nodeSamplescan.c

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/*-------------------------------------------------------------------------
 *
 * nodeSamplescan.c
 *    Support routines for sample scans of relations (table sampling).
 *
 * Portions Copyright (c) 1996-2020, 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/relscan.h"
#include "access/tableam.h"
#include "access/tsmapi.h"
#include "executor/executor.h"
#include "executor/nodeSamplescan.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "storage/bufmgr.h"
#include "storage/predicate.h"
#include "utils/builtins.h"
#include "utils/rel.h"

static TupleTableSlot *SampleNext(SampleScanState *node);
static void tablesample_init(SampleScanState *scanstate);
static TupleTableSlot *tablesample_getnext(SampleScanState *scanstate);

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

/* ----------------------------------------------------------------
 *      SampleNext
 *
 *      This is a workhorse for ExecSampleScan
 * ----------------------------------------------------------------
 */
static TupleTableSlot *
SampleNext(SampleScanState *node)
{
    /*
     * 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
     */
    return tablesample_getnext(node);
}

/*
 * 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.
 * ----------------------------------------------------------------
 */
static TupleTableSlot *
ExecSampleScan(PlanState *pstate)
{
    SampleScanState *node = castNode(SampleScanState, pstate);

    return ExecScan(&node->ss,
                    (ExecScanAccessMtd) SampleNext,
                    (ExecScanRecheckMtd) SampleRecheck);
}

/* ----------------------------------------------------------------
 *      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;
    scanstate->ss.ps.ExecProcNode = ExecSampleScan;

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

    /*
     * open the scan relation
     */
    scanstate->ss.ss_currentRelation =
        ExecOpenScanRelation(estate,
                             node->scan.scanrelid,
                             eflags);

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

    /* and create slot with appropriate rowtype */
    ExecInitScanTupleSlot(estate, &scanstate->ss,
                          RelationGetDescr(scanstate->ss.ss_currentRelation),
                          table_slot_callbacks(scanstate->ss.ss_currentRelation));

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

    /*
     * 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);

    /*
     * 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
     */
    if (node->ss.ps.ps_ResultTupleSlot)
        ExecClearTuple(node->ss.ps.ps_ResultTupleSlot);
    ExecClearTuple(node->ss.ss_ScanTupleSlot);

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

/* ----------------------------------------------------------------
 *      ExecReScanSampleScan
 *
 *      Rescans the relation.
 *
 * ----------------------------------------------------------------
 */
void
ExecReScanSampleScan(SampleScanState *node)
{
    /* Remember we need to do BeginSampleScan again (if we did it at all) */
    node->begun = false;
    node->done = false;
    node->haveblock = false;
    node->donetuples = 0;

    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;

    scanstate->donetuples = 0;
    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 =
            table_beginscan_sampling(scanstate->ss.ss_currentRelation,
                                     scanstate->ss.ps.state->es_snapshot,
                                     0, NULL,
                                     scanstate->use_bulkread,
                                     allow_sync,
                                     scanstate->use_pagemode);
    }
    else
    {
        table_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.
 */
static TupleTableSlot *
tablesample_getnext(SampleScanState *scanstate)
{
    TableScanDesc scan = scanstate->ss.ss_currentScanDesc;
    TupleTableSlot *slot = scanstate->ss.ss_ScanTupleSlot;

    ExecClearTuple(slot);

    if (scanstate->done)
        return NULL;

    for (;;)
    {
        if (!scanstate->haveblock)
        {
            if (!table_scan_sample_next_block(scan, scanstate))
            {
                scanstate->haveblock = false;
                scanstate->done = true;

                /* exhausted relation */
                return NULL;
            }

            scanstate->haveblock = true;
        }

        if (!table_scan_sample_next_tuple(scan, scanstate, slot))
        {
            /*
             * If we get here, it means we've exhausted the items on this page
             * and it's time to move to the next.
             */
            scanstate->haveblock = false;
            continue;
        }

        /* Found visible tuple, return it. */
        break;
    }

    scanstate->donetuples++;

    return slot;
}