execCurrent.c

Go to the documentation of this file.

/*-------------------------------------------------------------------------
 *
 * execCurrent.c
 *    executor support for WHERE CURRENT OF cursor
 *
 * Portions Copyright (c) 1996-2016, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *  src/backend/executor/execCurrent.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.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);


/*
 * 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 = PortalGetQueryDesc(portal);
    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;
        ListCell   *lc;

        /*
         * 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;
        foreach(lc, queryDesc->estate->es_rowMarks)
        {
            ExecRowMark *thiserm = (ExecRowMark *) lfirst(lc);

            if (!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
    {
        ScanState  *scanstate;
        bool        lisnull;
        Oid tuple_tableoid PG_USED_FOR_ASSERTS_ONLY;
        ItemPointer tuple_tid;

        /*
         * 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 = search_plan_tree(queryDesc->planstate, table_oid);
        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 */
        if (TupIsNull(scanstate->ss_ScanTupleSlot))
            return false;

        /* Use slot_getattr to catch any possible mistakes */
        tuple_tableoid =
            DatumGetObjectId(slot_getattr(scanstate->ss_ScanTupleSlot,
                                          TableOidAttributeNumber,
                                          &lisnull));
        Assert(!lisnull);
        tuple_tid = (ItemPointer)
            DatumGetPointer(slot_getattr(scanstate->ss_ScanTupleSlot,
                                         SelfItemPointerAttributeNumber,
                                         &lisnull));
        Assert(!lisnull);

        Assert(tuple_tableoid == table_oid);

        *current_tid = *tuple_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 = &paramInfo->params[paramId - 1];

        /* give hook a chance in case parameter is dynamic */
        if (!OidIsValid(prm->ptype) && paramInfo->paramFetch != NULL)
            (*paramInfo->paramFetch) (paramInfo, paramId);

        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.
 */
static ScanState *
search_plan_tree(PlanState *node, Oid table_oid)
{
    if (node == NULL)
        return NULL;
    switch (nodeTag(node))
    {
            /*
             * Relation scan nodes can all be treated alike
             */
        case T_SeqScanState:
        case T_SampleScanState:
        case T_IndexScanState:
        case T_IndexOnlyScanState:
        case T_BitmapHeapScanState:
        case T_TidScanState:
        case T_ForeignScanState:
        case T_CustomScanState:
            {
                ScanState  *sstate = (ScanState *) node;

                if (RelationGetRelid(sstate->ss_currentRelation) == table_oid)
                    return sstate;
                break;
            }

            /*
             * For Append, we must look through the members; watch out for
             * multiple matches (possible if it was from UNION ALL)
             */
        case T_AppendState:
            {
                AppendState *astate = (AppendState *) node;
                ScanState  *result = NULL;
                int         i;

                for (i = 0; i < astate->as_nplans; i++)
                {
                    ScanState  *elem = search_plan_tree(astate->appendplans[i],
                                                        table_oid);

                    if (!elem)
                        continue;
                    if (result)
                        return NULL;    /* multiple matches */
                    result = elem;
                }
                return result;
            }

            /*
             * Similarly for MergeAppend
             */
        case T_MergeAppendState:
            {
                MergeAppendState *mstate = (MergeAppendState *) node;
                ScanState  *result = NULL;
                int         i;

                for (i = 0; i < mstate->ms_nplans; i++)
                {
                    ScanState  *elem = search_plan_tree(mstate->mergeplans[i],
                                                        table_oid);

                    if (!elem)
                        continue;
                    if (result)
                        return NULL;    /* multiple matches */
                    result = elem;
                }
                return result;
            }

            /*
             * 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:
            return search_plan_tree(node->lefttree, table_oid);

            /*
             * SubqueryScan too, but it keeps the child in a different place
             */
        case T_SubqueryScanState:
            return search_plan_tree(((SubqueryScanState *) node)->subplan,
                                    table_oid);

        default:
            /* Otherwise, assume we can't descend through it */
            break;
    }
    return NULL;
}