portalcmds.c

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/*-------------------------------------------------------------------------
 *
 * portalcmds.c
 *    Utility commands affecting portals (that is, SQL cursor commands)
 *
 * Note: see also tcop/pquery.c, which implements portal operations for
 * the FE/BE protocol.  This module uses pquery.c for some operations.
 * And both modules depend on utils/mmgr/portalmem.c, which controls
 * storage management for portals (but doesn't run any queries in them).
 *
 *
 * Portions Copyright (c) 1996-2013, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/commands/portalcmds.c
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include <limits.h>

#include "access/xact.h"
#include "commands/portalcmds.h"
#include "executor/executor.h"
#include "executor/tstoreReceiver.h"
#include "tcop/pquery.h"
#include "utils/memutils.h"
#include "utils/snapmgr.h"


/*
 * PerformCursorOpen
 *      Execute SQL DECLARE CURSOR command.
 *
 * The query has already been through parse analysis, rewriting, and planning.
 * When it gets here, it looks like a SELECT PlannedStmt, except that the
 * utilityStmt field is set.
 */
void
PerformCursorOpen(PlannedStmt *stmt, ParamListInfo params,
                  const char *queryString, bool isTopLevel)
{
    DeclareCursorStmt *cstmt = (DeclareCursorStmt *) stmt->utilityStmt;
    Portal      portal;
    MemoryContext oldContext;

    if (cstmt == NULL || !IsA(cstmt, DeclareCursorStmt))
        elog(ERROR, "PerformCursorOpen called for non-cursor query");

    /*
     * Disallow empty-string cursor name (conflicts with protocol-level
     * unnamed portal).
     */
    if (!cstmt->portalname || cstmt->portalname[0] == '\0')
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_CURSOR_NAME),
                 errmsg("invalid cursor name: must not be empty")));

    /*
     * If this is a non-holdable cursor, we require that this statement has
     * been executed inside a transaction block (or else, it would have no
     * user-visible effect).
     */
    if (!(cstmt->options & CURSOR_OPT_HOLD))
        RequireTransactionChain(isTopLevel, "DECLARE CURSOR");

    /*
     * Create a portal and copy the plan and queryString into its memory.
     */
    portal = CreatePortal(cstmt->portalname, false, false);

    oldContext = MemoryContextSwitchTo(PortalGetHeapMemory(portal));

    stmt = copyObject(stmt);
    stmt->utilityStmt = NULL;   /* make it look like plain SELECT */

    queryString = pstrdup(queryString);

    PortalDefineQuery(portal,
                      NULL,
                      queryString,
                      "SELECT", /* cursor's query is always a SELECT */
                      list_make1(stmt),
                      NULL);

    /*----------
     * Also copy the outer portal's parameter list into the inner portal's
     * memory context.  We want to pass down the parameter values in case we
     * had a command like
     *      DECLARE c CURSOR FOR SELECT ... WHERE foo = $1
     * This will have been parsed using the outer parameter set and the
     * parameter value needs to be preserved for use when the cursor is
     * executed.
     *----------
     */
    params = copyParamList(params);

    MemoryContextSwitchTo(oldContext);

    /*
     * Set up options for portal.
     *
     * If the user didn't specify a SCROLL type, allow or disallow scrolling
     * based on whether it would require any additional runtime overhead to do
     * so.  Also, we disallow scrolling for FOR UPDATE cursors.
     */
    portal->cursorOptions = cstmt->options;
    if (!(portal->cursorOptions & (CURSOR_OPT_SCROLL | CURSOR_OPT_NO_SCROLL)))
    {
        if (stmt->rowMarks == NIL &&
            ExecSupportsBackwardScan(stmt->planTree))
            portal->cursorOptions |= CURSOR_OPT_SCROLL;
        else
            portal->cursorOptions |= CURSOR_OPT_NO_SCROLL;
    }

    /*
     * Start execution, inserting parameters if any.
     */
    PortalStart(portal, params, 0, GetActiveSnapshot());

    Assert(portal->strategy == PORTAL_ONE_SELECT);

    /*
     * We're done; the query won't actually be run until PerformPortalFetch is
     * called.
     */
}

/*
 * PerformPortalFetch
 *      Execute SQL FETCH or MOVE command.
 *
 *  stmt: parsetree node for command
 *  dest: where to send results
 *  completionTag: points to a buffer of size COMPLETION_TAG_BUFSIZE
 *      in which to store a command completion status string.
 *
 * completionTag may be NULL if caller doesn't want a status string.
 */
void
PerformPortalFetch(FetchStmt *stmt,
                   DestReceiver *dest,
                   char *completionTag)
{
    Portal      portal;
    long        nprocessed;

    /*
     * Disallow empty-string cursor name (conflicts with protocol-level
     * unnamed portal).
     */
    if (!stmt->portalname || stmt->portalname[0] == '\0')
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_CURSOR_NAME),
                 errmsg("invalid cursor name: must not be empty")));

    /* get the portal from the portal name */
    portal = GetPortalByName(stmt->portalname);
    if (!PortalIsValid(portal))
    {
        ereport(ERROR,
                (errcode(ERRCODE_UNDEFINED_CURSOR),
                 errmsg("cursor \"%s\" does not exist", stmt->portalname)));
        return;                 /* keep compiler happy */
    }

    /* Adjust dest if needed.  MOVE wants destination DestNone */
    if (stmt->ismove)
        dest = None_Receiver;

    /* Do it */
    nprocessed = PortalRunFetch(portal,
                                stmt->direction,
                                stmt->howMany,
                                dest);

    /* Return command status if wanted */
    if (completionTag)
        snprintf(completionTag, COMPLETION_TAG_BUFSIZE, "%s %ld",
                 stmt->ismove ? "MOVE" : "FETCH",
                 nprocessed);
}

/*
 * PerformPortalClose
 *      Close a cursor.
 */
void
PerformPortalClose(const char *name)
{
    Portal      portal;

    /* NULL means CLOSE ALL */
    if (name == NULL)
    {
        PortalHashTableDeleteAll();
        return;
    }

    /*
     * Disallow empty-string cursor name (conflicts with protocol-level
     * unnamed portal).
     */
    if (name[0] == '\0')
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_CURSOR_NAME),
                 errmsg("invalid cursor name: must not be empty")));

    /*
     * get the portal from the portal name
     */
    portal = GetPortalByName(name);
    if (!PortalIsValid(portal))
    {
        ereport(ERROR,
                (errcode(ERRCODE_UNDEFINED_CURSOR),
                 errmsg("cursor \"%s\" does not exist", name)));
        return;                 /* keep compiler happy */
    }

    /*
     * Note: PortalCleanup is called as a side-effect, if not already done.
     */
    PortalDrop(portal, false);
}

/*
 * PortalCleanup
 *
 * Clean up a portal when it's dropped.  This is the standard cleanup hook
 * for portals.
 *
 * Note: if portal->status is PORTAL_FAILED, we are probably being called
 * during error abort, and must be careful to avoid doing anything that
 * is likely to fail again.
 */
void
PortalCleanup(Portal portal)
{
    QueryDesc  *queryDesc;

    /*
     * sanity checks
     */
    AssertArg(PortalIsValid(portal));
    AssertArg(portal->cleanup == PortalCleanup);

    /*
     * Shut down executor, if still running.  We skip this during error abort,
     * since other mechanisms will take care of releasing executor resources,
     * and we can't be sure that ExecutorEnd itself wouldn't fail.
     */
    queryDesc = PortalGetQueryDesc(portal);
    if (queryDesc)
    {
        /*
         * Reset the queryDesc before anything else.  This prevents us from
         * trying to shut down the executor twice, in case of an error below.
         * The transaction abort mechanisms will take care of resource cleanup
         * in such a case.
         */
        portal->queryDesc = NULL;

        if (portal->status != PORTAL_FAILED)
        {
            ResourceOwner saveResourceOwner;

            /* We must make the portal's resource owner current */
            saveResourceOwner = CurrentResourceOwner;
            PG_TRY();
            {
                CurrentResourceOwner = portal->resowner;
                ExecutorFinish(queryDesc);
                ExecutorEnd(queryDesc);
                FreeQueryDesc(queryDesc);
            }
            PG_CATCH();
            {
                /* Ensure CurrentResourceOwner is restored on error */
                CurrentResourceOwner = saveResourceOwner;
                PG_RE_THROW();
            }
            PG_END_TRY();
            CurrentResourceOwner = saveResourceOwner;
        }
    }
}

/*
 * PersistHoldablePortal
 *
 * Prepare the specified Portal for access outside of the current
 * transaction. When this function returns, all future accesses to the
 * portal must be done via the Tuplestore (not by invoking the
 * executor).
 */
void
PersistHoldablePortal(Portal portal)
{
    QueryDesc  *queryDesc = PortalGetQueryDesc(portal);
    Portal      saveActivePortal;
    ResourceOwner saveResourceOwner;
    MemoryContext savePortalContext;
    MemoryContext oldcxt;

    /*
     * If we're preserving a holdable portal, we had better be inside the
     * transaction that originally created it.
     */
    Assert(portal->createSubid != InvalidSubTransactionId);
    Assert(queryDesc != NULL);

    /*
     * Caller must have created the tuplestore already.
     */
    Assert(portal->holdContext != NULL);
    Assert(portal->holdStore != NULL);

    /*
     * Before closing down the executor, we must copy the tupdesc into
     * long-term memory, since it was created in executor memory.
     */
    oldcxt = MemoryContextSwitchTo(portal->holdContext);

    portal->tupDesc = CreateTupleDescCopy(portal->tupDesc);

    MemoryContextSwitchTo(oldcxt);

    /*
     * Check for improper portal use, and mark portal active.
     */
    if (portal->status != PORTAL_READY)
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("portal \"%s\" cannot be run", portal->name)));
    portal->status = PORTAL_ACTIVE;

    /*
     * Set up global portal context pointers.
     */
    saveActivePortal = ActivePortal;
    saveResourceOwner = CurrentResourceOwner;
    savePortalContext = PortalContext;
    PG_TRY();
    {
        ActivePortal = portal;
        CurrentResourceOwner = portal->resowner;
        PortalContext = PortalGetHeapMemory(portal);

        MemoryContextSwitchTo(PortalContext);

        PushActiveSnapshot(queryDesc->snapshot);

        /*
         * Rewind the executor: we need to store the entire result set in the
         * tuplestore, so that subsequent backward FETCHs can be processed.
         */
        ExecutorRewind(queryDesc);

        /*
         * Change the destination to output to the tuplestore.  Note we tell
         * the tuplestore receiver to detoast all data passed through it.
         */
        queryDesc->dest = CreateDestReceiver(DestTuplestore);
        SetTuplestoreDestReceiverParams(queryDesc->dest,
                                        portal->holdStore,
                                        portal->holdContext,
                                        true);

        /* Fetch the result set into the tuplestore */
        ExecutorRun(queryDesc, ForwardScanDirection, 0L);

        (*queryDesc->dest->rDestroy) (queryDesc->dest);
        queryDesc->dest = NULL;

        /*
         * Now shut down the inner executor.
         */
        portal->queryDesc = NULL;       /* prevent double shutdown */
        ExecutorFinish(queryDesc);
        ExecutorEnd(queryDesc);
        FreeQueryDesc(queryDesc);

        /*
         * Set the position in the result set: ideally, this could be
         * implemented by just skipping straight to the tuple # that we need
         * to be at, but the tuplestore API doesn't support that. So we start
         * at the beginning of the tuplestore and iterate through it until we
         * reach where we need to be.  FIXME someday?  (Fortunately, the
         * typical case is that we're supposed to be at or near the start of
         * the result set, so this isn't as bad as it sounds.)
         */
        MemoryContextSwitchTo(portal->holdContext);

        if (portal->atEnd)
        {
            /* we can handle this case even if posOverflow */
            while (tuplestore_advance(portal->holdStore, true))
                 /* continue */ ;
        }
        else
        {
            long        store_pos;

            if (portal->posOverflow)    /* oops, cannot trust portalPos */
                ereport(ERROR,
                        (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                         errmsg("could not reposition held cursor")));

            tuplestore_rescan(portal->holdStore);

            for (store_pos = 0; store_pos < portal->portalPos; store_pos++)
            {
                if (!tuplestore_advance(portal->holdStore, true))
                    elog(ERROR, "unexpected end of tuple stream");
            }
        }
    }
    PG_CATCH();
    {
        /* Uncaught error while executing portal: mark it dead */
        MarkPortalFailed(portal);

        /* Restore global vars and propagate error */
        ActivePortal = saveActivePortal;
        CurrentResourceOwner = saveResourceOwner;
        PortalContext = savePortalContext;

        PG_RE_THROW();
    }
    PG_END_TRY();

    MemoryContextSwitchTo(oldcxt);

    /* Mark portal not active */
    portal->status = PORTAL_READY;

    ActivePortal = saveActivePortal;
    CurrentResourceOwner = saveResourceOwner;
    PortalContext = savePortalContext;

    PopActiveSnapshot();

    /*
     * We can now release any subsidiary memory of the portal's heap context;
     * we'll never use it again.  The executor already dropped its context,
     * but this will clean up anything that glommed onto the portal's heap via
     * PortalContext.
     */
    MemoryContextDeleteChildren(PortalGetHeapMemory(portal));
}