    (hasnull ? (bits8 *) tp + sizeof(IndexTupleData) : NULL));

#ifdef TOAST_INDEX_HACK
    for (i = 0; i < numberOfAttributes; i++)
    {
        if (untoasted_free[i])
            pfree(DatumGetPointer(untoasted_values[i]));
    }
#endif

    /*
     * We do this because heap_fill_tuple wants to initialize a "tupmask"
     * which is used for HeapTuples, but we want an indextuple infomask. The
     * only relevant info is the "has variable attributes" field. We have
     * already set the hasnull bit above.
     */
    if (tupmask & HEAP_HASVARWIDTH)
        infomask |= INDEX_VAR_MASK;

    /* Also assert we got rid of external attributes */
#ifdef TOAST_INDEX_HACK
    Assert((tupmask & HEAP_HASEXTERNAL) == 0);
#endif

    /*
     * Here we make sure that the size will fit in the field reserved for it
     * in t_info.
     */
    if ((size & INDEX_SIZE_MASK) != size)
        ereport(ERROR,
                (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
                 errmsg("index row requires %zu bytes, maximum size is %zu",
                        size, (Size) INDEX_SIZE_MASK)));

    infomask |= size;

    /*
     * initialize metadata
     */
    tuple->t_info = infomask;
    return tuple;
}

index_truncate_tuple()

IndexTuple index_truncate_tuple	(	TupleDesc	sourceDescriptor,
		IndexTuple	source,
		int	leavenatts
	)

Definition at line 557 of file indextuple.c.

References Assert, CopyIndexTuple(), index_deform_tuple(), index_form_tuple(), INDEX_MAX_KEYS, IndexTupleSize, TupleDescData::natts, palloc(), pfree(), IndexTupleData::t_tid, TupleDescCopy(), TupleDescSize, and values.

Referenced by _bt_truncate().

{
    TupleDesc   truncdesc;
    Datum       values[INDEX_MAX_KEYS];
    bool        isnull[INDEX_MAX_KEYS];
    IndexTuple  truncated;

    Assert(leavenatts <= sourceDescriptor->natts);

    /* Easy case: no truncation actually required */
    if (leavenatts == sourceDescriptor->natts)
        return CopyIndexTuple(source);

    /* Create temporary descriptor to scribble on */
    truncdesc = palloc(TupleDescSize(sourceDescriptor));
    TupleDescCopy(truncdesc, sourceDescriptor);
    truncdesc->natts = leavenatts;

    /* Deform, form copy of tuple with fewer attributes */
    index_deform_tuple(source, truncdesc, values, isnull);
    truncated = index_form_tuple(truncdesc, values, isnull);
    truncated->t_tid = source->t_tid;
    Assert(IndexTupleSize(truncated) <= IndexTupleSize(source));

    /*
     * Cannot leak memory here, TupleDescCopy() doesn't allocate any inner
     * structure, so, plain pfree() should clean all allocated memory
     */
    pfree(truncdesc);

    return truncated;
}

nocache_index_getattr()

Datum nocache_index_getattr	(	IndexTuple	tup,
		int	attnum,
		TupleDesc	tupleDesc
	)

Definition at line 222 of file indextuple.c.

References Assert, att_addlength_pointer, att_align_nominal, att_align_pointer, att_isnull, attlen, attnum, byte, fetchatt, i, IndexInfoFindDataOffset, IndexTupleHasNulls, IndexTupleHasVarwidths, TupleDescData::natts, IndexTupleData::t_info, and TupleDescAttr.

{
    char       *tp;             /* ptr to data part of tuple */
    bits8      *bp = NULL;      /* ptr to null bitmap in tuple */
    bool        slow = false;   /* do we have to walk attrs? */
    int         data_off;       /* tuple data offset */
    int         off;            /* current offset within data */

    /* ----------------
     *   Three cases:
     *
     *   1: No nulls and no variable-width attributes.
     *   2: Has a null or a var-width AFTER att.
     *   3: Has nulls or var-widths BEFORE att.
     * ----------------
     */

    data_off = IndexInfoFindDataOffset(tup->t_info);

    attnum--;

    if (IndexTupleHasNulls(tup))
    {
        /*
         * there's a null somewhere in the tuple
         *
         * check to see if desired att is null
         */

        /* XXX "knows" t_bits are just after fixed tuple header! */
        bp = (bits8 *) ((char *) tup + sizeof(IndexTupleData));

        /*
         * Now check to see if any preceding bits are null...
         */
        {
            int         byte = attnum >> 3;
            int         finalbit = attnum & 0x07;

            /* check for nulls "before" final bit of last byte */
            if ((~bp[byte]) & ((1 << finalbit) - 1))
                slow = true;
            else
            {
                /* check for nulls in any "earlier" bytes */
                int         i;

                for (i = 0; i < byte; i++)
                {
                    if (bp[i] != 0xFF)
                    {
                        slow = true;
                        break;
                    }
                }
            }
        }
    }

    tp = (char *) tup + data_off;

    if (!slow)
    {
        Form_pg_attribute att;

        /*
         * If we get here, there are no nulls up to and including the target
         * attribute.  If we have a cached offset, we can use it.
         */
        att = TupleDescAttr(tupleDesc, attnum);
        if (att->attcacheoff >= 0)
            return fetchatt(att, tp + att->attcacheoff);

        /*
         * Otherwise, check for non-fixed-length attrs up to and including
         * target.  If there aren't any, it's safe to cheaply initialize the
         * cached offsets for these attrs.
         */
        if (IndexTupleHasVarwidths(tup))
        {
            int         j;

            for (j = 0; j <= attnum; j++)
            {
                if (TupleDescAttr(tupleDesc, j)->attlen <= 0)
                {
                    slow = true;
                    break;
                }
            }
        }
    }

    if (!slow)
    {
        int         natts = tupleDesc->natts;
        int         j = 1;

        /*
         * If we get here, we have a tuple with no nulls or var-widths up to
         * and including the target attribute, so we can use the cached offset
         * ... only we don't have it yet, or we'd not have got here.  Since
         * it's cheap to compute offsets for fixed-width columns, we take the
         * opportunity to initialize the cached offsets for *all* the leading
         * fixed-width columns, in hope of avoiding future visits to this
         * routine.
         */
        TupleDescAttr(tupleDesc, 0)->attcacheoff = 0;

        /* we might have set some offsets in the slow path previously */
        while (j < natts && TupleDescAttr(tupleDesc, j)->attcacheoff > 0)
            j++;

        off = TupleDescAttr(tupleDesc, j - 1)->attcacheoff +
            TupleDescAttr(tupleDesc, j - 1)->attlen;

        for (; j < natts; j++)
        {
            Form_pg_attribute att = TupleDescAttr(tupleDesc, j);

            if (att->attlen <= 0)
                break;

            off = att_align_nominal(off, att->attalign);

            att->attcacheoff = off;

            off += att->attlen;
        }

        Assert(j > attnum);

        off = TupleDescAttr(tupleDesc, attnum)->attcacheoff;
    }
    else
    {
        bool        usecache = true;
        int         i;

        /*
         * Now we know that we have to walk the tuple CAREFULLY.  But we still
         * might be able to cache some offsets for next time.
         *
         * Note - This loop is a little tricky.  For each non-null attribute,
         * we have to first account for alignment padding before the attr,
         * then advance over the attr based on its length.  Nulls have no
         * storage and no alignment padding either.  We can use/set
         * attcacheoff until we reach either a null or a var-width attribute.
         */
        off = 0;
        for (i = 0;; i++)       /* loop exit is at "break" */
        {
            Form_pg_attribute att = TupleDescAttr(tupleDesc, i);

            if (IndexTupleHasNulls(tup) && att_isnull(i, bp))
            {
                usecache = false;
                continue;       /* this cannot be the target att */
            }

            /* If we know the next offset, we can skip the rest */
            if (usecache && att->attcacheoff >= 0)
                off = att->attcacheoff;
            else if (att->attlen == -1)
            {
                /*
                 * We can only cache the offset for a varlena attribute if the
                 * offset is already suitably aligned, so that there would be
                 * no pad bytes in any case: then the offset will be valid for
                 * either an aligned or unaligned value.
                 */
                if (usecache &&
                    off == att_align_nominal(off, att->attalign))
                    att->attcacheoff = off;
                else
                {
                    off = att_align_pointer(off, att->attalign, -1,
                                            tp + off);
                    usecache = false;
                }
            }
            else
            {
                /* not varlena, so safe to use att_align_nominal */
                off = att_align_nominal(off, att->attalign);

                if (usecache)
                    att->attcacheoff = off;
            }

            if (i == attnum)
                break;

            off = att_addlength_pointer(off, att->attlen, tp + off);

            if (usecache && att->attlen <= 0)
                usecache = false;
        }
    }

    return fetchatt(TupleDescAttr(tupleDesc, attnum), tp + off);
}