heaptoast.c

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/*-------------------------------------------------------------------------
 *
 * heaptoast.c
 *    Heap-specific definitions for external and compressed storage
 *    of variable size attributes.
 *
 * Copyright (c) 2000-2026, PostgreSQL Global Development Group
 *
 *
 * IDENTIFICATION
 *    src/backend/access/heap/heaptoast.c
 *
 *
 * INTERFACE ROUTINES
 *      heap_toast_insert_or_update -
 *          Try to make a given tuple fit into one page by compressing
 *          or moving off attributes
 *
 *      heap_toast_delete -
 *          Reclaim toast storage when a tuple is deleted
 *
 *-------------------------------------------------------------------------
 */
 
#include "postgres.h"
 
#include "access/detoast.h"
#include "access/genam.h"
#include "access/heapam.h"
#include "access/heaptoast.h"
#include "access/toast_helper.h"
#include "access/toast_internals.h"
#include "utils/fmgroids.h"
 
 
/* ----------
 * heap_toast_delete -
 *
 *  Cascaded delete toast-entries on DELETE
 * ----------
 */
void
heap_toast_delete(Relation rel, HeapTuple oldtup, bool is_speculative)
{
    TupleDesc   tupleDesc;
    Datum       toast_values[MaxHeapAttributeNumber];
    bool        toast_isnull[MaxHeapAttributeNumber];
 
    /*
     * We should only ever be called for tuples of plain relations or
     * materialized views --- recursing on a toast rel is bad news.
     */
    Assert(rel->rd_rel->relkind == RELKIND_RELATION ||
           rel->rd_rel->relkind == RELKIND_MATVIEW);
 
    /*
     * Get the tuple descriptor and break down the tuple into fields.
     *
     * NOTE: it's debatable whether to use heap_deform_tuple() here or just
     * heap_getattr() only the varlena columns.  The latter could win if there
     * are few varlena columns and many non-varlena ones. However,
     * heap_deform_tuple costs only O(N) while the heap_getattr way would cost
     * O(N^2) if there are many varlena columns, so it seems better to err on
     * the side of linear cost.  (We won't even be here unless there's at
     * least one varlena column, by the way.)
     */
    tupleDesc = rel->rd_att;
 
    Assert(tupleDesc->natts <= MaxHeapAttributeNumber);
    heap_deform_tuple(oldtup, tupleDesc, toast_values, toast_isnull);
 
    /* Do the real work. */
    toast_delete_external(rel, toast_values, toast_isnull, is_speculative);
}
 
 
/* ----------
 * heap_toast_insert_or_update -
 *
 *  Delete no-longer-used toast-entries and create new ones to
 *  make the new tuple fit on INSERT or UPDATE
 *
 * Inputs:
 *  newtup: the candidate new tuple to be inserted
 *  oldtup: the old row version for UPDATE, or NULL for INSERT
 *  options: options to be passed to heap_insert() for toast rows
 * Result:
 *  either newtup if no toasting is needed, or a palloc'd modified tuple
 *  that is what should actually get stored
 *
 * NOTE: neither newtup nor oldtup will be modified.  This is a change
 * from the pre-8.1 API of this routine.
 * ----------
 */
HeapTuple
heap_toast_insert_or_update(Relation rel, HeapTuple newtup, HeapTuple oldtup,
                            int options)
{
    HeapTuple   result_tuple;
    TupleDesc   tupleDesc;
    int         numAttrs;
 
    Size        maxDataLen;
    Size        hoff;
 
    bool        toast_isnull[MaxHeapAttributeNumber];
    bool        toast_oldisnull[MaxHeapAttributeNumber];
    Datum       toast_values[MaxHeapAttributeNumber];
    Datum       toast_oldvalues[MaxHeapAttributeNumber];
    ToastAttrInfo toast_attr[MaxHeapAttributeNumber];
    ToastTupleContext ttc;
 
    /*
     * Ignore the INSERT_SPECULATIVE option. Speculative insertions/super
     * deletions just normally insert/delete the toast values. It seems
     * easiest to deal with that here, instead on, potentially, multiple
     * callers.
     */
    options &= ~HEAP_INSERT_SPECULATIVE;
 
    /*
     * We should only ever be called for tuples of plain relations or
     * materialized views --- recursing on a toast rel is bad news.
     */
    Assert(rel->rd_rel->relkind == RELKIND_RELATION ||
           rel->rd_rel->relkind == RELKIND_MATVIEW);
 
    /*
     * Get the tuple descriptor and break down the tuple(s) into fields.
     */
    tupleDesc = rel->rd_att;
    numAttrs = tupleDesc->natts;
 
    Assert(numAttrs <= MaxHeapAttributeNumber);
    heap_deform_tuple(newtup, tupleDesc, toast_values, toast_isnull);
    if (oldtup != NULL)
        heap_deform_tuple(oldtup, tupleDesc, toast_oldvalues, toast_oldisnull);
 
    /* ----------
     * Prepare for toasting
     * ----------
     */
    ttc.ttc_rel = rel;
    ttc.ttc_values = toast_values;
    ttc.ttc_isnull = toast_isnull;
    if (oldtup == NULL)
    {
        ttc.ttc_oldvalues = NULL;
        ttc.ttc_oldisnull = NULL;
    }
    else
    {
        ttc.ttc_oldvalues = toast_oldvalues;
        ttc.ttc_oldisnull = toast_oldisnull;
    }
    ttc.ttc_attr = toast_attr;
    toast_tuple_init(&ttc);
 
    /* ----------
     * Compress and/or save external until data fits into target length
     *
     *  1: Inline compress attributes with attstorage EXTENDED, and store very
     *     large attributes with attstorage EXTENDED or EXTERNAL external
     *     immediately
     *  2: Store attributes with attstorage EXTENDED or EXTERNAL external
     *  3: Inline compress attributes with attstorage MAIN
     *  4: Store attributes with attstorage MAIN external
     * ----------
     */
 
    /* compute header overhead --- this should match heap_form_tuple() */
    hoff = SizeofHeapTupleHeader;
    if ((ttc.ttc_flags & TOAST_HAS_NULLS) != 0)
        hoff += BITMAPLEN(numAttrs);
    hoff = MAXALIGN(hoff);
    /* now convert to a limit on the tuple data size */
    maxDataLen = RelationGetToastTupleTarget(rel, TOAST_TUPLE_TARGET) - hoff;
 
    /*
     * Look for attributes with attstorage EXTENDED to compress.  Also find
     * large attributes with attstorage EXTENDED or EXTERNAL, and store them
     * external.
     */
    while (heap_compute_data_size(tupleDesc,
                                  toast_values, toast_isnull) > maxDataLen)
    {
        int         biggest_attno;
 
        biggest_attno = toast_tuple_find_biggest_attribute(&ttc, true, false);
        if (biggest_attno < 0)
            break;
 
        /*
         * Attempt to compress it inline, if it has attstorage EXTENDED
         */
        if (TupleDescAttr(tupleDesc, biggest_attno)->attstorage == TYPSTORAGE_EXTENDED)
            toast_tuple_try_compression(&ttc, biggest_attno);
        else
        {
            /*
             * has attstorage EXTERNAL, ignore on subsequent compression
             * passes
             */
            toast_attr[biggest_attno].tai_colflags |= TOASTCOL_INCOMPRESSIBLE;
        }
 
        /*
         * If this value is by itself more than maxDataLen (after compression
         * if any), push it out to the toast table immediately, if possible.
         * This avoids uselessly compressing other fields in the common case
         * where we have one long field and several short ones.
         *
         * XXX maybe the threshold should be less than maxDataLen?
         */
        if (toast_attr[biggest_attno].tai_size > maxDataLen &&
            rel->rd_rel->reltoastrelid != InvalidOid)
            toast_tuple_externalize(&ttc, biggest_attno, options);
    }
 
    /*
     * Second we look for attributes of attstorage EXTENDED or EXTERNAL that
     * are still inline, and make them external.  But skip this if there's no
     * toast table to push them to.
     */
    while (heap_compute_data_size(tupleDesc,
                                  toast_values, toast_isnull) > maxDataLen &&
           rel->rd_rel->reltoastrelid != InvalidOid)
    {
        int         biggest_attno;
 
        biggest_attno = toast_tuple_find_biggest_attribute(&ttc, false, false);
        if (biggest_attno < 0)
            break;
        toast_tuple_externalize(&ttc, biggest_attno, options);
    }
 
    /*
     * Round 3 - this time we take attributes with storage MAIN into
     * compression
     */
    while (heap_compute_data_size(tupleDesc,
                                  toast_values, toast_isnull) > maxDataLen)
    {
        int         biggest_attno;
 
        biggest_attno = toast_tuple_find_biggest_attribute(&ttc, true, true);
        if (biggest_attno < 0)
            break;
 
        toast_tuple_try_compression(&ttc, biggest_attno);
    }
 
    /*
     * Finally we store attributes of type MAIN externally.  At this point we
     * increase the target tuple size, so that MAIN attributes aren't stored
     * externally unless really necessary.
     */
    maxDataLen = TOAST_TUPLE_TARGET_MAIN - hoff;
 
    while (heap_compute_data_size(tupleDesc,
                                  toast_values, toast_isnull) > maxDataLen &&
           rel->rd_rel->reltoastrelid != InvalidOid)
    {
        int         biggest_attno;
 
        biggest_attno = toast_tuple_find_biggest_attribute(&ttc, false, true);
        if (biggest_attno < 0)
            break;
 
        toast_tuple_externalize(&ttc, biggest_attno, options);
    }
 
    /*
     * In the case we toasted any values, we need to build a new heap tuple
     * with the changed values.
     */
    if ((ttc.ttc_flags & TOAST_NEEDS_CHANGE) != 0)
    {
        HeapTupleHeader olddata = newtup->t_data;
        HeapTupleHeader new_data;
        int32       new_header_len;
        int32       new_data_len;
        int32       new_tuple_len;
 
        /*
         * Calculate the new size of the tuple.
         *
         * Note: we used to assume here that the old tuple's t_hoff must equal
         * the new_header_len value, but that was incorrect.  The old tuple
         * might have a smaller-than-current natts, if there's been an ALTER
         * TABLE ADD COLUMN since it was stored; and that would lead to a
         * different conclusion about the size of the null bitmap, or even
         * whether there needs to be one at all.
         */
        new_header_len = SizeofHeapTupleHeader;
        if ((ttc.ttc_flags & TOAST_HAS_NULLS) != 0)
            new_header_len += BITMAPLEN(numAttrs);
        new_header_len = MAXALIGN(new_header_len);
        new_data_len = heap_compute_data_size(tupleDesc,
                                              toast_values, toast_isnull);
        new_tuple_len = new_header_len + new_data_len;
 
        /*
         * Allocate and zero the space needed, and fill HeapTupleData fields.
         */
        result_tuple = (HeapTuple) palloc0(HEAPTUPLESIZE + new_tuple_len);
        result_tuple->t_len = new_tuple_len;
        result_tuple->t_self = newtup->t_self;
        result_tuple->t_tableOid = newtup->t_tableOid;
        new_data = (HeapTupleHeader) ((char *) result_tuple + HEAPTUPLESIZE);
        result_tuple->t_data = new_data;
 
        /*
         * Copy the existing tuple header, but adjust natts and t_hoff.
         */
        memcpy(new_data, olddata, SizeofHeapTupleHeader);
        HeapTupleHeaderSetNatts(new_data, numAttrs);
        new_data->t_hoff = new_header_len;
 
        /* Copy over the data, and fill the null bitmap if needed */
        heap_fill_tuple(tupleDesc,
                        toast_values,
                        toast_isnull,
                        (char *) new_data + new_header_len,
                        new_data_len,
                        &(new_data->t_infomask),
                        ((ttc.ttc_flags & TOAST_HAS_NULLS) != 0) ?
                        new_data->t_bits : NULL);
    }
    else
        result_tuple = newtup;
 
    toast_tuple_cleanup(&ttc);
 
    return result_tuple;
}
 
 
/* ----------
 * toast_flatten_tuple -
 *
 *  "Flatten" a tuple to contain no out-of-line toasted fields.
 *  (This does not eliminate compressed or short-header datums.)
 *
 *  Note: we expect the caller already checked HeapTupleHasExternal(tup),
 *  so there is no need for a short-circuit path.
 * ----------
 */
HeapTuple
toast_flatten_tuple(HeapTuple tup, TupleDesc tupleDesc)
{
    HeapTuple   new_tuple;
    int         numAttrs = tupleDesc->natts;
    int         i;
    Datum       toast_values[MaxTupleAttributeNumber];
    bool        toast_isnull[MaxTupleAttributeNumber];
    bool        toast_free[MaxTupleAttributeNumber];
 
    /*
     * Break down the tuple into fields.
     */
    Assert(numAttrs <= MaxTupleAttributeNumber);
    heap_deform_tuple(tup, tupleDesc, toast_values, toast_isnull);
 
    memset(toast_free, 0, numAttrs * sizeof(bool));
 
    for (i = 0; i < numAttrs; i++)
    {
        /*
         * Look at non-null varlena attributes
         */
        if (!toast_isnull[i] && TupleDescCompactAttr(tupleDesc, i)->attlen == -1)
        {
            struct varlena *new_value;
 
            new_value = (struct varlena *) DatumGetPointer(toast_values[i]);
            if (VARATT_IS_EXTERNAL(new_value))
            {
                new_value = detoast_external_attr(new_value);
                toast_values[i] = PointerGetDatum(new_value);
                toast_free[i] = true;
            }
        }
    }
 
    /*
     * Form the reconfigured tuple.
     */
    new_tuple = heap_form_tuple(tupleDesc, toast_values, toast_isnull);
 
    /*
     * Be sure to copy the tuple's identity fields.  We also make a point of
     * copying visibility info, just in case anybody looks at those fields in
     * a syscache entry.
     */
    new_tuple->t_self = tup->t_self;
    new_tuple->t_tableOid = tup->t_tableOid;
 
    new_tuple->t_data->t_choice = tup->t_data->t_choice;
    new_tuple->t_data->t_ctid = tup->t_data->t_ctid;
    new_tuple->t_data->t_infomask &= ~HEAP_XACT_MASK;
    new_tuple->t_data->t_infomask |=
        tup->t_data->t_infomask & HEAP_XACT_MASK;
    new_tuple->t_data->t_infomask2 &= ~HEAP2_XACT_MASK;
    new_tuple->t_data->t_infomask2 |=
        tup->t_data->t_infomask2 & HEAP2_XACT_MASK;
 
    /*
     * Free allocated temp values
     */
    for (i = 0; i < numAttrs; i++)
        if (toast_free[i])
            pfree(DatumGetPointer(toast_values[i]));
 
    return new_tuple;
}
 
 
/* ----------
 * toast_flatten_tuple_to_datum -
 *
 *  "Flatten" a tuple containing out-of-line toasted fields into a Datum.
 *  The result is always palloc'd in the current memory context.
 *
 *  We have a general rule that Datums of container types (rows, arrays,
 *  ranges, etc) must not contain any external TOAST pointers.  Without
 *  this rule, we'd have to look inside each Datum when preparing a tuple
 *  for storage, which would be expensive and would fail to extend cleanly
 *  to new sorts of container types.
 *
 *  However, we don't want to say that tuples represented as HeapTuples
 *  can't contain toasted fields, so instead this routine should be called
 *  when such a HeapTuple is being converted into a Datum.
 *
 *  While we're at it, we decompress any compressed fields too.  This is not
 *  necessary for correctness, but reflects an expectation that compression
 *  will be more effective if applied to the whole tuple not individual
 *  fields.  We are not so concerned about that that we want to deconstruct
 *  and reconstruct tuples just to get rid of compressed fields, however.
 *  So callers typically won't call this unless they see that the tuple has
 *  at least one external field.
 *
 *  On the other hand, in-line short-header varlena fields are left alone.
 *  If we "untoasted" them here, they'd just get changed back to short-header
 *  format anyway within heap_fill_tuple.
 * ----------
 */
Datum
toast_flatten_tuple_to_datum(HeapTupleHeader tup,
                             uint32 tup_len,
                             TupleDesc tupleDesc)
{
    HeapTupleHeader new_data;
    int32       new_header_len;
    int32       new_data_len;
    int32       new_tuple_len;
    HeapTupleData tmptup;
    int         numAttrs = tupleDesc->natts;
    int         i;
    bool        has_nulls = false;
    Datum       toast_values[MaxTupleAttributeNumber];
    bool        toast_isnull[MaxTupleAttributeNumber];
    bool        toast_free[MaxTupleAttributeNumber];
 
    /* Build a temporary HeapTuple control structure */
    tmptup.t_len = tup_len;
    ItemPointerSetInvalid(&(tmptup.t_self));
    tmptup.t_tableOid = InvalidOid;
    tmptup.t_data = tup;
 
    /*
     * Break down the tuple into fields.
     */
    Assert(numAttrs <= MaxTupleAttributeNumber);
    heap_deform_tuple(&tmptup, tupleDesc, toast_values, toast_isnull);
 
    memset(toast_free, 0, numAttrs * sizeof(bool));
 
    for (i = 0; i < numAttrs; i++)
    {
        /*
         * Look at non-null varlena attributes
         */
        if (toast_isnull[i])
            has_nulls = true;
        else if (TupleDescCompactAttr(tupleDesc, i)->attlen == -1)
        {
            struct varlena *new_value;
 
            new_value = (struct varlena *) DatumGetPointer(toast_values[i]);
            if (VARATT_IS_EXTERNAL(new_value) ||
                VARATT_IS_COMPRESSED(new_value))
            {
                new_value = detoast_attr(new_value);
                toast_values[i] = PointerGetDatum(new_value);
                toast_free[i] = true;
            }
        }
    }
 
    /*
     * Calculate the new size of the tuple.
     *
     * This should match the reconstruction code in
     * heap_toast_insert_or_update.
     */
    new_header_len = SizeofHeapTupleHeader;
    if (has_nulls)
        new_header_len += BITMAPLEN(numAttrs);
    new_header_len = MAXALIGN(new_header_len);
    new_data_len = heap_compute_data_size(tupleDesc,
                                          toast_values, toast_isnull);
    new_tuple_len = new_header_len + new_data_len;
 
    new_data = (HeapTupleHeader) palloc0(new_tuple_len);
 
    /*
     * Copy the existing tuple header, but adjust natts and t_hoff.
     */
    memcpy(new_data, tup, SizeofHeapTupleHeader);
    HeapTupleHeaderSetNatts(new_data, numAttrs);
    new_data->t_hoff = new_header_len;
 
    /* Set the composite-Datum header fields correctly */
    HeapTupleHeaderSetDatumLength(new_data, new_tuple_len);
    HeapTupleHeaderSetTypeId(new_data, tupleDesc->tdtypeid);
    HeapTupleHeaderSetTypMod(new_data, tupleDesc->tdtypmod);
 
    /* Copy over the data, and fill the null bitmap if needed */
    heap_fill_tuple(tupleDesc,
                    toast_values,
                    toast_isnull,
                    (char *) new_data + new_header_len,
                    new_data_len,
                    &(new_data->t_infomask),
                    has_nulls ? new_data->t_bits : NULL);
 
    /*
     * Free allocated temp values
     */
    for (i = 0; i < numAttrs; i++)
        if (toast_free[i])
            pfree(DatumGetPointer(toast_values[i]));
 
    return PointerGetDatum(new_data);
}
 
 
/* ----------
 * toast_build_flattened_tuple -
 *
 *  Build a tuple containing no out-of-line toasted fields.
 *  (This does not eliminate compressed or short-header datums.)
 *
 *  This is essentially just like heap_form_tuple, except that it will
 *  expand any external-data pointers beforehand.
 *
 *  It's not very clear whether it would be preferable to decompress
 *  in-line compressed datums while at it.  For now, we don't.
 * ----------
 */
HeapTuple
toast_build_flattened_tuple(TupleDesc tupleDesc,
                            const Datum *values,
                            const bool *isnull)
{
    HeapTuple   new_tuple;
    int         numAttrs = tupleDesc->natts;
    int         num_to_free;
    int         i;
    Datum       new_values[MaxTupleAttributeNumber];
    void       *freeable_values[MaxTupleAttributeNumber];
 
    /*
     * We can pass the caller's isnull array directly to heap_form_tuple, but
     * we potentially need to modify the values array.
     */
    Assert(numAttrs <= MaxTupleAttributeNumber);
    memcpy(new_values, values, numAttrs * sizeof(Datum));
 
    num_to_free = 0;
    for (i = 0; i < numAttrs; i++)
    {
        /*
         * Look at non-null varlena attributes
         */
        if (!isnull[i] && TupleDescCompactAttr(tupleDesc, i)->attlen == -1)
        {
            struct varlena *new_value;
 
            new_value = (struct varlena *) DatumGetPointer(new_values[i]);
            if (VARATT_IS_EXTERNAL(new_value))
            {
                new_value = detoast_external_attr(new_value);
                new_values[i] = PointerGetDatum(new_value);
                freeable_values[num_to_free++] = new_value;
            }
        }
    }
 
    /*
     * Form the reconfigured tuple.
     */
    new_tuple = heap_form_tuple(tupleDesc, new_values, isnull);
 
    /*
     * Free allocated temp values
     */
    for (i = 0; i < num_to_free; i++)
        pfree(freeable_values[i]);
 
    return new_tuple;
}
 
/*
 * Fetch a TOAST slice from a heap table.
 *
 * toastrel is the relation from which chunks are to be fetched.
 * valueid identifies the TOAST value from which chunks are being fetched.
 * attrsize is the total size of the TOAST value.
 * sliceoffset is the byte offset within the TOAST value from which to fetch.
 * slicelength is the number of bytes to be fetched from the TOAST value.
 * result is the varlena into which the results should be written.
 */
void
heap_fetch_toast_slice(Relation toastrel, Oid valueid, int32 attrsize,
                       int32 sliceoffset, int32 slicelength,
                       struct varlena *result)
{
    Relation   *toastidxs;
    ScanKeyData toastkey[3];
    TupleDesc   toasttupDesc = toastrel->rd_att;
    int         nscankeys;
    SysScanDesc toastscan;
    HeapTuple   ttup;
    int32       expectedchunk;
    int32       totalchunks = ((attrsize - 1) / TOAST_MAX_CHUNK_SIZE) + 1;
    int         startchunk;
    int         endchunk;
    int         num_indexes;
    int         validIndex;
 
    /* Look for the valid index of toast relation */
    validIndex = toast_open_indexes(toastrel,
                                    AccessShareLock,
                                    &toastidxs,
                                    &num_indexes);
 
    startchunk = sliceoffset / TOAST_MAX_CHUNK_SIZE;
    endchunk = (sliceoffset + slicelength - 1) / TOAST_MAX_CHUNK_SIZE;
    Assert(endchunk <= totalchunks);
 
    /* Set up a scan key to fetch from the index. */
    ScanKeyInit(&toastkey[0],
                (AttrNumber) 1,
                BTEqualStrategyNumber, F_OIDEQ,
                ObjectIdGetDatum(valueid));
 
    /*
     * No additional condition if fetching all chunks. Otherwise, use an
     * equality condition for one chunk, and a range condition otherwise.
     */
    if (startchunk == 0 && endchunk == totalchunks - 1)
        nscankeys = 1;
    else if (startchunk == endchunk)
    {
        ScanKeyInit(&toastkey[1],
                    (AttrNumber) 2,
                    BTEqualStrategyNumber, F_INT4EQ,
                    Int32GetDatum(startchunk));
        nscankeys = 2;
    }
    else
    {
        ScanKeyInit(&toastkey[1],
                    (AttrNumber) 2,
                    BTGreaterEqualStrategyNumber, F_INT4GE,
                    Int32GetDatum(startchunk));
        ScanKeyInit(&toastkey[2],
                    (AttrNumber) 2,
                    BTLessEqualStrategyNumber, F_INT4LE,
                    Int32GetDatum(endchunk));
        nscankeys = 3;
    }
 
    /* Prepare for scan */
    toastscan = systable_beginscan_ordered(toastrel, toastidxs[validIndex],
                                           get_toast_snapshot(), nscankeys, toastkey);
 
    /*
     * Read the chunks by index
     *
     * The index is on (valueid, chunkidx) so they will come in order
     */
    expectedchunk = startchunk;
    while ((ttup = systable_getnext_ordered(toastscan, ForwardScanDirection)) != NULL)
    {
        int32       curchunk;
        Pointer     chunk;
        bool        isnull;
        char       *chunkdata;
        int32       chunksize;
        int32       expected_size;
        int32       chcpystrt;
        int32       chcpyend;
 
        /*
         * Have a chunk, extract the sequence number and the data
         */
        curchunk = DatumGetInt32(fastgetattr(ttup, 2, toasttupDesc, &isnull));
        Assert(!isnull);
        chunk = DatumGetPointer(fastgetattr(ttup, 3, toasttupDesc, &isnull));
        Assert(!isnull);
        if (!VARATT_IS_EXTENDED(chunk))
        {
            chunksize = VARSIZE(chunk) - VARHDRSZ;
            chunkdata = VARDATA(chunk);
        }
        else if (VARATT_IS_SHORT(chunk))
        {
            /* could happen due to heap_form_tuple doing its thing */
            chunksize = VARSIZE_SHORT(chunk) - VARHDRSZ_SHORT;
            chunkdata = VARDATA_SHORT(chunk);
        }
        else
        {
            /* should never happen */
            elog(ERROR, "found toasted toast chunk for toast value %u in %s",
                 valueid, RelationGetRelationName(toastrel));
            chunksize = 0;      /* keep compiler quiet */
            chunkdata = NULL;
        }
 
        /*
         * Some checks on the data we've found
         */
        if (curchunk != expectedchunk)
            ereport(ERROR,
                    (errcode(ERRCODE_DATA_CORRUPTED),
                     errmsg_internal("unexpected chunk number %d (expected %d) for toast value %u in %s",
                                     curchunk, expectedchunk, valueid,
                                     RelationGetRelationName(toastrel))));
        if (curchunk > endchunk)
            ereport(ERROR,
                    (errcode(ERRCODE_DATA_CORRUPTED),
                     errmsg_internal("unexpected chunk number %d (out of range %d..%d) for toast value %u in %s",
                                     curchunk,
                                     startchunk, endchunk, valueid,
                                     RelationGetRelationName(toastrel))));
        expected_size = curchunk < totalchunks - 1 ? TOAST_MAX_CHUNK_SIZE
            : attrsize - ((totalchunks - 1) * TOAST_MAX_CHUNK_SIZE);
        if (chunksize != expected_size)
            ereport(ERROR,
                    (errcode(ERRCODE_DATA_CORRUPTED),
                     errmsg_internal("unexpected chunk size %d (expected %d) in chunk %d of %d for toast value %u in %s",
                                     chunksize, expected_size,
                                     curchunk, totalchunks, valueid,
                                     RelationGetRelationName(toastrel))));
 
        /*
         * Copy the data into proper place in our result
         */
        chcpystrt = 0;
        chcpyend = chunksize - 1;
        if (curchunk == startchunk)
            chcpystrt = sliceoffset % TOAST_MAX_CHUNK_SIZE;
        if (curchunk == endchunk)
            chcpyend = (sliceoffset + slicelength - 1) % TOAST_MAX_CHUNK_SIZE;
 
        memcpy(VARDATA(result) +
               (curchunk * TOAST_MAX_CHUNK_SIZE - sliceoffset) + chcpystrt,
               chunkdata + chcpystrt,
               (chcpyend - chcpystrt) + 1);
 
        expectedchunk++;
    }
 
    /*
     * Final checks that we successfully fetched the datum
     */
    if (expectedchunk != (endchunk + 1))
        ereport(ERROR,
                (errcode(ERRCODE_DATA_CORRUPTED),
                 errmsg_internal("missing chunk number %d for toast value %u in %s",
                                 expectedchunk, valueid,
                                 RelationGetRelationName(toastrel))));
 
    /* End scan and close indexes. */
    systable_endscan_ordered(toastscan);
    toast_close_indexes(toastidxs, num_indexes, AccessShareLock);
}