nodeTidscan.c

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/*-------------------------------------------------------------------------
 *
 * nodeTidscan.c
 *    Routines to support direct tid scans of relations
 *
 * Portions Copyright (c) 1996-2013, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/executor/nodeTidscan.c
 *
 *-------------------------------------------------------------------------
 */
/*
 * INTERFACE ROUTINES
 *
 *      ExecTidScan         scans a relation using tids
 *      ExecInitTidScan     creates and initializes state info.
 *      ExecReScanTidScan   rescans the tid relation.
 *      ExecEndTidScan      releases all storage.
 *      ExecTidMarkPos      marks scan position.
 *      ExecTidRestrPos     restores scan position.
 */
#include "postgres.h"

#include "access/sysattr.h"
#include "catalog/pg_type.h"
#include "executor/execdebug.h"
#include "executor/nodeTidscan.h"
#include "optimizer/clauses.h"
#include "storage/bufmgr.h"
#include "utils/array.h"
#include "utils/rel.h"


#define IsCTIDVar(node)  \
    ((node) != NULL && \
     IsA((node), Var) && \
     ((Var *) (node))->varattno == SelfItemPointerAttributeNumber && \
     ((Var *) (node))->varlevelsup == 0)

static void TidListCreate(TidScanState *tidstate);
static int  itemptr_comparator(const void *a, const void *b);
static TupleTableSlot *TidNext(TidScanState *node);


/*
 * Compute the list of TIDs to be visited, by evaluating the expressions
 * for them.
 *
 * (The result is actually an array, not a list.)
 */
static void
TidListCreate(TidScanState *tidstate)
{
    List       *evalList = tidstate->tss_tidquals;
    ExprContext *econtext = tidstate->ss.ps.ps_ExprContext;
    BlockNumber nblocks;
    ItemPointerData *tidList;
    int         numAllocTids;
    int         numTids;
    ListCell   *l;

    /*
     * We silently discard any TIDs that are out of range at the time of scan
     * start.  (Since we hold at least AccessShareLock on the table, it won't
     * be possible for someone to truncate away the blocks we intend to
     * visit.)
     */
    nblocks = RelationGetNumberOfBlocks(tidstate->ss.ss_currentRelation);

    /*
     * We initialize the array with enough slots for the case that all quals
     * are simple OpExprs or CurrentOfExprs.  If there are any
     * ScalarArrayOpExprs, we may have to enlarge the array.
     */
    numAllocTids = list_length(evalList);
    tidList = (ItemPointerData *)
        palloc(numAllocTids * sizeof(ItemPointerData));
    numTids = 0;
    tidstate->tss_isCurrentOf = false;

    foreach(l, evalList)
    {
        ExprState  *exstate = (ExprState *) lfirst(l);
        Expr       *expr = exstate->expr;
        ItemPointer itemptr;
        bool        isNull;

        if (is_opclause(expr))
        {
            FuncExprState *fexstate = (FuncExprState *) exstate;
            Node       *arg1;
            Node       *arg2;

            arg1 = get_leftop(expr);
            arg2 = get_rightop(expr);
            if (IsCTIDVar(arg1))
                exstate = (ExprState *) lsecond(fexstate->args);
            else if (IsCTIDVar(arg2))
                exstate = (ExprState *) linitial(fexstate->args);
            else
                elog(ERROR, "could not identify CTID variable");

            itemptr = (ItemPointer)
                DatumGetPointer(ExecEvalExprSwitchContext(exstate,
                                                          econtext,
                                                          &isNull,
                                                          NULL));
            if (!isNull &&
                ItemPointerIsValid(itemptr) &&
                ItemPointerGetBlockNumber(itemptr) < nblocks)
            {
                if (numTids >= numAllocTids)
                {
                    numAllocTids *= 2;
                    tidList = (ItemPointerData *)
                        repalloc(tidList,
                                 numAllocTids * sizeof(ItemPointerData));
                }
                tidList[numTids++] = *itemptr;
            }
        }
        else if (expr && IsA(expr, ScalarArrayOpExpr))
        {
            ScalarArrayOpExprState *saexstate = (ScalarArrayOpExprState *) exstate;
            Datum       arraydatum;
            ArrayType  *itemarray;
            Datum      *ipdatums;
            bool       *ipnulls;
            int         ndatums;
            int         i;

            exstate = (ExprState *) lsecond(saexstate->fxprstate.args);
            arraydatum = ExecEvalExprSwitchContext(exstate,
                                                   econtext,
                                                   &isNull,
                                                   NULL);
            if (isNull)
                continue;
            itemarray = DatumGetArrayTypeP(arraydatum);
            deconstruct_array(itemarray,
                              TIDOID, SizeOfIptrData, false, 's',
                              &ipdatums, &ipnulls, &ndatums);
            if (numTids + ndatums > numAllocTids)
            {
                numAllocTids = numTids + ndatums;
                tidList = (ItemPointerData *)
                    repalloc(tidList,
                             numAllocTids * sizeof(ItemPointerData));
            }
            for (i = 0; i < ndatums; i++)
            {
                if (!ipnulls[i])
                {
                    itemptr = (ItemPointer) DatumGetPointer(ipdatums[i]);
                    if (ItemPointerIsValid(itemptr) &&
                        ItemPointerGetBlockNumber(itemptr) < nblocks)
                        tidList[numTids++] = *itemptr;
                }
            }
            pfree(ipdatums);
            pfree(ipnulls);
        }
        else if (expr && IsA(expr, CurrentOfExpr))
        {
            CurrentOfExpr *cexpr = (CurrentOfExpr *) expr;
            ItemPointerData cursor_tid;

            if (execCurrentOf(cexpr, econtext,
                           RelationGetRelid(tidstate->ss.ss_currentRelation),
                              &cursor_tid))
            {
                if (numTids >= numAllocTids)
                {
                    numAllocTids *= 2;
                    tidList = (ItemPointerData *)
                        repalloc(tidList,
                                 numAllocTids * sizeof(ItemPointerData));
                }
                tidList[numTids++] = cursor_tid;
                tidstate->tss_isCurrentOf = true;
            }
        }
        else
            elog(ERROR, "could not identify CTID expression");
    }

    /*
     * Sort the array of TIDs into order, and eliminate duplicates.
     * Eliminating duplicates is necessary since we want OR semantics across
     * the list.  Sorting makes it easier to detect duplicates, and as a bonus
     * ensures that we will visit the heap in the most efficient way.
     */
    if (numTids > 1)
    {
        int         lastTid;
        int         i;

        /* CurrentOfExpr could never appear OR'd with something else */
        Assert(!tidstate->tss_isCurrentOf);

        qsort((void *) tidList, numTids, sizeof(ItemPointerData),
              itemptr_comparator);
        lastTid = 0;
        for (i = 1; i < numTids; i++)
        {
            if (!ItemPointerEquals(&tidList[lastTid], &tidList[i]))
                tidList[++lastTid] = tidList[i];
        }
        numTids = lastTid + 1;
    }

    tidstate->tss_TidList = tidList;
    tidstate->tss_NumTids = numTids;
    tidstate->tss_TidPtr = -1;
}

/*
 * qsort comparator for ItemPointerData items
 */
static int
itemptr_comparator(const void *a, const void *b)
{
    const ItemPointerData *ipa = (const ItemPointerData *) a;
    const ItemPointerData *ipb = (const ItemPointerData *) b;
    BlockNumber ba = ItemPointerGetBlockNumber(ipa);
    BlockNumber bb = ItemPointerGetBlockNumber(ipb);
    OffsetNumber oa = ItemPointerGetOffsetNumber(ipa);
    OffsetNumber ob = ItemPointerGetOffsetNumber(ipb);

    if (ba < bb)
        return -1;
    if (ba > bb)
        return 1;
    if (oa < ob)
        return -1;
    if (oa > ob)
        return 1;
    return 0;
}

/* ----------------------------------------------------------------
 *      TidNext
 *
 *      Retrieve a tuple from the TidScan node's currentRelation
 *      using the tids in the TidScanState information.
 *
 * ----------------------------------------------------------------
 */
static TupleTableSlot *
TidNext(TidScanState *node)
{
    EState     *estate;
    ScanDirection direction;
    Snapshot    snapshot;
    Relation    heapRelation;
    HeapTuple   tuple;
    TupleTableSlot *slot;
    Buffer      buffer = InvalidBuffer;
    ItemPointerData *tidList;
    int         numTids;
    bool        bBackward;

    /*
     * extract necessary information from tid scan node
     */
    estate = node->ss.ps.state;
    direction = estate->es_direction;
    snapshot = estate->es_snapshot;
    heapRelation = node->ss.ss_currentRelation;
    slot = node->ss.ss_ScanTupleSlot;

    /*
     * First time through, compute the list of TIDs to be visited
     */
    if (node->tss_TidList == NULL)
        TidListCreate(node);

    tidList = node->tss_TidList;
    numTids = node->tss_NumTids;

    tuple = &(node->tss_htup);

    /*
     * Initialize or advance scan position, depending on direction.
     */
    bBackward = ScanDirectionIsBackward(direction);
    if (bBackward)
    {
        if (node->tss_TidPtr < 0)
        {
            /* initialize for backward scan */
            node->tss_TidPtr = numTids - 1;
        }
        else
            node->tss_TidPtr--;
    }
    else
    {
        if (node->tss_TidPtr < 0)
        {
            /* initialize for forward scan */
            node->tss_TidPtr = 0;
        }
        else
            node->tss_TidPtr++;
    }

    while (node->tss_TidPtr >= 0 && node->tss_TidPtr < numTids)
    {
        tuple->t_self = tidList[node->tss_TidPtr];

        /*
         * For WHERE CURRENT OF, the tuple retrieved from the cursor might
         * since have been updated; if so, we should fetch the version that is
         * current according to our snapshot.
         */
        if (node->tss_isCurrentOf)
            heap_get_latest_tid(heapRelation, snapshot, &tuple->t_self);

        if (heap_fetch(heapRelation, snapshot, tuple, &buffer, false, NULL))
        {
            /*
             * store the scanned tuple in the scan tuple slot of the scan
             * state.  Eventually we will only do this and not return a tuple.
             * Note: we pass 'false' because tuples returned by amgetnext are
             * pointers onto disk pages and were not created with palloc() and
             * so should not be pfree()'d.
             */
            ExecStoreTuple(tuple,       /* tuple to store */
                           slot,    /* slot to store in */
                           buffer,      /* buffer associated with tuple  */
                           false);      /* don't pfree */

            /*
             * At this point we have an extra pin on the buffer, because
             * ExecStoreTuple incremented the pin count. Drop our local pin.
             */
            ReleaseBuffer(buffer);

            return slot;
        }
        /* Bad TID or failed snapshot qual; try next */
        if (bBackward)
            node->tss_TidPtr--;
        else
            node->tss_TidPtr++;
    }

    /*
     * if we get here it means the tid scan failed so we are at the end of the
     * scan..
     */
    return ExecClearTuple(slot);
}

/*
 * TidRecheck -- access method routine to recheck a tuple in EvalPlanQual
 */
static bool
TidRecheck(TidScanState *node, TupleTableSlot *slot)
{
    /*
     * XXX shouldn't we check here to make sure tuple matches TID list? In
     * runtime-key case this is not certain, is it?  However, in the WHERE
     * CURRENT OF case it might not match anyway ...
     */
    return true;
}


/* ----------------------------------------------------------------
 *      ExecTidScan(node)
 *
 *      Scans the relation using tids and returns
 *         the next qualifying tuple in the direction specified.
 *      We call the ExecScan() routine and pass it the appropriate
 *      access method functions.
 *
 *      Conditions:
 *        -- the "cursor" maintained by the AMI is positioned at the tuple
 *           returned previously.
 *
 *      Initial States:
 *        -- the relation indicated is opened for scanning so that the
 *           "cursor" is positioned before the first qualifying tuple.
 *        -- tidPtr is -1.
 * ----------------------------------------------------------------
 */
TupleTableSlot *
ExecTidScan(TidScanState *node)
{
    return ExecScan(&node->ss,
                    (ExecScanAccessMtd) TidNext,
                    (ExecScanRecheckMtd) TidRecheck);
}

/* ----------------------------------------------------------------
 *      ExecReScanTidScan(node)
 * ----------------------------------------------------------------
 */
void
ExecReScanTidScan(TidScanState *node)
{
    if (node->tss_TidList)
        pfree(node->tss_TidList);
    node->tss_TidList = NULL;
    node->tss_NumTids = 0;
    node->tss_TidPtr = -1;

    ExecScanReScan(&node->ss);
}

/* ----------------------------------------------------------------
 *      ExecEndTidScan
 *
 *      Releases any storage allocated through C routines.
 *      Returns nothing.
 * ----------------------------------------------------------------
 */
void
ExecEndTidScan(TidScanState *node)
{
    /*
     * Free the exprcontext
     */
    ExecFreeExprContext(&node->ss.ps);

    /*
     * clear out tuple table slots
     */
    ExecClearTuple(node->ss.ps.ps_ResultTupleSlot);
    ExecClearTuple(node->ss.ss_ScanTupleSlot);

    /*
     * close the heap relation.
     */
    ExecCloseScanRelation(node->ss.ss_currentRelation);
}

/* ----------------------------------------------------------------
 *      ExecTidMarkPos
 *
 *      Marks scan position by marking the current tid.
 *      Returns nothing.
 * ----------------------------------------------------------------
 */
void
ExecTidMarkPos(TidScanState *node)
{
    node->tss_MarkTidPtr = node->tss_TidPtr;
}

/* ----------------------------------------------------------------
 *      ExecTidRestrPos
 *
 *      Restores scan position by restoring the current tid.
 *      Returns nothing.
 *
 *      XXX Assumes previously marked scan position belongs to current tid
 * ----------------------------------------------------------------
 */
void
ExecTidRestrPos(TidScanState *node)
{
    node->tss_TidPtr = node->tss_MarkTidPtr;
}

/* ----------------------------------------------------------------
 *      ExecInitTidScan
 *
 *      Initializes the tid scan's state information, creates
 *      scan keys, and opens the base and tid relations.
 *
 *      Parameters:
 *        node: TidNode node produced by the planner.
 *        estate: the execution state initialized in InitPlan.
 * ----------------------------------------------------------------
 */
TidScanState *
ExecInitTidScan(TidScan *node, EState *estate, int eflags)
{
    TidScanState *tidstate;
    Relation    currentRelation;

    /*
     * create state structure
     */
    tidstate = makeNode(TidScanState);
    tidstate->ss.ps.plan = (Plan *) node;
    tidstate->ss.ps.state = estate;

    /*
     * Miscellaneous initialization
     *
     * create expression context for node
     */
    ExecAssignExprContext(estate, &tidstate->ss.ps);

    tidstate->ss.ps.ps_TupFromTlist = false;

    /*
     * initialize child expressions
     */
    tidstate->ss.ps.targetlist = (List *)
        ExecInitExpr((Expr *) node->scan.plan.targetlist,
                     (PlanState *) tidstate);
    tidstate->ss.ps.qual = (List *)
        ExecInitExpr((Expr *) node->scan.plan.qual,
                     (PlanState *) tidstate);

    tidstate->tss_tidquals = (List *)
        ExecInitExpr((Expr *) node->tidquals,
                     (PlanState *) tidstate);

    /*
     * tuple table initialization
     */
    ExecInitResultTupleSlot(estate, &tidstate->ss.ps);
    ExecInitScanTupleSlot(estate, &tidstate->ss);

    /*
     * mark tid list as not computed yet
     */
    tidstate->tss_TidList = NULL;
    tidstate->tss_NumTids = 0;
    tidstate->tss_TidPtr = -1;

    /*
     * open the base relation and acquire appropriate lock on it.
     */
    currentRelation = ExecOpenScanRelation(estate, node->scan.scanrelid, eflags);

    tidstate->ss.ss_currentRelation = currentRelation;
    tidstate->ss.ss_currentScanDesc = NULL;     /* no heap scan here */

    /*
     * get the scan type from the relation descriptor.
     */
    ExecAssignScanType(&tidstate->ss, RelationGetDescr(currentRelation));

    /*
     * Initialize result tuple type and projection info.
     */
    ExecAssignResultTypeFromTL(&tidstate->ss.ps);
    ExecAssignScanProjectionInfo(&tidstate->ss);

    /*
     * all done.
     */
    return tidstate;
}