detoast.c

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/*-------------------------------------------------------------------------
 *
 * detoast.c
 *    Retrieve compressed or external variable size attributes.
 *
 * Copyright (c) 2000-2025, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *    src/backend/access/common/detoast.c
 *
 *-------------------------------------------------------------------------
 */
 
#include "postgres.h"
 
#include "access/detoast.h"
#include "access/table.h"
#include "access/tableam.h"
#include "access/toast_internals.h"
#include "common/int.h"
#include "common/pg_lzcompress.h"
#include "utils/expandeddatum.h"
#include "utils/rel.h"
 
static struct varlena *toast_fetch_datum(struct varlena *attr);
static struct varlena *toast_fetch_datum_slice(struct varlena *attr,
                                               int32 sliceoffset,
                                               int32 slicelength);
static struct varlena *toast_decompress_datum(struct varlena *attr);
static struct varlena *toast_decompress_datum_slice(struct varlena *attr, int32 slicelength);
 
/* ----------
 * detoast_external_attr -
 *
 *  Public entry point to get back a toasted value from
 *  external source (possibly still in compressed format).
 *
 * This will return a datum that contains all the data internally, ie, not
 * relying on external storage or memory, but it can still be compressed or
 * have a short header.  Note some callers assume that if the input is an
 * EXTERNAL datum, the result will be a pfree'able chunk.
 * ----------
 */
struct varlena *
detoast_external_attr(struct varlena *attr)
{
    struct varlena *result;
 
    if (VARATT_IS_EXTERNAL_ONDISK(attr))
    {
        /*
         * This is an external stored plain value
         */
        result = toast_fetch_datum(attr);
    }
    else if (VARATT_IS_EXTERNAL_INDIRECT(attr))
    {
        /*
         * This is an indirect pointer --- dereference it
         */
        struct varatt_indirect redirect;
 
        VARATT_EXTERNAL_GET_POINTER(redirect, attr);
        attr = (struct varlena *) redirect.pointer;
 
        /* nested indirect Datums aren't allowed */
        Assert(!VARATT_IS_EXTERNAL_INDIRECT(attr));
 
        /* recurse if value is still external in some other way */
        if (VARATT_IS_EXTERNAL(attr))
            return detoast_external_attr(attr);
 
        /*
         * Copy into the caller's memory context, in case caller tries to
         * pfree the result.
         */
        result = (struct varlena *) palloc(VARSIZE_ANY(attr));
        memcpy(result, attr, VARSIZE_ANY(attr));
    }
    else if (VARATT_IS_EXTERNAL_EXPANDED(attr))
    {
        /*
         * This is an expanded-object pointer --- get flat format
         */
        ExpandedObjectHeader *eoh;
        Size        resultsize;
 
        eoh = DatumGetEOHP(PointerGetDatum(attr));
        resultsize = EOH_get_flat_size(eoh);
        result = (struct varlena *) palloc(resultsize);
        EOH_flatten_into(eoh, result, resultsize);
    }
    else
    {
        /*
         * This is a plain value inside of the main tuple - why am I called?
         */
        result = attr;
    }
 
    return result;
}
 
 
/* ----------
 * detoast_attr -
 *
 *  Public entry point to get back a toasted value from compression
 *  or external storage.  The result is always non-extended varlena form.
 *
 * Note some callers assume that if the input is an EXTERNAL or COMPRESSED
 * datum, the result will be a pfree'able chunk.
 * ----------
 */
struct varlena *
detoast_attr(struct varlena *attr)
{
    if (VARATT_IS_EXTERNAL_ONDISK(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))
        {
            struct varlena *tmp = attr;
 
            attr = toast_decompress_datum(tmp);
            pfree(tmp);
        }
    }
    else if (VARATT_IS_EXTERNAL_INDIRECT(attr))
    {
        /*
         * This is an indirect pointer --- dereference it
         */
        struct varatt_indirect redirect;
 
        VARATT_EXTERNAL_GET_POINTER(redirect, attr);
        attr = (struct varlena *) redirect.pointer;
 
        /* nested indirect Datums aren't allowed */
        Assert(!VARATT_IS_EXTERNAL_INDIRECT(attr));
 
        /* recurse in case value is still extended in some other way */
        attr = detoast_attr(attr);
 
        /* if it isn't, we'd better copy it */
        if (attr == (struct varlena *) redirect.pointer)
        {
            struct varlena *result;
 
            result = (struct varlena *) palloc(VARSIZE_ANY(attr));
            memcpy(result, attr, VARSIZE_ANY(attr));
            attr = result;
        }
    }
    else if (VARATT_IS_EXTERNAL_EXPANDED(attr))
    {
        /*
         * This is an expanded-object pointer --- get flat format
         */
        attr = detoast_external_attr(attr);
        /* flatteners are not allowed to produce compressed/short output */
        Assert(!VARATT_IS_EXTENDED(attr));
    }
    else if (VARATT_IS_COMPRESSED(attr))
    {
        /*
         * This is a compressed value inside of the main tuple
         */
        attr = toast_decompress_datum(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;
}
 
 
/* ----------
 * detoast_attr_slice -
 *
 *      Public entry point to get back part of a toasted value
 *      from compression or external storage.
 *
 * sliceoffset is where to start (zero or more)
 * If slicelength < 0, return everything beyond sliceoffset
 * ----------
 */
struct varlena *
detoast_attr_slice(struct varlena *attr,
                   int32 sliceoffset, int32 slicelength)
{
    struct varlena *preslice;
    struct varlena *result;
    char       *attrdata;
    int32       slicelimit;
    int32       attrsize;
 
    if (sliceoffset < 0)
        elog(ERROR, "invalid sliceoffset: %d", sliceoffset);
 
    /*
     * Compute slicelimit = offset + length, or -1 if we must fetch all of the
     * value.  In case of integer overflow, we must fetch all.
     */
    if (slicelength < 0)
        slicelimit = -1;
    else if (pg_add_s32_overflow(sliceoffset, slicelength, &slicelimit))
        slicelength = slicelimit = -1;
 
    if (VARATT_IS_EXTERNAL_ONDISK(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);
 
        /*
         * For compressed values, we need to fetch enough slices to decompress
         * at least the requested part (when a prefix is requested).
         * Otherwise, just fetch all slices.
         */
        if (slicelimit >= 0)
        {
            int32       max_size = VARATT_EXTERNAL_GET_EXTSIZE(toast_pointer);
 
            /*
             * Determine maximum amount of compressed data needed for a prefix
             * of a given length (after decompression).
             *
             * At least for now, if it's LZ4 data, we'll have to fetch the
             * whole thing, because there doesn't seem to be an API call to
             * determine how much compressed data we need to be sure of being
             * able to decompress the required slice.
             */
            if (VARATT_EXTERNAL_GET_COMPRESS_METHOD(toast_pointer) ==
                TOAST_PGLZ_COMPRESSION_ID)
                max_size = pglz_maximum_compressed_size(slicelimit, max_size);
 
            /*
             * Fetch enough compressed slices (compressed marker will get set
             * automatically).
             */
            preslice = toast_fetch_datum_slice(attr, 0, max_size);
        }
        else
            preslice = toast_fetch_datum(attr);
    }
    else if (VARATT_IS_EXTERNAL_INDIRECT(attr))
    {
        struct varatt_indirect redirect;
 
        VARATT_EXTERNAL_GET_POINTER(redirect, attr);
 
        /* nested indirect Datums aren't allowed */
        Assert(!VARATT_IS_EXTERNAL_INDIRECT(redirect.pointer));
 
        return detoast_attr_slice(redirect.pointer,
                                  sliceoffset, slicelength);
    }
    else if (VARATT_IS_EXTERNAL_EXPANDED(attr))
    {
        /* pass it off to detoast_external_attr to flatten */
        preslice = detoast_external_attr(attr);
    }
    else
        preslice = attr;
 
    Assert(!VARATT_IS_EXTERNAL(preslice));
 
    if (VARATT_IS_COMPRESSED(preslice))
    {
        struct varlena *tmp = preslice;
 
        /* Decompress enough to encompass the slice and the offset */
        if (slicelimit >= 0)
            preslice = toast_decompress_datum_slice(tmp, slicelimit);
        else
            preslice = toast_decompress_datum(tmp);
 
        if (tmp != 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;
    }
    else if (slicelength < 0 || slicelimit > attrsize)
        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_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;
    struct varlena *result;
    struct varatt_external toast_pointer;
    int32       attrsize;
 
    if (!VARATT_IS_EXTERNAL_ONDISK(attr))
        elog(ERROR, "toast_fetch_datum shouldn't be called for non-ondisk datums");
 
    /* Must copy to access aligned fields */
    VARATT_EXTERNAL_GET_POINTER(toast_pointer, attr);
 
    attrsize = VARATT_EXTERNAL_GET_EXTSIZE(toast_pointer);
 
    result = (struct varlena *) palloc(attrsize + VARHDRSZ);
 
    if (VARATT_EXTERNAL_IS_COMPRESSED(toast_pointer))
        SET_VARSIZE_COMPRESSED(result, attrsize + VARHDRSZ);
    else
        SET_VARSIZE(result, attrsize + VARHDRSZ);
 
    if (attrsize == 0)
        return result;          /* Probably shouldn't happen, but just in
                                 * case. */
 
    /*
     * Open the toast relation and its indexes
     */
    toastrel = table_open(toast_pointer.va_toastrelid, AccessShareLock);
 
    /* Fetch all chunks */
    table_relation_fetch_toast_slice(toastrel, toast_pointer.va_valueid,
                                     attrsize, 0, attrsize, result);
 
    /* Close toast table */
    table_close(toastrel, AccessShareLock);
 
    return result;
}
 
/* ----------
 * toast_fetch_datum_slice -
 *
 *  Reconstruct a segment of a Datum from the chunks saved
 *  in the toast relation
 *
 *  Note that this function supports non-compressed external datums
 *  and compressed external datums (in which case the requested slice
 *  has to be a prefix, i.e. sliceoffset has to be 0).
 * ----------
 */
static struct varlena *
toast_fetch_datum_slice(struct varlena *attr, int32 sliceoffset,
                        int32 slicelength)
{
    Relation    toastrel;
    struct varlena *result;
    struct varatt_external toast_pointer;
    int32       attrsize;
 
    if (!VARATT_IS_EXTERNAL_ONDISK(attr))
        elog(ERROR, "toast_fetch_datum_slice shouldn't be called for non-ondisk datums");
 
    /* Must copy to access aligned fields */
    VARATT_EXTERNAL_GET_POINTER(toast_pointer, attr);
 
    /*
     * It's nonsense to fetch slices of a compressed datum unless when it's a
     * prefix -- this isn't lo_* we can't return a compressed datum which is
     * meaningful to toast later.
     */
    Assert(!VARATT_EXTERNAL_IS_COMPRESSED(toast_pointer) || 0 == sliceoffset);
 
    attrsize = VARATT_EXTERNAL_GET_EXTSIZE(toast_pointer);
 
    if (sliceoffset >= attrsize)
    {
        sliceoffset = 0;
        slicelength = 0;
    }
 
    /*
     * When fetching a prefix of a compressed external datum, account for the
     * space required by va_tcinfo, which is stored at the beginning as an
     * int32 value.
     */
    if (VARATT_EXTERNAL_IS_COMPRESSED(toast_pointer) && slicelength > 0)
        slicelength = slicelength + sizeof(int32);
 
    /*
     * Adjust length request if needed.  (Note: our sole caller,
     * detoast_attr_slice, protects us against sliceoffset + slicelength
     * overflowing.)
     */
    if (((sliceoffset + slicelength) > attrsize) || slicelength < 0)
        slicelength = attrsize - sliceoffset;
 
    result = (struct varlena *) palloc(slicelength + VARHDRSZ);
 
    if (VARATT_EXTERNAL_IS_COMPRESSED(toast_pointer))
        SET_VARSIZE_COMPRESSED(result, slicelength + VARHDRSZ);
    else
        SET_VARSIZE(result, slicelength + VARHDRSZ);
 
    if (slicelength == 0)
        return result;          /* Can save a lot of work at this point! */
 
    /* Open the toast relation */
    toastrel = table_open(toast_pointer.va_toastrelid, AccessShareLock);
 
    /* Fetch all chunks */
    table_relation_fetch_toast_slice(toastrel, toast_pointer.va_valueid,
                                     attrsize, sliceoffset, slicelength,
                                     result);
 
    /* Close toast table */
    table_close(toastrel, AccessShareLock);
 
    return result;
}
 
/* ----------
 * toast_decompress_datum -
 *
 * Decompress a compressed version of a varlena datum
 */
static struct varlena *
toast_decompress_datum(struct varlena *attr)
{
    ToastCompressionId cmid;
 
    Assert(VARATT_IS_COMPRESSED(attr));
 
    /*
     * Fetch the compression method id stored in the compression header and
     * decompress the data using the appropriate decompression routine.
     */
    cmid = TOAST_COMPRESS_METHOD(attr);
    switch (cmid)
    {
        case TOAST_PGLZ_COMPRESSION_ID:
            return pglz_decompress_datum(attr);
        case TOAST_LZ4_COMPRESSION_ID:
            return lz4_decompress_datum(attr);
        default:
            elog(ERROR, "invalid compression method id %d", cmid);
            return NULL;        /* keep compiler quiet */
    }
}
 
 
/* ----------
 * toast_decompress_datum_slice -
 *
 * Decompress the front of a compressed version of a varlena datum.
 * offset handling happens in detoast_attr_slice.
 * Here we just decompress a slice from the front.
 */
static struct varlena *
toast_decompress_datum_slice(struct varlena *attr, int32 slicelength)
{
    ToastCompressionId cmid;
 
    Assert(VARATT_IS_COMPRESSED(attr));
 
    /*
     * Some callers may pass a slicelength that's more than the actual
     * decompressed size.  If so, just decompress normally.  This avoids
     * possibly allocating a larger-than-necessary result object, and may be
     * faster and/or more robust as well.  Notably, some versions of liblz4
     * have been seen to give wrong results if passed an output size that is
     * more than the data's true decompressed size.
     */
    if ((uint32) slicelength >= TOAST_COMPRESS_EXTSIZE(attr))
        return toast_decompress_datum(attr);
 
    /*
     * Fetch the compression method id stored in the compression header and
     * decompress the data slice using the appropriate decompression routine.
     */
    cmid = TOAST_COMPRESS_METHOD(attr);
    switch (cmid)
    {
        case TOAST_PGLZ_COMPRESSION_ID:
            return pglz_decompress_datum_slice(attr, slicelength);
        case TOAST_LZ4_COMPRESSION_ID:
            return lz4_decompress_datum_slice(attr, slicelength);
        default:
            elog(ERROR, "invalid compression method id %d", cmid);
            return NULL;        /* keep compiler quiet */
    }
}
 
/* ----------
 * 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_ONDISK(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_EXTERNAL_INDIRECT(attr))
    {
        struct varatt_indirect toast_pointer;
 
        VARATT_EXTERNAL_GET_POINTER(toast_pointer, attr);
 
        /* nested indirect Datums aren't allowed */
        Assert(!VARATT_IS_EXTERNAL_INDIRECT(toast_pointer.pointer));
 
        return toast_raw_datum_size(PointerGetDatum(toast_pointer.pointer));
    }
    else if (VARATT_IS_EXTERNAL_EXPANDED(attr))
    {
        result = EOH_get_flat_size(DatumGetEOHP(value));
    }
    else if (VARATT_IS_COMPRESSED(attr))
    {
        /* here, va_rawsize is just the payload size */
        result = VARDATA_COMPRESSED_GET_EXTSIZE(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_ONDISK(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 = VARATT_EXTERNAL_GET_EXTSIZE(toast_pointer);
    }
    else if (VARATT_IS_EXTERNAL_INDIRECT(attr))
    {
        struct varatt_indirect toast_pointer;
 
        VARATT_EXTERNAL_GET_POINTER(toast_pointer, attr);
 
        /* nested indirect Datums aren't allowed */
        Assert(!VARATT_IS_EXTERNAL_INDIRECT(attr));
 
        return toast_datum_size(PointerGetDatum(toast_pointer.pointer));
    }
    else if (VARATT_IS_EXTERNAL_EXPANDED(attr))
    {
        result = EOH_get_flat_size(DatumGetEOHP(value));
    }
    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;
}