nodeSamplescan.c

Go to the documentation of this file.

/*-------------------------------------------------------------------------
 *
 * nodeSamplescan.c
 *    Support routines for sample scans of relations (table sampling).
 *
 * Portions Copyright (c) 1996-2017, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/executor/nodeSamplescan.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "access/hash.h"
#include "access/relscan.h"
#include "access/tsmapi.h"
#include "executor/executor.h"
#include "executor/nodeSamplescan.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "storage/predicate.h"
#include "utils/builtins.h"
#include "utils/rel.h"
#include "utils/tqual.h"

static void InitScanRelation(SampleScanState *node, EState *estate, int eflags);
static TupleTableSlot *SampleNext(SampleScanState *node);
static void tablesample_init(SampleScanState *scanstate);
static HeapTuple tablesample_getnext(SampleScanState *scanstate);
static bool SampleTupleVisible(HeapTuple tuple, OffsetNumber tupoffset,
                   HeapScanDesc scan);

/* ----------------------------------------------------------------
 *                      Scan Support
 * ----------------------------------------------------------------
 */

/* ----------------------------------------------------------------
 *      SampleNext
 *
 *      This is a workhorse for ExecSampleScan
 * ----------------------------------------------------------------
 */
static TupleTableSlot *
SampleNext(SampleScanState *node)
{
    HeapTuple   tuple;
    TupleTableSlot *slot;

    /*
     * if this is first call within a scan, initialize
     */
    if (!node->begun)
        tablesample_init(node);

    /*
     * get the next tuple, and store it in our result slot
     */
    tuple = tablesample_getnext(node);

    slot = node->ss.ss_ScanTupleSlot;

    if (tuple)
        ExecStoreTuple(tuple,   /* tuple to store */
                       slot,    /* slot to store in */
                       node->ss.ss_currentScanDesc->rs_cbuf,    /* tuple's buffer */
                       false);  /* don't pfree this pointer */
    else
        ExecClearTuple(slot);

    return slot;
}

/*
 * SampleRecheck -- access method routine to recheck a tuple in EvalPlanQual
 */
static bool
SampleRecheck(SampleScanState *node, TupleTableSlot *slot)
{
    /*
     * No need to recheck for SampleScan, since like SeqScan we don't pass any
     * checkable keys to heap_beginscan.
     */
    return true;
}

/* ----------------------------------------------------------------
 *      ExecSampleScan(node)
 *
 *      Scans the relation using the sampling method and returns
 *      the next qualifying tuple.
 *      We call the ExecScan() routine and pass it the appropriate
 *      access method functions.
 * ----------------------------------------------------------------
 */
TupleTableSlot *
ExecSampleScan(SampleScanState *node)
{
    return ExecScan((ScanState *) node,
                    (ExecScanAccessMtd) SampleNext,
                    (ExecScanRecheckMtd) SampleRecheck);
}

/* ----------------------------------------------------------------
 *      InitScanRelation
 *
 *      Set up to access the scan relation.
 * ----------------------------------------------------------------
 */
static void
InitScanRelation(SampleScanState *node, EState *estate, int eflags)
{
    Relation    currentRelation;

    /*
     * get the relation object id from the relid'th entry in the range table,
     * open that relation and acquire appropriate lock on it.
     */
    currentRelation = ExecOpenScanRelation(estate,
                                           ((SampleScan *) node->ss.ps.plan)->scan.scanrelid,
                                           eflags);

    node->ss.ss_currentRelation = currentRelation;

    /* we won't set up the HeapScanDesc till later */
    node->ss.ss_currentScanDesc = NULL;

    /* and report the scan tuple slot's rowtype */
    ExecAssignScanType(&node->ss, RelationGetDescr(currentRelation));
}


/* ----------------------------------------------------------------
 *      ExecInitSampleScan
 * ----------------------------------------------------------------
 */
SampleScanState *
ExecInitSampleScan(SampleScan *node, EState *estate, int eflags)
{
    SampleScanState *scanstate;
    TableSampleClause *tsc = node->tablesample;
    TsmRoutine *tsm;

    Assert(outerPlan(node) == NULL);
    Assert(innerPlan(node) == NULL);

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

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

    /*
     * initialize child expressions
     */
    scanstate->ss.ps.qual =
        ExecInitQual(node->scan.plan.qual, (PlanState *) scanstate);

    scanstate->args = ExecInitExprList(tsc->args, (PlanState *) scanstate);
    scanstate->repeatable =
        ExecInitExpr(tsc->repeatable, (PlanState *) scanstate);

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

    /*
     * initialize scan relation
     */
    InitScanRelation(scanstate, estate, eflags);

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

    /*
     * If we don't have a REPEATABLE clause, select a random seed.  We want to
     * do this just once, since the seed shouldn't change over rescans.
     */
    if (tsc->repeatable == NULL)
        scanstate->seed = random();

    /*
     * Finally, initialize the TABLESAMPLE method handler.
     */
    tsm = GetTsmRoutine(tsc->tsmhandler);
    scanstate->tsmroutine = tsm;
    scanstate->tsm_state = NULL;

    if (tsm->InitSampleScan)
        tsm->InitSampleScan(scanstate, eflags);

    /* We'll do BeginSampleScan later; we can't evaluate params yet */
    scanstate->begun = false;

    return scanstate;
}

/* ----------------------------------------------------------------
 *      ExecEndSampleScan
 *
 *      frees any storage allocated through C routines.
 * ----------------------------------------------------------------
 */
void
ExecEndSampleScan(SampleScanState *node)
{
    /*
     * Tell sampling function that we finished the scan.
     */
    if (node->tsmroutine->EndSampleScan)
        node->tsmroutine->EndSampleScan(node);

    /*
     * Free the exprcontext
     */
    ExecFreeExprContext(&node->ss.ps);

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

    /*
     * close heap scan
     */
    if (node->ss.ss_currentScanDesc)
        heap_endscan(node->ss.ss_currentScanDesc);

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

/* ----------------------------------------------------------------
 *      ExecReScanSampleScan
 *
 *      Rescans the relation.
 *
 * ----------------------------------------------------------------
 */
void
ExecReScanSampleScan(SampleScanState *node)
{
    /* Remember we need to do BeginSampleScan again (if we did it at all) */
    node->begun = false;

    ExecScanReScan(&node->ss);
}


/*
 * Initialize the TABLESAMPLE method: evaluate params and call BeginSampleScan.
 */
static void
tablesample_init(SampleScanState *scanstate)
{
    TsmRoutine *tsm = scanstate->tsmroutine;
    ExprContext *econtext = scanstate->ss.ps.ps_ExprContext;
    Datum      *params;
    Datum       datum;
    bool        isnull;
    uint32      seed;
    bool        allow_sync;
    int         i;
    ListCell   *arg;

    params = (Datum *) palloc(list_length(scanstate->args) * sizeof(Datum));

    i = 0;
    foreach(arg, scanstate->args)
    {
        ExprState  *argstate = (ExprState *) lfirst(arg);

        params[i] = ExecEvalExprSwitchContext(argstate,
                                              econtext,
                                              &isnull);
        if (isnull)
            ereport(ERROR,
                    (errcode(ERRCODE_INVALID_TABLESAMPLE_ARGUMENT),
                     errmsg("TABLESAMPLE parameter cannot be null")));
        i++;
    }

    if (scanstate->repeatable)
    {
        datum = ExecEvalExprSwitchContext(scanstate->repeatable,
                                          econtext,
                                          &isnull);
        if (isnull)
            ereport(ERROR,
                    (errcode(ERRCODE_INVALID_TABLESAMPLE_REPEAT),
                     errmsg("TABLESAMPLE REPEATABLE parameter cannot be null")));

        /*
         * The REPEATABLE parameter has been coerced to float8 by the parser.
         * The reason for using float8 at the SQL level is that it will
         * produce unsurprising results both for users used to databases that
         * accept only integers in the REPEATABLE clause and for those who
         * might expect that REPEATABLE works like setseed() (a float in the
         * range from -1 to 1).
         *
         * We use hashfloat8() to convert the supplied value into a suitable
         * seed.  For regression-testing purposes, that has the convenient
         * property that REPEATABLE(0) gives a machine-independent result.
         */
        seed = DatumGetUInt32(DirectFunctionCall1(hashfloat8, datum));
    }
    else
    {
        /* Use the seed selected by ExecInitSampleScan */
        seed = scanstate->seed;
    }

    /* Set default values for params that BeginSampleScan can adjust */
    scanstate->use_bulkread = true;
    scanstate->use_pagemode = true;

    /* Let tablesample method do its thing */
    tsm->BeginSampleScan(scanstate,
                         params,
                         list_length(scanstate->args),
                         seed);

    /* We'll use syncscan if there's no NextSampleBlock function */
    allow_sync = (tsm->NextSampleBlock == NULL);

    /* Now we can create or reset the HeapScanDesc */
    if (scanstate->ss.ss_currentScanDesc == NULL)
    {
        scanstate->ss.ss_currentScanDesc =
            heap_beginscan_sampling(scanstate->ss.ss_currentRelation,
                                    scanstate->ss.ps.state->es_snapshot,
                                    0, NULL,
                                    scanstate->use_bulkread,
                                    allow_sync,
                                    scanstate->use_pagemode);
    }
    else
    {
        heap_rescan_set_params(scanstate->ss.ss_currentScanDesc, NULL,
                               scanstate->use_bulkread,
                               allow_sync,
                               scanstate->use_pagemode);
    }

    pfree(params);

    /* And we're initialized. */
    scanstate->begun = true;
}

/*
 * Get next tuple from TABLESAMPLE method.
 *
 * Note: an awful lot of this is copied-and-pasted from heapam.c.  It would
 * perhaps be better to refactor to share more code.
 */
static HeapTuple
tablesample_getnext(SampleScanState *scanstate)
{
    TsmRoutine *tsm = scanstate->tsmroutine;
    HeapScanDesc scan = scanstate->ss.ss_currentScanDesc;
    HeapTuple   tuple = &(scan->rs_ctup);
    Snapshot    snapshot = scan->rs_snapshot;
    bool        pagemode = scan->rs_pageatatime;
    BlockNumber blockno;
    Page        page;
    bool        all_visible;
    OffsetNumber maxoffset;

    if (!scan->rs_inited)
    {
        /*
         * return null immediately if relation is empty
         */
        if (scan->rs_nblocks == 0)
        {
            Assert(!BufferIsValid(scan->rs_cbuf));
            tuple->t_data = NULL;
            return NULL;
        }
        if (tsm->NextSampleBlock)
        {
            blockno = tsm->NextSampleBlock(scanstate);
            if (!BlockNumberIsValid(blockno))
            {
                tuple->t_data = NULL;
                return NULL;
            }
        }
        else
            blockno = scan->rs_startblock;
        Assert(blockno < scan->rs_nblocks);
        heapgetpage(scan, blockno);
        scan->rs_inited = true;
    }
    else
    {
        /* continue from previously returned page/tuple */
        blockno = scan->rs_cblock;  /* current page */
    }

    /*
     * When not using pagemode, we must lock the buffer during tuple
     * visibility checks.
     */
    if (!pagemode)
        LockBuffer(scan->rs_cbuf, BUFFER_LOCK_SHARE);

    page = (Page) BufferGetPage(scan->rs_cbuf);
    all_visible = PageIsAllVisible(page) && !snapshot->takenDuringRecovery;
    maxoffset = PageGetMaxOffsetNumber(page);

    for (;;)
    {
        OffsetNumber tupoffset;
        bool        finished;

        CHECK_FOR_INTERRUPTS();

        /* Ask the tablesample method which tuples to check on this page. */
        tupoffset = tsm->NextSampleTuple(scanstate,
                                         blockno,
                                         maxoffset);

        if (OffsetNumberIsValid(tupoffset))
        {
            ItemId      itemid;
            bool        visible;

            /* Skip invalid tuple pointers. */
            itemid = PageGetItemId(page, tupoffset);
            if (!ItemIdIsNormal(itemid))
                continue;

            tuple->t_data = (HeapTupleHeader) PageGetItem(page, itemid);
            tuple->t_len = ItemIdGetLength(itemid);
            ItemPointerSet(&(tuple->t_self), blockno, tupoffset);

            if (all_visible)
                visible = true;
            else
                visible = SampleTupleVisible(tuple, tupoffset, scan);

            /* in pagemode, heapgetpage did this for us */
            if (!pagemode)
                CheckForSerializableConflictOut(visible, scan->rs_rd, tuple,
                                                scan->rs_cbuf, snapshot);

            if (visible)
            {
                /* Found visible tuple, return it. */
                if (!pagemode)
                    LockBuffer(scan->rs_cbuf, BUFFER_LOCK_UNLOCK);
                break;
            }
            else
            {
                /* Try next tuple from same page. */
                continue;
            }
        }

        /*
         * if we get here, it means we've exhausted the items on this page and
         * it's time to move to the next.
         */
        if (!pagemode)
            LockBuffer(scan->rs_cbuf, BUFFER_LOCK_UNLOCK);

        if (tsm->NextSampleBlock)
        {
            blockno = tsm->NextSampleBlock(scanstate);
            Assert(!scan->rs_syncscan);
            finished = !BlockNumberIsValid(blockno);
        }
        else
        {
            /* Without NextSampleBlock, just do a plain forward seqscan. */
            blockno++;
            if (blockno >= scan->rs_nblocks)
                blockno = 0;

            /*
             * Report our new scan position for synchronization purposes.
             *
             * Note: we do this before checking for end of scan so that the
             * final state of the position hint is back at the start of the
             * rel.  That's not strictly necessary, but otherwise when you run
             * the same query multiple times the starting position would shift
             * a little bit backwards on every invocation, which is confusing.
             * We don't guarantee any specific ordering in general, though.
             */
            if (scan->rs_syncscan)
                ss_report_location(scan->rs_rd, blockno);

            finished = (blockno == scan->rs_startblock);
        }

        /*
         * Reached end of scan?
         */
        if (finished)
        {
            if (BufferIsValid(scan->rs_cbuf))
                ReleaseBuffer(scan->rs_cbuf);
            scan->rs_cbuf = InvalidBuffer;
            scan->rs_cblock = InvalidBlockNumber;
            tuple->t_data = NULL;
            scan->rs_inited = false;
            return NULL;
        }

        Assert(blockno < scan->rs_nblocks);
        heapgetpage(scan, blockno);

        /* Re-establish state for new page */
        if (!pagemode)
            LockBuffer(scan->rs_cbuf, BUFFER_LOCK_SHARE);

        page = (Page) BufferGetPage(scan->rs_cbuf);
        all_visible = PageIsAllVisible(page) && !snapshot->takenDuringRecovery;
        maxoffset = PageGetMaxOffsetNumber(page);
    }

    /* Count successfully-fetched tuples as heap fetches */
    pgstat_count_heap_getnext(scan->rs_rd);

    return &(scan->rs_ctup);
}

/*
 * Check visibility of the tuple.
 */
static bool
SampleTupleVisible(HeapTuple tuple, OffsetNumber tupoffset, HeapScanDesc scan)
{
    if (scan->rs_pageatatime)
    {
        /*
         * In pageatatime mode, heapgetpage() already did visibility checks,
         * so just look at the info it left in rs_vistuples[].
         *
         * We use a binary search over the known-sorted array.  Note: we could
         * save some effort if we insisted that NextSampleTuple select tuples
         * in increasing order, but it's not clear that there would be enough
         * gain to justify the restriction.
         */
        int         start = 0,
                    end = scan->rs_ntuples - 1;

        while (start <= end)
        {
            int         mid = (start + end) / 2;
            OffsetNumber curoffset = scan->rs_vistuples[mid];

            if (tupoffset == curoffset)
                return true;
            else if (tupoffset < curoffset)
                end = mid - 1;
            else
                start = mid + 1;
        }

        return false;
    }
    else
    {
        /* Otherwise, we have to check the tuple individually. */
        return HeapTupleSatisfiesVisibility(tuple,
                                            scan->rs_snapshot,
                                            scan->rs_cbuf);
    }
}