toast_helper.c

Go to the documentation of this file.

/*-------------------------------------------------------------------------
 *
 * toast_helper.c
 *    Helper functions for table AMs implementing compressed or
 *    out-of-line storage of varlena attributes.
 *
 * Copyright (c) 2000-2026, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *    src/backend/access/table/toast_helper.c
 *
 *-------------------------------------------------------------------------
 */
 
#include "postgres.h"
 
#include "access/detoast.h"
#include "access/toast_helper.h"
#include "access/toast_internals.h"
#include "catalog/pg_type_d.h"
#include "varatt.h"
 
 
/*
 * Prepare to TOAST a tuple.
 *
 * tupleDesc, toast_values, and toast_isnull are required parameters; they
 * provide the necessary details about the tuple to be toasted.
 *
 * toast_oldvalues and toast_oldisnull should be NULL for a newly-inserted
 * tuple; for an update, they should describe the existing tuple.
 *
 * All of these arrays should have a length equal to tupleDesc->natts.
 *
 * On return, toast_flags and toast_attr will have been initialized.
 * toast_flags is just a single uint8, but toast_attr is a caller-provided
 * array with a length equal to tupleDesc->natts.  The caller need not
 * perform any initialization of the array before calling this function.
 */
void
toast_tuple_init(ToastTupleContext *ttc)
{
    TupleDesc   tupleDesc = ttc->ttc_rel->rd_att;
    int         numAttrs = tupleDesc->natts;
    int         i;
 
    ttc->ttc_flags = 0;
 
    for (i = 0; i < numAttrs; i++)
    {
        Form_pg_attribute att = TupleDescAttr(tupleDesc, i);
        struct varlena *old_value;
        struct varlena *new_value;
 
        ttc->ttc_attr[i].tai_colflags = 0;
        ttc->ttc_attr[i].tai_oldexternal = NULL;
        ttc->ttc_attr[i].tai_compression = att->attcompression;
 
        if (ttc->ttc_oldvalues != NULL)
        {
            /*
             * For UPDATE get the old and new values of this attribute
             */
            old_value =
                (struct varlena *) DatumGetPointer(ttc->ttc_oldvalues[i]);
            new_value =
                (struct varlena *) DatumGetPointer(ttc->ttc_values[i]);
 
            /*
             * If the old value is stored on disk, check if it has changed so
             * we have to delete it later.
             */
            if (att->attlen == -1 && !ttc->ttc_oldisnull[i] &&
                VARATT_IS_EXTERNAL_ONDISK(old_value))
            {
                if (ttc->ttc_isnull[i] ||
                    !VARATT_IS_EXTERNAL_ONDISK(new_value) ||
                    memcmp(old_value, new_value,
                           VARSIZE_EXTERNAL(old_value)) != 0)
                {
                    /*
                     * The old external stored value isn't needed any more
                     * after the update
                     */
                    ttc->ttc_attr[i].tai_colflags |= TOASTCOL_NEEDS_DELETE_OLD;
                    ttc->ttc_flags |= TOAST_NEEDS_DELETE_OLD;
                }
                else
                {
                    /*
                     * This attribute isn't changed by this update so we reuse
                     * the original reference to the old value in the new
                     * tuple.
                     */
                    ttc->ttc_attr[i].tai_colflags |= TOASTCOL_IGNORE;
                    continue;
                }
            }
        }
        else
        {
            /*
             * For INSERT simply get the new value
             */
            new_value = (struct varlena *) DatumGetPointer(ttc->ttc_values[i]);
        }
 
        /*
         * Handle NULL attributes
         */
        if (ttc->ttc_isnull[i])
        {
            ttc->ttc_attr[i].tai_colflags |= TOASTCOL_IGNORE;
            ttc->ttc_flags |= TOAST_HAS_NULLS;
            continue;
        }
 
        /*
         * Now look at varlena attributes
         */
        if (att->attlen == -1)
        {
            /*
             * If the table's attribute says PLAIN always, force it so.
             */
            if (att->attstorage == TYPSTORAGE_PLAIN)
                ttc->ttc_attr[i].tai_colflags |= TOASTCOL_IGNORE;
 
            /*
             * We took care of UPDATE above, so any external value we find
             * still in the tuple must be someone else's that we cannot reuse
             * (this includes the case of an out-of-line in-memory datum).
             * 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))
            {
                ttc->ttc_attr[i].tai_oldexternal = new_value;
                if (att->attstorage == TYPSTORAGE_PLAIN)
                    new_value = detoast_attr(new_value);
                else
                    new_value = detoast_external_attr(new_value);
                ttc->ttc_values[i] = PointerGetDatum(new_value);
                ttc->ttc_attr[i].tai_colflags |= TOASTCOL_NEEDS_FREE;
                ttc->ttc_flags |= (TOAST_NEEDS_CHANGE | TOAST_NEEDS_FREE);
            }
 
            /*
             * Remember the size of this attribute
             */
            ttc->ttc_attr[i].tai_size = VARSIZE_ANY(new_value);
        }
        else
        {
            /*
             * Not a varlena attribute, plain storage always
             */
            ttc->ttc_attr[i].tai_colflags |= TOASTCOL_IGNORE;
        }
    }
}
 
/*
 * Find the largest varlena attribute that satisfies certain criteria.
 *
 * The relevant column must not be marked TOASTCOL_IGNORE, and if the
 * for_compression flag is passed as true, it must also not be marked
 * TOASTCOL_INCOMPRESSIBLE.
 *
 * The column must have attstorage EXTERNAL or EXTENDED if check_main is
 * false, and must have attstorage MAIN if check_main is true.
 *
 * The column must have a minimum size of MAXALIGN(TOAST_POINTER_SIZE);
 * if not, no benefit is to be expected by compressing it.
 *
 * The return value is the index of the biggest suitable column, or
 * -1 if there is none.
 */
int
toast_tuple_find_biggest_attribute(ToastTupleContext *ttc,
                                   bool for_compression, bool check_main)
{
    TupleDesc   tupleDesc = ttc->ttc_rel->rd_att;
    int         numAttrs = tupleDesc->natts;
    int         biggest_attno = -1;
    int32       biggest_size = MAXALIGN(TOAST_POINTER_SIZE);
    int32       skip_colflags = TOASTCOL_IGNORE;
    int         i;
 
    if (for_compression)
        skip_colflags |= TOASTCOL_INCOMPRESSIBLE;
 
    for (i = 0; i < numAttrs; i++)
    {
        Form_pg_attribute att = TupleDescAttr(tupleDesc, i);
 
        if ((ttc->ttc_attr[i].tai_colflags & skip_colflags) != 0)
            continue;
        if (VARATT_IS_EXTERNAL(DatumGetPointer(ttc->ttc_values[i])))
            continue;           /* can't happen, toast_action would be PLAIN */
        if (for_compression &&
            VARATT_IS_COMPRESSED(DatumGetPointer(ttc->ttc_values[i])))
            continue;
        if (check_main && att->attstorage != TYPSTORAGE_MAIN)
            continue;
        if (!check_main && att->attstorage != TYPSTORAGE_EXTENDED &&
            att->attstorage != TYPSTORAGE_EXTERNAL)
            continue;
 
        if (ttc->ttc_attr[i].tai_size > biggest_size)
        {
            biggest_attno = i;
            biggest_size = ttc->ttc_attr[i].tai_size;
        }
    }
 
    return biggest_attno;
}
 
/*
 * Try compression for an attribute.
 *
 * If we find that the attribute is not compressible, mark it so.
 */
void
toast_tuple_try_compression(ToastTupleContext *ttc, int attribute)
{
    Datum      *value = &ttc->ttc_values[attribute];
    Datum       new_value;
    ToastAttrInfo *attr = &ttc->ttc_attr[attribute];
 
    new_value = toast_compress_datum(*value, attr->tai_compression);
 
    if (DatumGetPointer(new_value) != NULL)
    {
        /* successful compression */
        if ((attr->tai_colflags & TOASTCOL_NEEDS_FREE) != 0)
            pfree(DatumGetPointer(*value));
        *value = new_value;
        attr->tai_colflags |= TOASTCOL_NEEDS_FREE;
        attr->tai_size = VARSIZE(DatumGetPointer(*value));
        ttc->ttc_flags |= (TOAST_NEEDS_CHANGE | TOAST_NEEDS_FREE);
    }
    else
    {
        /* incompressible, ignore on subsequent compression passes */
        attr->tai_colflags |= TOASTCOL_INCOMPRESSIBLE;
    }
}
 
/*
 * Move an attribute to external storage.
 */
void
toast_tuple_externalize(ToastTupleContext *ttc, int attribute, int options)
{
    Datum      *value = &ttc->ttc_values[attribute];
    Datum       old_value = *value;
    ToastAttrInfo *attr = &ttc->ttc_attr[attribute];
 
    attr->tai_colflags |= TOASTCOL_IGNORE;
    *value = toast_save_datum(ttc->ttc_rel, old_value, attr->tai_oldexternal,
                              options);
    if ((attr->tai_colflags & TOASTCOL_NEEDS_FREE) != 0)
        pfree(DatumGetPointer(old_value));
    attr->tai_colflags |= TOASTCOL_NEEDS_FREE;
    ttc->ttc_flags |= (TOAST_NEEDS_CHANGE | TOAST_NEEDS_FREE);
}
 
/*
 * Perform appropriate cleanup after one tuple has been subjected to TOAST.
 */
void
toast_tuple_cleanup(ToastTupleContext *ttc)
{
    TupleDesc   tupleDesc = ttc->ttc_rel->rd_att;
    int         numAttrs = tupleDesc->natts;
 
    /*
     * Free allocated temp values
     */
    if ((ttc->ttc_flags & TOAST_NEEDS_FREE) != 0)
    {
        int         i;
 
        for (i = 0; i < numAttrs; i++)
        {
            ToastAttrInfo *attr = &ttc->ttc_attr[i];
 
            if ((attr->tai_colflags & TOASTCOL_NEEDS_FREE) != 0)
                pfree(DatumGetPointer(ttc->ttc_values[i]));
        }
    }
 
    /*
     * Delete external values from the old tuple
     */
    if ((ttc->ttc_flags & TOAST_NEEDS_DELETE_OLD) != 0)
    {
        int         i;
 
        for (i = 0; i < numAttrs; i++)
        {
            ToastAttrInfo *attr = &ttc->ttc_attr[i];
 
            if ((attr->tai_colflags & TOASTCOL_NEEDS_DELETE_OLD) != 0)
                toast_delete_datum(ttc->ttc_rel, ttc->ttc_oldvalues[i], false);
        }
    }
}
 
/*
 * Check for external stored attributes and delete them from the secondary
 * relation.
 */
void
toast_delete_external(Relation rel, const Datum *values, const bool *isnull,
                      bool is_speculative)
{
    TupleDesc   tupleDesc = rel->rd_att;
    int         numAttrs = tupleDesc->natts;
    int         i;
 
    for (i = 0; i < numAttrs; i++)
    {
        if (TupleDescCompactAttr(tupleDesc, i)->attlen == -1)
        {
            Datum       value = values[i];
 
            if (isnull[i])
                continue;
            else if (VARATT_IS_EXTERNAL_ONDISK(DatumGetPointer(value)))
                toast_delete_datum(rel, value, is_speculative);
        }
    }
}