tuptoaster.c

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/*-------------------------------------------------------------------------
 *
 * tuptoaster.c
 *    Support routines for external and compressed storage of
 *    variable size attributes.
 *
 * Copyright (c) 2000-2013, PostgreSQL Global Development Group
 *
 *
 * IDENTIFICATION
 *    src/backend/access/heap/tuptoaster.c
 *
 *
 * INTERFACE ROUTINES
 *      toast_insert_or_update -
 *          Try to make a given tuple fit into one page by compressing
 *          or moving off attributes
 *
 *      toast_delete -
 *          Reclaim toast storage when a tuple is deleted
 *
 *      heap_tuple_untoast_attr -
 *          Fetch back a given value from the "secondary" relation
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include <unistd.h>
#include <fcntl.h>

#include "access/genam.h"
#include "access/heapam.h"
#include "access/tuptoaster.h"
#include "access/xact.h"
#include "catalog/catalog.h"
#include "utils/fmgroids.h"
#include "utils/pg_lzcompress.h"
#include "utils/rel.h"
#include "utils/typcache.h"
#include "utils/tqual.h"


#undef TOAST_DEBUG

/* Size of an EXTERNAL datum that contains a standard TOAST pointer */
#define TOAST_POINTER_SIZE (VARHDRSZ_EXTERNAL + sizeof(struct varatt_external))

/*
 * Testing whether an externally-stored value is compressed now requires
 * comparing extsize (the actual length of the external data) to rawsize
 * (the original uncompressed datum's size).  The latter includes VARHDRSZ
 * overhead, the former doesn't.  We never use compression unless it actually
 * saves space, so we expect either equality or less-than.
 */
#define VARATT_EXTERNAL_IS_COMPRESSED(toast_pointer) \
    ((toast_pointer).va_extsize < (toast_pointer).va_rawsize - VARHDRSZ)

/*
 * Macro to fetch the possibly-unaligned contents of an EXTERNAL datum
 * into a local "struct varatt_external" toast pointer.  This should be
 * just a memcpy, but some versions of gcc seem to produce broken code
 * that assumes the datum contents are aligned.  Introducing an explicit
 * intermediate "varattrib_1b_e *" variable seems to fix it.
 */
#define VARATT_EXTERNAL_GET_POINTER(toast_pointer, attr) \
do { \
    varattrib_1b_e *attre = (varattrib_1b_e *) (attr); \
    Assert(VARATT_IS_EXTERNAL(attre)); \
    Assert(VARSIZE_EXTERNAL(attre) == sizeof(toast_pointer) + VARHDRSZ_EXTERNAL); \
    memcpy(&(toast_pointer), VARDATA_EXTERNAL(attre), sizeof(toast_pointer)); \
} while (0)


static void toast_delete_datum(Relation rel, Datum value);
static Datum toast_save_datum(Relation rel, Datum value,
                 struct varlena * oldexternal, int options);
static bool toastrel_valueid_exists(Relation toastrel, Oid valueid);
static bool toastid_valueid_exists(Oid toastrelid, Oid valueid);
static struct varlena *toast_fetch_datum(struct varlena * attr);
static struct varlena *toast_fetch_datum_slice(struct varlena * attr,
                        int32 sliceoffset, int32 length);


/* ----------
 * heap_tuple_fetch_attr -
 *
 *  Public entry point to get back a toasted value from
 *  external storage (possibly still in compressed format).
 *
 * This will return a datum that contains all the data internally, ie, not
 * relying on external storage, but it can still be compressed or have a short
 * header.
 ----------
 */
struct varlena *
heap_tuple_fetch_attr(struct varlena * attr)
{
    struct varlena *result;

    if (VARATT_IS_EXTERNAL(attr))
    {
        /*
         * This is an external stored plain value
         */
        result = toast_fetch_datum(attr);
    }
    else
    {
        /*
         * This is a plain value inside of the main tuple - why am I called?
         */
        result = attr;
    }

    return result;
}


/* ----------
 * heap_tuple_untoast_attr -
 *
 *  Public entry point to get back a toasted value from compression
 *  or external storage.
 * ----------
 */
struct varlena *
heap_tuple_untoast_attr(struct varlena * attr)
{
    if (VARATT_IS_EXTERNAL(attr))
    {
        /*
         * This is an externally stored datum --- fetch it back from there
         */
        attr = toast_fetch_datum(attr);
        /* If it's compressed, decompress it */
        if (VARATT_IS_COMPRESSED(attr))
        {
            PGLZ_Header *tmp = (PGLZ_Header *) attr;

            attr = (struct varlena *) palloc(PGLZ_RAW_SIZE(tmp) + VARHDRSZ);
            SET_VARSIZE(attr, PGLZ_RAW_SIZE(tmp) + VARHDRSZ);
            pglz_decompress(tmp, VARDATA(attr));
            pfree(tmp);
        }
    }
    else if (VARATT_IS_COMPRESSED(attr))
    {
        /*
         * This is a compressed value inside of the main tuple
         */
        PGLZ_Header *tmp = (PGLZ_Header *) attr;

        attr = (struct varlena *) palloc(PGLZ_RAW_SIZE(tmp) + VARHDRSZ);
        SET_VARSIZE(attr, PGLZ_RAW_SIZE(tmp) + VARHDRSZ);
        pglz_decompress(tmp, VARDATA(attr));
    }
    else if (VARATT_IS_SHORT(attr))
    {
        /*
         * This is a short-header varlena --- convert to 4-byte header format
         */
        Size        data_size = VARSIZE_SHORT(attr) - VARHDRSZ_SHORT;
        Size        new_size = data_size + VARHDRSZ;
        struct varlena *new_attr;

        new_attr = (struct varlena *) palloc(new_size);
        SET_VARSIZE(new_attr, new_size);
        memcpy(VARDATA(new_attr), VARDATA_SHORT(attr), data_size);
        attr = new_attr;
    }

    return attr;
}


/* ----------
 * heap_tuple_untoast_attr_slice -
 *
 *      Public entry point to get back part of a toasted value
 *      from compression or external storage.
 * ----------
 */
struct varlena *
heap_tuple_untoast_attr_slice(struct varlena * attr,
                              int32 sliceoffset, int32 slicelength)
{
    struct varlena *preslice;
    struct varlena *result;
    char       *attrdata;
    int32       attrsize;

    if (VARATT_IS_EXTERNAL(attr))
    {
        struct varatt_external toast_pointer;

        VARATT_EXTERNAL_GET_POINTER(toast_pointer, attr);

        /* fast path for non-compressed external datums */
        if (!VARATT_EXTERNAL_IS_COMPRESSED(toast_pointer))
            return toast_fetch_datum_slice(attr, sliceoffset, slicelength);

        /* fetch it back (compressed marker will get set automatically) */
        preslice = toast_fetch_datum(attr);
    }
    else
        preslice = attr;

    if (VARATT_IS_COMPRESSED(preslice))
    {
        PGLZ_Header *tmp = (PGLZ_Header *) preslice;
        Size        size = PGLZ_RAW_SIZE(tmp) + VARHDRSZ;

        preslice = (struct varlena *) palloc(size);
        SET_VARSIZE(preslice, size);
        pglz_decompress(tmp, VARDATA(preslice));

        if (tmp != (PGLZ_Header *) attr)
            pfree(tmp);
    }

    if (VARATT_IS_SHORT(preslice))
    {
        attrdata = VARDATA_SHORT(preslice);
        attrsize = VARSIZE_SHORT(preslice) - VARHDRSZ_SHORT;
    }
    else
    {
        attrdata = VARDATA(preslice);
        attrsize = VARSIZE(preslice) - VARHDRSZ;
    }

    /* slicing of datum for compressed cases and plain value */

    if (sliceoffset >= attrsize)
    {
        sliceoffset = 0;
        slicelength = 0;
    }

    if (((sliceoffset + slicelength) > attrsize) || slicelength < 0)
        slicelength = attrsize - sliceoffset;

    result = (struct varlena *) palloc(slicelength + VARHDRSZ);
    SET_VARSIZE(result, slicelength + VARHDRSZ);

    memcpy(VARDATA(result), attrdata + sliceoffset, slicelength);

    if (preslice != attr)
        pfree(preslice);

    return result;
}


/* ----------
 * toast_raw_datum_size -
 *
 *  Return the raw (detoasted) size of a varlena datum
 *  (including the VARHDRSZ header)
 * ----------
 */
Size
toast_raw_datum_size(Datum value)
{
    struct varlena *attr = (struct varlena *) DatumGetPointer(value);
    Size        result;

    if (VARATT_IS_EXTERNAL(attr))
    {
        /* va_rawsize is the size of the original datum -- including header */
        struct varatt_external toast_pointer;

        VARATT_EXTERNAL_GET_POINTER(toast_pointer, attr);
        result = toast_pointer.va_rawsize;
    }
    else if (VARATT_IS_COMPRESSED(attr))
    {
        /* here, va_rawsize is just the payload size */
        result = VARRAWSIZE_4B_C(attr) + VARHDRSZ;
    }
    else if (VARATT_IS_SHORT(attr))
    {
        /*
         * we have to normalize the header length to VARHDRSZ or else the
         * callers of this function will be confused.
         */
        result = VARSIZE_SHORT(attr) - VARHDRSZ_SHORT + VARHDRSZ;
    }
    else
    {
        /* plain untoasted datum */
        result = VARSIZE(attr);
    }
    return result;
}

/* ----------
 * toast_datum_size
 *
 *  Return the physical storage size (possibly compressed) of a varlena datum
 * ----------
 */
Size
toast_datum_size(Datum value)
{
    struct varlena *attr = (struct varlena *) DatumGetPointer(value);
    Size        result;

    if (VARATT_IS_EXTERNAL(attr))
    {
        /*
         * Attribute is stored externally - return the extsize whether
         * compressed or not.  We do not count the size of the toast pointer
         * ... should we?
         */
        struct varatt_external toast_pointer;

        VARATT_EXTERNAL_GET_POINTER(toast_pointer, attr);
        result = toast_pointer.va_extsize;
    }
    else if (VARATT_IS_SHORT(attr))
    {
        result = VARSIZE_SHORT(attr);
    }
    else
    {
        /*
         * Attribute is stored inline either compressed or not, just calculate
         * the size of the datum in either case.
         */
        result = VARSIZE(attr);
    }
    return result;
}


/* ----------
 * toast_delete -
 *
 *  Cascaded delete toast-entries on DELETE
 * ----------
 */
void
toast_delete(Relation rel, HeapTuple oldtup)
{
    TupleDesc   tupleDesc;
    Form_pg_attribute *att;
    int         numAttrs;
    int         i;
    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;
    att = tupleDesc->attrs;
    numAttrs = tupleDesc->natts;

    Assert(numAttrs <= MaxHeapAttributeNumber);
    heap_deform_tuple(oldtup, tupleDesc, toast_values, toast_isnull);

    /*
     * Check for external stored attributes and delete them from the secondary
     * relation.
     */
    for (i = 0; i < numAttrs; i++)
    {
        if (att[i]->attlen == -1)
        {
            Datum       value = toast_values[i];

            if (!toast_isnull[i] && VARATT_IS_EXTERNAL(PointerGetDatum(value)))
                toast_delete_datum(rel, value);
        }
    }
}


/* ----------
 * 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
toast_insert_or_update(Relation rel, HeapTuple newtup, HeapTuple oldtup,
                       int options)
{
    HeapTuple   result_tuple;
    TupleDesc   tupleDesc;
    Form_pg_attribute *att;
    int         numAttrs;
    int         i;

    bool        need_change = false;
    bool        need_free = false;
    bool        need_delold = false;
    bool        has_nulls = false;

    Size        maxDataLen;
    Size        hoff;

    char        toast_action[MaxHeapAttributeNumber];
    bool        toast_isnull[MaxHeapAttributeNumber];
    bool        toast_oldisnull[MaxHeapAttributeNumber];
    Datum       toast_values[MaxHeapAttributeNumber];
    Datum       toast_oldvalues[MaxHeapAttributeNumber];
    struct varlena *toast_oldexternal[MaxHeapAttributeNumber];
    int32       toast_sizes[MaxHeapAttributeNumber];
    bool        toast_free[MaxHeapAttributeNumber];
    bool        toast_delold[MaxHeapAttributeNumber];

    /*
     * We should only ever be called for tuples of plain relations ---
     * 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;
    att = tupleDesc->attrs;
    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);

    /* ----------
     * Then collect information about the values given
     *
     * NOTE: toast_action[i] can have these values:
     *      ' '     default handling
     *      'p'     already processed --- don't touch it
     *      'x'     incompressible, but OK to move off
     *
     * NOTE: toast_sizes[i] is only made valid for varlena attributes with
     *      toast_action[i] different from 'p'.
     * ----------
     */
    memset(toast_action, ' ', numAttrs * sizeof(char));
    memset(toast_oldexternal, 0, numAttrs * sizeof(struct varlena *));
    memset(toast_free, 0, numAttrs * sizeof(bool));
    memset(toast_delold, 0, numAttrs * sizeof(bool));

    for (i = 0; i < numAttrs; i++)
    {
        struct varlena *old_value;
        struct varlena *new_value;

        if (oldtup != NULL)
        {
            /*
             * For UPDATE get the old and new values of this attribute
             */
            old_value = (struct varlena *) DatumGetPointer(toast_oldvalues[i]);
            new_value = (struct varlena *) DatumGetPointer(toast_values[i]);

            /*
             * If the old value is an external stored one, check if it has
             * changed so we have to delete it later.
             */
            if (att[i]->attlen == -1 && !toast_oldisnull[i] &&
                VARATT_IS_EXTERNAL(old_value))
            {
                if (toast_isnull[i] || !VARATT_IS_EXTERNAL(new_value) ||
                    memcmp((char *) old_value, (char *) new_value,
                           VARSIZE_EXTERNAL(old_value)) != 0)
                {
                    /*
                     * The old external stored value isn't needed any more
                     * after the update
                     */
                    toast_delold[i] = true;
                    need_delold = true;
                }
                else
                {
                    /*
                     * This attribute isn't changed by this update so we reuse
                     * the original reference to the old value in the new
                     * tuple.
                     */
                    toast_action[i] = 'p';
                    continue;
                }
            }
        }
        else
        {
            /*
             * For INSERT simply get the new value
             */
            new_value = (struct varlena *) DatumGetPointer(toast_values[i]);
        }

        /*
         * Handle NULL attributes
         */
        if (toast_isnull[i])
        {
            toast_action[i] = 'p';
            has_nulls = true;
            continue;
        }

        /*
         * Now look at varlena attributes
         */
        if (att[i]->attlen == -1)
        {
            /*
             * If the table's attribute says PLAIN always, force it so.
             */
            if (att[i]->attstorage == 'p')
                toast_action[i] = 'p';

            /*
             * We took care of UPDATE above, so any external value we find
             * still in the tuple must be someone else's we cannot reuse.
             * Fetch it back (without decompression, unless we are forcing
             * PLAIN storage).  If necessary, we'll push it out as a new
             * external value below.
             */
            if (VARATT_IS_EXTERNAL(new_value))
            {
                toast_oldexternal[i] = new_value;
                if (att[i]->attstorage == 'p')
                    new_value = heap_tuple_untoast_attr(new_value);
                else
                    new_value = heap_tuple_fetch_attr(new_value);
                toast_values[i] = PointerGetDatum(new_value);
                toast_free[i] = true;
                need_change = true;
                need_free = true;
            }

            /*
             * Remember the size of this attribute
             */
            toast_sizes[i] = VARSIZE_ANY(new_value);
        }
        else
        {
            /*
             * Not a varlena attribute, plain storage always
             */
            toast_action[i] = 'p';
        }
    }

    /* ----------
     * Compress and/or save external until data fits into target length
     *
     *  1: Inline compress attributes with attstorage 'x', and store very
     *     large attributes with attstorage 'x' or 'e' external immediately
     *  2: Store attributes with attstorage 'x' or 'e' external
     *  3: Inline compress attributes with attstorage 'm'
     *  4: Store attributes with attstorage 'm' external
     * ----------
     */

    /* compute header overhead --- this should match heap_form_tuple() */
    hoff = offsetof(HeapTupleHeaderData, t_bits);
    if (has_nulls)
        hoff += BITMAPLEN(numAttrs);
    if (newtup->t_data->t_infomask & HEAP_HASOID)
        hoff += sizeof(Oid);
    hoff = MAXALIGN(hoff);
    /* now convert to a limit on the tuple data size */
    maxDataLen = TOAST_TUPLE_TARGET - hoff;

    /*
     * Look for attributes with attstorage 'x' to compress.  Also find large
     * attributes with attstorage 'x' or 'e', and store them external.
     */
    while (heap_compute_data_size(tupleDesc,
                                  toast_values, toast_isnull) > maxDataLen)
    {
        int         biggest_attno = -1;
        int32       biggest_size = MAXALIGN(TOAST_POINTER_SIZE);
        Datum       old_value;
        Datum       new_value;

        /*
         * Search for the biggest yet unprocessed internal attribute
         */
        for (i = 0; i < numAttrs; i++)
        {
            if (toast_action[i] != ' ')
                continue;
            if (VARATT_IS_EXTERNAL(DatumGetPointer(toast_values[i])))
                continue;       /* can't happen, toast_action would be 'p' */
            if (VARATT_IS_COMPRESSED(DatumGetPointer(toast_values[i])))
                continue;
            if (att[i]->attstorage != 'x' && att[i]->attstorage != 'e')
                continue;
            if (toast_sizes[i] > biggest_size)
            {
                biggest_attno = i;
                biggest_size = toast_sizes[i];
            }
        }

        if (biggest_attno < 0)
            break;

        /*
         * Attempt to compress it inline, if it has attstorage 'x'
         */
        i = biggest_attno;
        if (att[i]->attstorage == 'x')
        {
            old_value = toast_values[i];
            new_value = toast_compress_datum(old_value);

            if (DatumGetPointer(new_value) != NULL)
            {
                /* successful compression */
                if (toast_free[i])
                    pfree(DatumGetPointer(old_value));
                toast_values[i] = new_value;
                toast_free[i] = true;
                toast_sizes[i] = VARSIZE(DatumGetPointer(toast_values[i]));
                need_change = true;
                need_free = true;
            }
            else
            {
                /* incompressible, ignore on subsequent compression passes */
                toast_action[i] = 'x';
            }
        }
        else
        {
            /* has attstorage 'e', ignore on subsequent compression passes */
            toast_action[i] = 'x';
        }

        /*
         * 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_sizes[i] > maxDataLen &&
            rel->rd_rel->reltoastrelid != InvalidOid)
        {
            old_value = toast_values[i];
            toast_action[i] = 'p';
            toast_values[i] = toast_save_datum(rel, toast_values[i],
                                               toast_oldexternal[i], options);
            if (toast_free[i])
                pfree(DatumGetPointer(old_value));
            toast_free[i] = true;
            need_change = true;
            need_free = true;
        }
    }

    /*
     * Second we look for attributes of attstorage 'x' or 'e' that are still
     * inline.  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 = -1;
        int32       biggest_size = MAXALIGN(TOAST_POINTER_SIZE);
        Datum       old_value;

        /*------
         * Search for the biggest yet inlined attribute with
         * attstorage equals 'x' or 'e'
         *------
         */
        for (i = 0; i < numAttrs; i++)
        {
            if (toast_action[i] == 'p')
                continue;
            if (VARATT_IS_EXTERNAL(DatumGetPointer(toast_values[i])))
                continue;       /* can't happen, toast_action would be 'p' */
            if (att[i]->attstorage != 'x' && att[i]->attstorage != 'e')
                continue;
            if (toast_sizes[i] > biggest_size)
            {
                biggest_attno = i;
                biggest_size = toast_sizes[i];
            }
        }

        if (biggest_attno < 0)
            break;

        /*
         * Store this external
         */
        i = biggest_attno;
        old_value = toast_values[i];
        toast_action[i] = 'p';
        toast_values[i] = toast_save_datum(rel, toast_values[i],
                                           toast_oldexternal[i], options);
        if (toast_free[i])
            pfree(DatumGetPointer(old_value));
        toast_free[i] = true;

        need_change = true;
        need_free = true;
    }

    /*
     * Round 3 - this time we take attributes with storage 'm' into
     * compression
     */
    while (heap_compute_data_size(tupleDesc,
                                  toast_values, toast_isnull) > maxDataLen)
    {
        int         biggest_attno = -1;
        int32       biggest_size = MAXALIGN(TOAST_POINTER_SIZE);
        Datum       old_value;
        Datum       new_value;

        /*
         * Search for the biggest yet uncompressed internal attribute
         */
        for (i = 0; i < numAttrs; i++)
        {
            if (toast_action[i] != ' ')
                continue;
            if (VARATT_IS_EXTERNAL(DatumGetPointer(toast_values[i])))
                continue;       /* can't happen, toast_action would be 'p' */
            if (VARATT_IS_COMPRESSED(DatumGetPointer(toast_values[i])))
                continue;
            if (att[i]->attstorage != 'm')
                continue;
            if (toast_sizes[i] > biggest_size)
            {
                biggest_attno = i;
                biggest_size = toast_sizes[i];
            }
        }

        if (biggest_attno < 0)
            break;

        /*
         * Attempt to compress it inline
         */
        i = biggest_attno;
        old_value = toast_values[i];
        new_value = toast_compress_datum(old_value);

        if (DatumGetPointer(new_value) != NULL)
        {
            /* successful compression */
            if (toast_free[i])
                pfree(DatumGetPointer(old_value));
            toast_values[i] = new_value;
            toast_free[i] = true;
            toast_sizes[i] = VARSIZE(DatumGetPointer(toast_values[i]));
            need_change = true;
            need_free = true;
        }
        else
        {
            /* incompressible, ignore on subsequent compression passes */
            toast_action[i] = 'x';
        }
    }

    /*
     * Finally we store attributes of type 'm' externally.  At this point we
     * increase the target tuple size, so that 'm' 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 = -1;
        int32       biggest_size = MAXALIGN(TOAST_POINTER_SIZE);
        Datum       old_value;

        /*--------
         * Search for the biggest yet inlined attribute with
         * attstorage = 'm'
         *--------
         */
        for (i = 0; i < numAttrs; i++)
        {
            if (toast_action[i] == 'p')
                continue;
            if (VARATT_IS_EXTERNAL(DatumGetPointer(toast_values[i])))
                continue;       /* can't happen, toast_action would be 'p' */
            if (att[i]->attstorage != 'm')
                continue;
            if (toast_sizes[i] > biggest_size)
            {
                biggest_attno = i;
                biggest_size = toast_sizes[i];
            }
        }

        if (biggest_attno < 0)
            break;

        /*
         * Store this external
         */
        i = biggest_attno;
        old_value = toast_values[i];
        toast_action[i] = 'p';
        toast_values[i] = toast_save_datum(rel, toast_values[i],
                                           toast_oldexternal[i], options);
        if (toast_free[i])
            pfree(DatumGetPointer(old_value));
        toast_free[i] = true;

        need_change = true;
        need_free = true;
    }

    /*
     * In the case we toasted any values, we need to build a new heap tuple
     * with the changed values.
     */
    if (need_change)
    {
        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 = offsetof(HeapTupleHeaderData, t_bits);
        if (has_nulls)
            new_header_len += BITMAPLEN(numAttrs);
        if (olddata->t_infomask & HEAP_HASOID)
            new_header_len += sizeof(Oid);
        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, offsetof(HeapTupleHeaderData, t_bits));
        HeapTupleHeaderSetNatts(new_data, numAttrs);
        new_data->t_hoff = new_header_len;
        if (olddata->t_infomask & HEAP_HASOID)
            HeapTupleHeaderSetOid(new_data, HeapTupleHeaderGetOid(olddata));

        /* 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);
    }
    else
        result_tuple = newtup;

    /*
     * Free allocated temp values
     */
    if (need_free)
        for (i = 0; i < numAttrs; i++)
            if (toast_free[i])
                pfree(DatumGetPointer(toast_values[i]));

    /*
     * Delete external values from the old tuple
     */
    if (need_delold)
        for (i = 0; i < numAttrs; i++)
            if (toast_delold[i])
                toast_delete_datum(rel, toast_oldvalues[i]);

    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.)
 * ----------
 */
HeapTuple
toast_flatten_tuple(HeapTuple tup, TupleDesc tupleDesc)
{
    HeapTuple   new_tuple;
    Form_pg_attribute *att = tupleDesc->attrs;
    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] && att[i]->attlen == -1)
        {
            struct varlena *new_value;

            new_value = (struct varlena *) DatumGetPointer(toast_values[i]);
            if (VARATT_IS_EXTERNAL(new_value))
            {
                new_value = toast_fetch_datum(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 OID and identity fields.  We also make a
     * point of copying visibility info, just in case anybody looks at those
     * fields in a syscache entry.
     */
    if (tupleDesc->tdhasoid)
        HeapTupleSetOid(new_tuple, HeapTupleGetOid(tup));

    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_attribute -
 *
 *  If a Datum is of composite type, "flatten" it to contain no toasted fields.
 *  This must be invoked on any potentially-composite field that is to be
 *  inserted into a tuple.  Doing this preserves the invariant that toasting
 *  goes only one level deep in a tuple.
 *
 *  Note that flattening does not mean expansion of short-header varlenas,
 *  so in one sense toasting is allowed within composite datums.
 * ----------
 */
Datum
toast_flatten_tuple_attribute(Datum value,
                              Oid typeId, int32 typeMod)
{
    TupleDesc   tupleDesc;
    HeapTupleHeader olddata;
    HeapTupleHeader new_data;
    int32       new_header_len;
    int32       new_data_len;
    int32       new_tuple_len;
    HeapTupleData tmptup;
    Form_pg_attribute *att;
    int         numAttrs;
    int         i;
    bool        need_change = false;
    bool        has_nulls = false;
    Datum       toast_values[MaxTupleAttributeNumber];
    bool        toast_isnull[MaxTupleAttributeNumber];
    bool        toast_free[MaxTupleAttributeNumber];

    /*
     * See if it's a composite type, and get the tupdesc if so.
     */
    tupleDesc = lookup_rowtype_tupdesc_noerror(typeId, typeMod, true);
    if (tupleDesc == NULL)
        return value;           /* not a composite type */

    att = tupleDesc->attrs;
    numAttrs = tupleDesc->natts;

    /*
     * Break down the tuple into fields.
     */
    olddata = DatumGetHeapTupleHeader(value);
    Assert(typeId == HeapTupleHeaderGetTypeId(olddata));
    Assert(typeMod == HeapTupleHeaderGetTypMod(olddata));
    /* Build a temporary HeapTuple control structure */
    tmptup.t_len = HeapTupleHeaderGetDatumLength(olddata);
    ItemPointerSetInvalid(&(tmptup.t_self));
    tmptup.t_tableOid = InvalidOid;
    tmptup.t_data = olddata;

    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 (att[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 = heap_tuple_untoast_attr(new_value);
                toast_values[i] = PointerGetDatum(new_value);
                toast_free[i] = true;
                need_change = true;
            }
        }
    }

    /*
     * If nothing to untoast, just return the original tuple.
     */
    if (!need_change)
    {
        ReleaseTupleDesc(tupleDesc);
        return value;
    }

    /*
     * Calculate the new size of the tuple.
     *
     * This should match the reconstruction code in toast_insert_or_update.
     */
    new_header_len = offsetof(HeapTupleHeaderData, t_bits);
    if (has_nulls)
        new_header_len += BITMAPLEN(numAttrs);
    if (olddata->t_infomask & HEAP_HASOID)
        new_header_len += sizeof(Oid);
    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, olddata, offsetof(HeapTupleHeaderData, t_bits));
    HeapTupleHeaderSetNatts(new_data, numAttrs);
    new_data->t_hoff = new_header_len;
    if (olddata->t_infomask & HEAP_HASOID)
        HeapTupleHeaderSetOid(new_data, HeapTupleHeaderGetOid(olddata));

    /* Reset the datum length field, too */
    HeapTupleHeaderSetDatumLength(new_data, new_tuple_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),
                    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]));
    ReleaseTupleDesc(tupleDesc);

    return PointerGetDatum(new_data);
}


/* ----------
 * toast_compress_datum -
 *
 *  Create a compressed version of a varlena datum
 *
 *  If we fail (ie, compressed result is actually bigger than original)
 *  then return NULL.  We must not use compressed data if it'd expand
 *  the tuple!
 *
 *  We use VAR{SIZE,DATA}_ANY so we can handle short varlenas here without
 *  copying them.  But we can't handle external or compressed datums.
 * ----------
 */
Datum
toast_compress_datum(Datum value)
{
    struct varlena *tmp;
    int32       valsize = VARSIZE_ANY_EXHDR(DatumGetPointer(value));

    Assert(!VARATT_IS_EXTERNAL(DatumGetPointer(value)));
    Assert(!VARATT_IS_COMPRESSED(DatumGetPointer(value)));

    /*
     * No point in wasting a palloc cycle if value size is out of the allowed
     * range for compression
     */
    if (valsize < PGLZ_strategy_default->min_input_size ||
        valsize > PGLZ_strategy_default->max_input_size)
        return PointerGetDatum(NULL);

    tmp = (struct varlena *) palloc(PGLZ_MAX_OUTPUT(valsize));

    /*
     * We recheck the actual size even if pglz_compress() reports success,
     * because it might be satisfied with having saved as little as one byte
     * in the compressed data --- which could turn into a net loss once you
     * consider header and alignment padding.  Worst case, the compressed
     * format might require three padding bytes (plus header, which is
     * included in VARSIZE(tmp)), whereas the uncompressed format would take
     * only one header byte and no padding if the value is short enough.  So
     * we insist on a savings of more than 2 bytes to ensure we have a gain.
     */
    if (pglz_compress(VARDATA_ANY(DatumGetPointer(value)), valsize,
                      (PGLZ_Header *) tmp, PGLZ_strategy_default) &&
        VARSIZE(tmp) < valsize - 2)
    {
        /* successful compression */
        return PointerGetDatum(tmp);
    }
    else
    {
        /* incompressible data */
        pfree(tmp);
        return PointerGetDatum(NULL);
    }
}


/* ----------
 * toast_save_datum -
 *
 *  Save one single datum into the secondary relation and return
 *  a Datum reference for it.
 *
 * rel: the main relation we're working with (not the toast rel!)
 * value: datum to be pushed to toast storage
 * oldexternal: if not NULL, toast pointer previously representing the datum
 * options: options to be passed to heap_insert() for toast rows
 * ----------
 */
static Datum
toast_save_datum(Relation rel, Datum value,
                 struct varlena * oldexternal, int options)
{
    Relation    toastrel;
    Relation    toastidx;
    HeapTuple   toasttup;
    TupleDesc   toasttupDesc;
    Datum       t_values[3];
    bool        t_isnull[3];
    CommandId   mycid = GetCurrentCommandId(true);
    struct varlena *result;
    struct varatt_external toast_pointer;
    struct
    {
        struct varlena hdr;
        char        data[TOAST_MAX_CHUNK_SIZE]; /* make struct big enough */
        int32       align_it;   /* ensure struct is aligned well enough */
    }           chunk_data;
    int32       chunk_size;
    int32       chunk_seq = 0;
    char       *data_p;
    int32       data_todo;
    Pointer     dval = DatumGetPointer(value);

    /*
     * Open the toast relation and its index.  We can use the index to check
     * uniqueness of the OID we assign to the toasted item, even though it has
     * additional columns besides OID.
     */
    toastrel = heap_open(rel->rd_rel->reltoastrelid, RowExclusiveLock);
    toasttupDesc = toastrel->rd_att;
    toastidx = index_open(toastrel->rd_rel->reltoastidxid, RowExclusiveLock);

    /*
     * Get the data pointer and length, and compute va_rawsize and va_extsize.
     *
     * va_rawsize is the size of the equivalent fully uncompressed datum, so
     * we have to adjust for short headers.
     *
     * va_extsize is the actual size of the data payload in the toast records.
     */
    if (VARATT_IS_SHORT(dval))
    {
        data_p = VARDATA_SHORT(dval);
        data_todo = VARSIZE_SHORT(dval) - VARHDRSZ_SHORT;
        toast_pointer.va_rawsize = data_todo + VARHDRSZ;        /* as if not short */
        toast_pointer.va_extsize = data_todo;
    }
    else if (VARATT_IS_COMPRESSED(dval))
    {
        data_p = VARDATA(dval);
        data_todo = VARSIZE(dval) - VARHDRSZ;
        /* rawsize in a compressed datum is just the size of the payload */
        toast_pointer.va_rawsize = VARRAWSIZE_4B_C(dval) + VARHDRSZ;
        toast_pointer.va_extsize = data_todo;
        /* Assert that the numbers look like it's compressed */
        Assert(VARATT_EXTERNAL_IS_COMPRESSED(toast_pointer));
    }
    else
    {
        data_p = VARDATA(dval);
        data_todo = VARSIZE(dval) - VARHDRSZ;
        toast_pointer.va_rawsize = VARSIZE(dval);
        toast_pointer.va_extsize = data_todo;
    }

    /*
     * Insert the correct table OID into the result TOAST pointer.
     *
     * Normally this is the actual OID of the target toast table, but during
     * table-rewriting operations such as CLUSTER, we have to insert the OID
     * of the table's real permanent toast table instead.  rd_toastoid is set
     * if we have to substitute such an OID.
     */
    if (OidIsValid(rel->rd_toastoid))
        toast_pointer.va_toastrelid = rel->rd_toastoid;
    else
        toast_pointer.va_toastrelid = RelationGetRelid(toastrel);

    /*
     * Choose an OID to use as the value ID for this toast value.
     *
     * Normally we just choose an unused OID within the toast table.  But
     * during table-rewriting operations where we are preserving an existing
     * toast table OID, we want to preserve toast value OIDs too.  So, if
     * rd_toastoid is set and we had a prior external value from that same
     * toast table, re-use its value ID.  If we didn't have a prior external
     * value (which is a corner case, but possible if the table's attstorage
     * options have been changed), we have to pick a value ID that doesn't
     * conflict with either new or existing toast value OIDs.
     */
    if (!OidIsValid(rel->rd_toastoid))
    {
        /* normal case: just choose an unused OID */
        toast_pointer.va_valueid =
            GetNewOidWithIndex(toastrel,
                               RelationGetRelid(toastidx),
                               (AttrNumber) 1);
    }
    else
    {
        /* rewrite case: check to see if value was in old toast table */
        toast_pointer.va_valueid = InvalidOid;
        if (oldexternal != NULL)
        {
            struct varatt_external old_toast_pointer;

            Assert(VARATT_IS_EXTERNAL(oldexternal));
            /* Must copy to access aligned fields */
            VARATT_EXTERNAL_GET_POINTER(old_toast_pointer, oldexternal);
            if (old_toast_pointer.va_toastrelid == rel->rd_toastoid)
            {
                /* This value came from the old toast table; reuse its OID */
                toast_pointer.va_valueid = old_toast_pointer.va_valueid;

                /*
                 * There is a corner case here: the table rewrite might have
                 * to copy both live and recently-dead versions of a row, and
                 * those versions could easily reference the same toast value.
                 * When we copy the second or later version of such a row,
                 * reusing the OID will mean we select an OID that's already
                 * in the new toast table.  Check for that, and if so, just
                 * fall through without writing the data again.
                 *
                 * While annoying and ugly-looking, this is a good thing
                 * because it ensures that we wind up with only one copy of
                 * the toast value when there is only one copy in the old
                 * toast table.  Before we detected this case, we'd have made
                 * multiple copies, wasting space; and what's worse, the
                 * copies belonging to already-deleted heap tuples would not
                 * be reclaimed by VACUUM.
                 */
                if (toastrel_valueid_exists(toastrel,
                                            toast_pointer.va_valueid))
                {
                    /* Match, so short-circuit the data storage loop below */
                    data_todo = 0;
                }
            }
        }
        if (toast_pointer.va_valueid == InvalidOid)
        {
            /*
             * new value; must choose an OID that doesn't conflict in either
             * old or new toast table
             */
            do
            {
                toast_pointer.va_valueid =
                    GetNewOidWithIndex(toastrel,
                                       RelationGetRelid(toastidx),
                                       (AttrNumber) 1);
            } while (toastid_valueid_exists(rel->rd_toastoid,
                                            toast_pointer.va_valueid));
        }
    }

    /*
     * Initialize constant parts of the tuple data
     */
    t_values[0] = ObjectIdGetDatum(toast_pointer.va_valueid);
    t_values[2] = PointerGetDatum(&chunk_data);
    t_isnull[0] = false;
    t_isnull[1] = false;
    t_isnull[2] = false;

    /*
     * Split up the item into chunks
     */
    while (data_todo > 0)
    {
        /*
         * Calculate the size of this chunk
         */
        chunk_size = Min(TOAST_MAX_CHUNK_SIZE, data_todo);

        /*
         * Build a tuple and store it
         */
        t_values[1] = Int32GetDatum(chunk_seq++);
        SET_VARSIZE(&chunk_data, chunk_size + VARHDRSZ);
        memcpy(VARDATA(&chunk_data), data_p, chunk_size);
        toasttup = heap_form_tuple(toasttupDesc, t_values, t_isnull);

        heap_insert(toastrel, toasttup, mycid, options, NULL);

        /*
         * Create the index entry.  We cheat a little here by not using
         * FormIndexDatum: this relies on the knowledge that the index columns
         * are the same as the initial columns of the table.
         *
         * Note also that there had better not be any user-created index on
         * the TOAST table, since we don't bother to update anything else.
         */
        index_insert(toastidx, t_values, t_isnull,
                     &(toasttup->t_self),
                     toastrel,
                     toastidx->rd_index->indisunique ?
                     UNIQUE_CHECK_YES : UNIQUE_CHECK_NO);

        /*
         * Free memory
         */
        heap_freetuple(toasttup);

        /*
         * Move on to next chunk
         */
        data_todo -= chunk_size;
        data_p += chunk_size;
    }

    /*
     * Done - close toast relation
     */
    index_close(toastidx, RowExclusiveLock);
    heap_close(toastrel, RowExclusiveLock);

    /*
     * Create the TOAST pointer value that we'll return
     */
    result = (struct varlena *) palloc(TOAST_POINTER_SIZE);
    SET_VARSIZE_EXTERNAL(result, TOAST_POINTER_SIZE);
    memcpy(VARDATA_EXTERNAL(result), &toast_pointer, sizeof(toast_pointer));

    return PointerGetDatum(result);
}


/* ----------
 * toast_delete_datum -
 *
 *  Delete a single external stored value.
 * ----------
 */
static void
toast_delete_datum(Relation rel, Datum value)
{
    struct varlena *attr = (struct varlena *) DatumGetPointer(value);
    struct varatt_external toast_pointer;
    Relation    toastrel;
    Relation    toastidx;
    ScanKeyData toastkey;
    SysScanDesc toastscan;
    HeapTuple   toasttup;

    if (!VARATT_IS_EXTERNAL(attr))
        return;

    /* Must copy to access aligned fields */
    VARATT_EXTERNAL_GET_POINTER(toast_pointer, attr);

    /*
     * Open the toast relation and its index
     */
    toastrel = heap_open(toast_pointer.va_toastrelid, RowExclusiveLock);
    toastidx = index_open(toastrel->rd_rel->reltoastidxid, RowExclusiveLock);

    /*
     * Setup a scan key to find chunks with matching va_valueid
     */
    ScanKeyInit(&toastkey,
                (AttrNumber) 1,
                BTEqualStrategyNumber, F_OIDEQ,
                ObjectIdGetDatum(toast_pointer.va_valueid));

    /*
     * Find all the chunks.  (We don't actually care whether we see them in
     * sequence or not, but since we've already locked the index we might as
     * well use systable_beginscan_ordered.)
     */
    toastscan = systable_beginscan_ordered(toastrel, toastidx,
                                           SnapshotToast, 1, &toastkey);
    while ((toasttup = systable_getnext_ordered(toastscan, ForwardScanDirection)) != NULL)
    {
        /*
         * Have a chunk, delete it
         */
        simple_heap_delete(toastrel, &toasttup->t_self);
    }

    /*
     * End scan and close relations
     */
    systable_endscan_ordered(toastscan);
    index_close(toastidx, RowExclusiveLock);
    heap_close(toastrel, RowExclusiveLock);
}


/* ----------
 * toastrel_valueid_exists -
 *
 *  Test whether a toast value with the given ID exists in the toast relation
 * ----------
 */
static bool
toastrel_valueid_exists(Relation toastrel, Oid valueid)
{
    bool        result = false;
    ScanKeyData toastkey;
    SysScanDesc toastscan;

    /*
     * Setup a scan key to find chunks with matching va_valueid
     */
    ScanKeyInit(&toastkey,
                (AttrNumber) 1,
                BTEqualStrategyNumber, F_OIDEQ,
                ObjectIdGetDatum(valueid));

    /*
     * Is there any such chunk?
     */
    toastscan = systable_beginscan(toastrel, toastrel->rd_rel->reltoastidxid,
                                   true, SnapshotToast, 1, &toastkey);

    if (systable_getnext(toastscan) != NULL)
        result = true;

    systable_endscan(toastscan);

    return result;
}

/* ----------
 * toastid_valueid_exists -
 *
 *  As above, but work from toast rel's OID not an open relation
 * ----------
 */
static bool
toastid_valueid_exists(Oid toastrelid, Oid valueid)
{
    bool        result;
    Relation    toastrel;

    toastrel = heap_open(toastrelid, AccessShareLock);

    result = toastrel_valueid_exists(toastrel, valueid);

    heap_close(toastrel, AccessShareLock);

    return result;
}


/* ----------
 * toast_fetch_datum -
 *
 *  Reconstruct an in memory Datum from the chunks saved
 *  in the toast relation
 * ----------
 */
static struct varlena *
toast_fetch_datum(struct varlena * attr)
{
    Relation    toastrel;
    Relation    toastidx;
    ScanKeyData toastkey;
    SysScanDesc toastscan;
    HeapTuple   ttup;
    TupleDesc   toasttupDesc;
    struct varlena *result;
    struct varatt_external toast_pointer;
    int32       ressize;
    int32       residx,
                nextidx;
    int32       numchunks;
    Pointer     chunk;
    bool        isnull;
    char       *chunkdata;
    int32       chunksize;

    /* Must copy to access aligned fields */
    VARATT_EXTERNAL_GET_POINTER(toast_pointer, attr);

    ressize = toast_pointer.va_extsize;
    numchunks = ((ressize - 1) / TOAST_MAX_CHUNK_SIZE) + 1;

    result = (struct varlena *) palloc(ressize + VARHDRSZ);

    if (VARATT_EXTERNAL_IS_COMPRESSED(toast_pointer))
        SET_VARSIZE_COMPRESSED(result, ressize + VARHDRSZ);
    else
        SET_VARSIZE(result, ressize + VARHDRSZ);

    /*
     * Open the toast relation and its index
     */
    toastrel = heap_open(toast_pointer.va_toastrelid, AccessShareLock);
    toasttupDesc = toastrel->rd_att;
    toastidx = index_open(toastrel->rd_rel->reltoastidxid, AccessShareLock);

    /*
     * Setup a scan key to fetch from the index by va_valueid
     */
    ScanKeyInit(&toastkey,
                (AttrNumber) 1,
                BTEqualStrategyNumber, F_OIDEQ,
                ObjectIdGetDatum(toast_pointer.va_valueid));

    /*
     * Read the chunks by index
     *
     * Note that because the index is actually on (valueid, chunkidx) we will
     * see the chunks in chunkidx order, even though we didn't explicitly ask
     * for it.
     */
    nextidx = 0;

    toastscan = systable_beginscan_ordered(toastrel, toastidx,
                                           SnapshotToast, 1, &toastkey);
    while ((ttup = systable_getnext_ordered(toastscan, ForwardScanDirection)) != NULL)
    {
        /*
         * Have a chunk, extract the sequence number and the data
         */
        residx = 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",
                 toast_pointer.va_valueid,
                 RelationGetRelationName(toastrel));
            chunksize = 0;      /* keep compiler quiet */
            chunkdata = NULL;
        }

        /*
         * Some checks on the data we've found
         */
        if (residx != nextidx)
            elog(ERROR, "unexpected chunk number %d (expected %d) for toast value %u in %s",
                 residx, nextidx,
                 toast_pointer.va_valueid,
                 RelationGetRelationName(toastrel));
        if (residx < numchunks - 1)
        {
            if (chunksize != TOAST_MAX_CHUNK_SIZE)
                elog(ERROR, "unexpected chunk size %d (expected %d) in chunk %d of %d for toast value %u in %s",
                     chunksize, (int) TOAST_MAX_CHUNK_SIZE,
                     residx, numchunks,
                     toast_pointer.va_valueid,
                     RelationGetRelationName(toastrel));
        }
        else if (residx == numchunks - 1)
        {
            if ((residx * TOAST_MAX_CHUNK_SIZE + chunksize) != ressize)
                elog(ERROR, "unexpected chunk size %d (expected %d) in final chunk %d for toast value %u in %s",
                     chunksize,
                     (int) (ressize - residx * TOAST_MAX_CHUNK_SIZE),
                     residx,
                     toast_pointer.va_valueid,
                     RelationGetRelationName(toastrel));
        }
        else
            elog(ERROR, "unexpected chunk number %d (out of range %d..%d) for toast value %u in %s",
                 residx,
                 0, numchunks - 1,
                 toast_pointer.va_valueid,
                 RelationGetRelationName(toastrel));

        /*
         * Copy the data into proper place in our result
         */
        memcpy(VARDATA(result) + residx * TOAST_MAX_CHUNK_SIZE,
               chunkdata,
               chunksize);

        nextidx++;
    }

    /*
     * Final checks that we successfully fetched the datum
     */
    if (nextidx != numchunks)
        elog(ERROR, "missing chunk number %d for toast value %u in %s",
             nextidx,
             toast_pointer.va_valueid,
             RelationGetRelationName(toastrel));

    /*
     * End scan and close relations
     */
    systable_endscan_ordered(toastscan);
    index_close(toastidx, AccessShareLock);
    heap_close(toastrel, AccessShareLock);

    return result;
}

/* ----------
 * toast_fetch_datum_slice -
 *
 *  Reconstruct a segment of a Datum from the chunks saved
 *  in the toast relation
 * ----------
 */
static struct varlena *
toast_fetch_datum_slice(struct varlena * attr, int32 sliceoffset, int32 length)
{
    Relation    toastrel;
    Relation    toastidx;
    ScanKeyData toastkey[3];
    int         nscankeys;
    SysScanDesc toastscan;
    HeapTuple   ttup;
    TupleDesc   toasttupDesc;
    struct varlena *result;
    struct varatt_external toast_pointer;
    int32       attrsize;
    int32       residx;
    int32       nextidx;
    int         numchunks;
    int         startchunk;
    int         endchunk;
    int32       startoffset;
    int32       endoffset;
    int         totalchunks;
    Pointer     chunk;
    bool        isnull;
    char       *chunkdata;
    int32       chunksize;
    int32       chcpystrt;
    int32       chcpyend;

    Assert(VARATT_IS_EXTERNAL(attr));

    /* Must copy to access aligned fields */
    VARATT_EXTERNAL_GET_POINTER(toast_pointer, attr);

    /*
     * It's nonsense to fetch slices of a compressed datum -- this isn't lo_*
     * we can't return a compressed datum which is meaningful to toast later
     */
    Assert(!VARATT_EXTERNAL_IS_COMPRESSED(toast_pointer));

    attrsize = toast_pointer.va_extsize;
    totalchunks = ((attrsize - 1) / TOAST_MAX_CHUNK_SIZE) + 1;

    if (sliceoffset >= attrsize)
    {
        sliceoffset = 0;
        length = 0;
    }

    if (((sliceoffset + length) > attrsize) || length < 0)
        length = attrsize - sliceoffset;

    result = (struct varlena *) palloc(length + VARHDRSZ);

    if (VARATT_EXTERNAL_IS_COMPRESSED(toast_pointer))
        SET_VARSIZE_COMPRESSED(result, length + VARHDRSZ);
    else
        SET_VARSIZE(result, length + VARHDRSZ);

    if (length == 0)
        return result;          /* Can save a lot of work at this point! */

    startchunk = sliceoffset / TOAST_MAX_CHUNK_SIZE;
    endchunk = (sliceoffset + length - 1) / TOAST_MAX_CHUNK_SIZE;
    numchunks = (endchunk - startchunk) + 1;

    startoffset = sliceoffset % TOAST_MAX_CHUNK_SIZE;
    endoffset = (sliceoffset + length - 1) % TOAST_MAX_CHUNK_SIZE;

    /*
     * Open the toast relation and its index
     */
    toastrel = heap_open(toast_pointer.va_toastrelid, AccessShareLock);
    toasttupDesc = toastrel->rd_att;
    toastidx = index_open(toastrel->rd_rel->reltoastidxid, AccessShareLock);

    /*
     * Setup a scan key to fetch from the index. This is either two keys or
     * three depending on the number of chunks.
     */
    ScanKeyInit(&toastkey[0],
                (AttrNumber) 1,
                BTEqualStrategyNumber, F_OIDEQ,
                ObjectIdGetDatum(toast_pointer.va_valueid));

    /*
     * Use equality condition for one chunk, a range condition otherwise:
     */
    if (numchunks == 1)
    {
        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;
    }

    /*
     * Read the chunks by index
     *
     * The index is on (valueid, chunkidx) so they will come in order
     */
    nextidx = startchunk;
    toastscan = systable_beginscan_ordered(toastrel, toastidx,
                                         SnapshotToast, nscankeys, toastkey);
    while ((ttup = systable_getnext_ordered(toastscan, ForwardScanDirection)) != NULL)
    {
        /*
         * Have a chunk, extract the sequence number and the data
         */
        residx = 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",
                 toast_pointer.va_valueid,
                 RelationGetRelationName(toastrel));
            chunksize = 0;      /* keep compiler quiet */
            chunkdata = NULL;
        }

        /*
         * Some checks on the data we've found
         */
        if ((residx != nextidx) || (residx > endchunk) || (residx < startchunk))
            elog(ERROR, "unexpected chunk number %d (expected %d) for toast value %u in %s",
                 residx, nextidx,
                 toast_pointer.va_valueid,
                 RelationGetRelationName(toastrel));
        if (residx < totalchunks - 1)
        {
            if (chunksize != TOAST_MAX_CHUNK_SIZE)
                elog(ERROR, "unexpected chunk size %d (expected %d) in chunk %d of %d for toast value %u in %s when fetching slice",
                     chunksize, (int) TOAST_MAX_CHUNK_SIZE,
                     residx, totalchunks,
                     toast_pointer.va_valueid,
                     RelationGetRelationName(toastrel));
        }
        else if (residx == totalchunks - 1)
        {
            if ((residx * TOAST_MAX_CHUNK_SIZE + chunksize) != attrsize)
                elog(ERROR, "unexpected chunk size %d (expected %d) in final chunk %d for toast value %u in %s when fetching slice",
                     chunksize,
                     (int) (attrsize - residx * TOAST_MAX_CHUNK_SIZE),
                     residx,
                     toast_pointer.va_valueid,
                     RelationGetRelationName(toastrel));
        }
        else
            elog(ERROR, "unexpected chunk number %d (out of range %d..%d) for toast value %u in %s",
                 residx,
                 0, totalchunks - 1,
                 toast_pointer.va_valueid,
                 RelationGetRelationName(toastrel));

        /*
         * Copy the data into proper place in our result
         */
        chcpystrt = 0;
        chcpyend = chunksize - 1;
        if (residx == startchunk)
            chcpystrt = startoffset;
        if (residx == endchunk)
            chcpyend = endoffset;

        memcpy(VARDATA(result) +
               (residx * TOAST_MAX_CHUNK_SIZE - sliceoffset) + chcpystrt,
               chunkdata + chcpystrt,
               (chcpyend - chcpystrt) + 1);

        nextidx++;
    }

    /*
     * Final checks that we successfully fetched the datum
     */
    if (nextidx != (endchunk + 1))
        elog(ERROR, "missing chunk number %d for toast value %u in %s",
             nextidx,
             toast_pointer.va_valueid,
             RelationGetRelationName(toastrel));

    /*
     * End scan and close relations
     */
    systable_endscan_ordered(toastscan);
    index_close(toastidx, AccessShareLock);
    heap_close(toastrel, AccessShareLock);

    return result;
}