execCurrent.c

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/*-------------------------------------------------------------------------
 *
 * execCurrent.c
 *    executor support for WHERE CURRENT OF cursor
 *
 * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *  src/backend/executor/execCurrent.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include "access/genam.h"
#include "access/relscan.h"
#include "access/sysattr.h"
#include "catalog/pg_type.h"
#include "executor/executor.h"
#include "utils/builtins.h"
#include "utils/lsyscache.h"
#include "utils/portal.h"
#include "utils/rel.h"
 
 
static char *fetch_cursor_param_value(ExprContext *econtext, int paramId);
static ScanState *search_plan_tree(PlanState *node, Oid table_oid,
                                   bool *pending_rescan);
 
 
/*
 * execCurrentOf
 *
 * Given a CURRENT OF expression and the OID of a table, determine which row
 * of the table is currently being scanned by the cursor named by CURRENT OF,
 * and return the row's TID into *current_tid.
 *
 * Returns true if a row was identified.  Returns false if the cursor is valid
 * for the table but is not currently scanning a row of the table (this is a
 * legal situation in inheritance cases).  Raises error if cursor is not a
 * valid updatable scan of the specified table.
 */
bool
execCurrentOf(CurrentOfExpr *cexpr,
              ExprContext *econtext,
              Oid table_oid,
              ItemPointer current_tid)
{
    char       *cursor_name;
    char       *table_name;
    Portal      portal;
    QueryDesc  *queryDesc;
 
    /* Get the cursor name --- may have to look up a parameter reference */
    if (cexpr->cursor_name)
        cursor_name = cexpr->cursor_name;
    else
        cursor_name = fetch_cursor_param_value(econtext, cexpr->cursor_param);
 
    /* Fetch table name for possible use in error messages */
    table_name = get_rel_name(table_oid);
    if (table_name == NULL)
        elog(ERROR, "cache lookup failed for relation %u", table_oid);
 
    /* Find the cursor's portal */
    portal = GetPortalByName(cursor_name);
    if (!PortalIsValid(portal))
        ereport(ERROR,
                (errcode(ERRCODE_UNDEFINED_CURSOR),
                 errmsg("cursor \"%s\" does not exist", cursor_name)));
 
    /*
     * We have to watch out for non-SELECT queries as well as held cursors,
     * both of which may have null queryDesc.
     */
    if (portal->strategy != PORTAL_ONE_SELECT)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_CURSOR_STATE),
                 errmsg("cursor \"%s\" is not a SELECT query",
                        cursor_name)));
    queryDesc = portal->queryDesc;
    if (queryDesc == NULL || queryDesc->estate == NULL)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_CURSOR_STATE),
                 errmsg("cursor \"%s\" is held from a previous transaction",
                        cursor_name)));
 
    /*
     * We have two different strategies depending on whether the cursor uses
     * FOR UPDATE/SHARE or not.  The reason for supporting both is that the
     * FOR UPDATE code is able to identify a target table in many cases where
     * the other code can't, while the non-FOR-UPDATE case allows use of WHERE
     * CURRENT OF with an insensitive cursor.
     */
    if (queryDesc->estate->es_rowmarks)
    {
        ExecRowMark *erm;
        Index       i;
 
        /*
         * Here, the query must have exactly one FOR UPDATE/SHARE reference to
         * the target table, and we dig the ctid info out of that.
         */
        erm = NULL;
        for (i = 0; i < queryDesc->estate->es_range_table_size; i++)
        {
            ExecRowMark *thiserm = queryDesc->estate->es_rowmarks[i];
 
            if (thiserm == NULL ||
                !RowMarkRequiresRowShareLock(thiserm->markType))
                continue;       /* ignore non-FOR UPDATE/SHARE items */
 
            if (thiserm->relid == table_oid)
            {
                if (erm)
                    ereport(ERROR,
                            (errcode(ERRCODE_INVALID_CURSOR_STATE),
                             errmsg("cursor \"%s\" has multiple FOR UPDATE/SHARE references to table \"%s\"",
                                    cursor_name, table_name)));
                erm = thiserm;
            }
        }
 
        if (erm == NULL)
            ereport(ERROR,
                    (errcode(ERRCODE_INVALID_CURSOR_STATE),
                     errmsg("cursor \"%s\" does not have a FOR UPDATE/SHARE reference to table \"%s\"",
                            cursor_name, table_name)));
 
        /*
         * The cursor must have a current result row: per the SQL spec, it's
         * an error if not.
         */
        if (portal->atStart || portal->atEnd)
            ereport(ERROR,
                    (errcode(ERRCODE_INVALID_CURSOR_STATE),
                     errmsg("cursor \"%s\" is not positioned on a row",
                            cursor_name)));
 
        /* Return the currently scanned TID, if there is one */
        if (ItemPointerIsValid(&(erm->curCtid)))
        {
            *current_tid = erm->curCtid;
            return true;
        }
 
        /*
         * This table didn't produce the cursor's current row; some other
         * inheritance child of the same parent must have.  Signal caller to
         * do nothing on this table.
         */
        return false;
    }
    else
    {
        /*
         * Without FOR UPDATE, we dig through the cursor's plan to find the
         * scan node.  Fail if it's not there or buried underneath
         * aggregation.
         */
        ScanState  *scanstate;
        bool        pending_rescan = false;
 
        scanstate = search_plan_tree(queryDesc->planstate, table_oid,
                                     &pending_rescan);
        if (!scanstate)
            ereport(ERROR,
                    (errcode(ERRCODE_INVALID_CURSOR_STATE),
                     errmsg("cursor \"%s\" is not a simply updatable scan of table \"%s\"",
                            cursor_name, table_name)));
 
        /*
         * The cursor must have a current result row: per the SQL spec, it's
         * an error if not.  We test this at the top level, rather than at the
         * scan node level, because in inheritance cases any one table scan
         * could easily not be on a row. We want to return false, not raise
         * error, if the passed-in table OID is for one of the inactive scans.
         */
        if (portal->atStart || portal->atEnd)
            ereport(ERROR,
                    (errcode(ERRCODE_INVALID_CURSOR_STATE),
                     errmsg("cursor \"%s\" is not positioned on a row",
                            cursor_name)));
 
        /*
         * Now OK to return false if we found an inactive scan.  It is
         * inactive either if it's not positioned on a row, or there's a
         * rescan pending for it.
         */
        if (TupIsNull(scanstate->ss_ScanTupleSlot) || pending_rescan)
            return false;
 
        /*
         * Extract TID of the scan's current row.  The mechanism for this is
         * in principle scan-type-dependent, but for most scan types, we can
         * just dig the TID out of the physical scan tuple.
         */
        if (IsA(scanstate, IndexOnlyScanState))
        {
            /*
             * For IndexOnlyScan, the tuple stored in ss_ScanTupleSlot may be
             * a virtual tuple that does not have the ctid column, so we have
             * to get the TID from xs_heaptid.
             */
            IndexScanDesc scan = ((IndexOnlyScanState *) scanstate)->ioss_ScanDesc;
 
            *current_tid = scan->xs_heaptid;
        }
        else
        {
            /*
             * Default case: try to fetch TID from the scan node's current
             * tuple.  As an extra cross-check, verify tableoid in the current
             * tuple.  If the scan hasn't provided a physical tuple, we have
             * to fail.
             */
            Datum       ldatum;
            bool        lisnull;
            ItemPointer tuple_tid;
 
#ifdef USE_ASSERT_CHECKING
            ldatum = slot_getsysattr(scanstate->ss_ScanTupleSlot,
                                     TableOidAttributeNumber,
                                     &lisnull);
            if (lisnull)
                ereport(ERROR,
                        (errcode(ERRCODE_INVALID_CURSOR_STATE),
                         errmsg("cursor \"%s\" is not a simply updatable scan of table \"%s\"",
                                cursor_name, table_name)));
            Assert(DatumGetObjectId(ldatum) == table_oid);
#endif
 
            ldatum = slot_getsysattr(scanstate->ss_ScanTupleSlot,
                                     SelfItemPointerAttributeNumber,
                                     &lisnull);
            if (lisnull)
                ereport(ERROR,
                        (errcode(ERRCODE_INVALID_CURSOR_STATE),
                         errmsg("cursor \"%s\" is not a simply updatable scan of table \"%s\"",
                                cursor_name, table_name)));
            tuple_tid = (ItemPointer) DatumGetPointer(ldatum);
 
            *current_tid = *tuple_tid;
        }
 
        Assert(ItemPointerIsValid(current_tid));
 
        return true;
    }
}
 
/*
 * fetch_cursor_param_value
 *
 * Fetch the string value of a param, verifying it is of type REFCURSOR.
 */
static char *
fetch_cursor_param_value(ExprContext *econtext, int paramId)
{
    ParamListInfo paramInfo = econtext->ecxt_param_list_info;
 
    if (paramInfo &&
        paramId > 0 && paramId <= paramInfo->numParams)
    {
        ParamExternData *prm;
        ParamExternData prmdata;
 
        /* give hook a chance in case parameter is dynamic */
        if (paramInfo->paramFetch != NULL)
            prm = paramInfo->paramFetch(paramInfo, paramId, false, &prmdata);
        else
            prm = &paramInfo->params[paramId - 1];
 
        if (OidIsValid(prm->ptype) && !prm->isnull)
        {
            /* safety check in case hook did something unexpected */
            if (prm->ptype != REFCURSOROID)
                ereport(ERROR,
                        (errcode(ERRCODE_DATATYPE_MISMATCH),
                         errmsg("type of parameter %d (%s) does not match that when preparing the plan (%s)",
                                paramId,
                                format_type_be(prm->ptype),
                                format_type_be(REFCURSOROID))));
 
            /* We know that refcursor uses text's I/O routines */
            return TextDatumGetCString(prm->value);
        }
    }
 
    ereport(ERROR,
            (errcode(ERRCODE_UNDEFINED_OBJECT),
             errmsg("no value found for parameter %d", paramId)));
    return NULL;
}
 
/*
 * search_plan_tree
 *
 * Search through a PlanState tree for a scan node on the specified table.
 * Return NULL if not found or multiple candidates.
 *
 * CAUTION: this function is not charged simply with finding some candidate
 * scan, but with ensuring that that scan returned the plan tree's current
 * output row.  That's why we must reject multiple-match cases.
 *
 * If a candidate is found, set *pending_rescan to true if that candidate
 * or any node above it has a pending rescan action, i.e. chgParam != NULL.
 * That indicates that we shouldn't consider the node to be positioned on a
 * valid tuple, even if its own state would indicate that it is.  (Caller
 * must initialize *pending_rescan to false, and should not trust its state
 * if multiple candidates are found.)
 */
static ScanState *
search_plan_tree(PlanState *node, Oid table_oid,
                 bool *pending_rescan)
{
    ScanState  *result = NULL;
 
    if (node == NULL)
        return NULL;
    switch (nodeTag(node))
    {
            /*
             * Relation scan nodes can all be treated alike: check to see if
             * they are scanning the specified table.
             *
             * ForeignScan and CustomScan might not have a currentRelation, in
             * which case we just ignore them.  (We dare not descend to any
             * child plan nodes they might have, since we do not know the
             * relationship of such a node's current output tuple to the
             * children's current outputs.)
             */
        case T_SeqScanState:
        case T_SampleScanState:
        case T_IndexScanState:
        case T_IndexOnlyScanState:
        case T_BitmapHeapScanState:
        case T_TidScanState:
        case T_TidRangeScanState:
        case T_ForeignScanState:
        case T_CustomScanState:
            {
                ScanState  *sstate = (ScanState *) node;
 
                if (sstate->ss_currentRelation &&
                    RelationGetRelid(sstate->ss_currentRelation) == table_oid)
                    result = sstate;
                break;
            }
 
            /*
             * For Append, we can check each input node.  It is safe to
             * descend to the inputs because only the input that resulted in
             * the Append's current output node could be positioned on a tuple
             * at all; the other inputs are either at EOF or not yet started.
             * Hence, if the desired table is scanned by some
             * currently-inactive input node, we will find that node but then
             * our caller will realize that it didn't emit the tuple of
             * interest.
             *
             * We do need to watch out for multiple matches (possible if
             * Append was from UNION ALL rather than an inheritance tree).
             *
             * Note: we can NOT descend through MergeAppend similarly, since
             * its inputs are likely all active, and we don't know which one
             * returned the current output tuple.  (Perhaps that could be
             * fixed if we were to let this code know more about MergeAppend's
             * internal state, but it does not seem worth the trouble.  Users
             * should not expect plans for ORDER BY queries to be considered
             * simply-updatable, since they won't be if the sorting is
             * implemented by a Sort node.)
             */
        case T_AppendState:
            {
                AppendState *astate = (AppendState *) node;
                int         i;
 
                for (i = 0; i < astate->as_nplans; i++)
                {
                    ScanState  *elem = search_plan_tree(astate->appendplans[i],
                                                        table_oid,
                                                        pending_rescan);
 
                    if (!elem)
                        continue;
                    if (result)
                        return NULL;    /* multiple matches */
                    result = elem;
                }
                break;
            }
 
            /*
             * Result and Limit can be descended through (these are safe
             * because they always return their input's current row)
             */
        case T_ResultState:
        case T_LimitState:
            result = search_plan_tree(outerPlanState(node),
                                      table_oid,
                                      pending_rescan);
            break;
 
            /*
             * SubqueryScan too, but it keeps the child in a different place
             */
        case T_SubqueryScanState:
            result = search_plan_tree(((SubqueryScanState *) node)->subplan,
                                      table_oid,
                                      pending_rescan);
            break;
 
        default:
            /* Otherwise, assume we can't descend through it */
            break;
    }
 
    /*
     * If we found a candidate at or below this node, then this node's
     * chgParam indicates a pending rescan that will affect the candidate.
     */
    if (result && node->chgParam != NULL)
        *pending_rescan = true;
 
    return result;
}