nodeTidscan.c

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/*-------------------------------------------------------------------------
 *
 * nodeTidscan.c
 *    Routines to support direct tid scans of relations
 *
 * Portions Copyright (c) 1996-2023, 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.
 */
#include "postgres.h"
 
#include "access/sysattr.h"
#include "access/tableam.h"
#include "catalog/pg_type.h"
#include "executor/execdebug.h"
#include "executor/nodeTidscan.h"
#include "lib/qunique.h"
#include "miscadmin.h"
#include "nodes/nodeFuncs.h"
#include "storage/bufmgr.h"
#include "utils/array.h"
#include "utils/rel.h"
 
 
/*
 * It's sufficient to check varattno to identify the CTID variable, as any
 * Var in the relation scan qual must be for our table.  (Even if it's a
 * parameterized scan referencing some other table's CTID, the other table's
 * Var would have become a Param by the time it gets here.)
 */
#define IsCTIDVar(node)  \
    ((node) != NULL && \
     IsA((node), Var) && \
     ((Var *) (node))->varattno == SelfItemPointerAttributeNumber)
 
/* one element in tss_tidexprs */
typedef struct TidExpr
{
    ExprState  *exprstate;      /* ExprState for a TID-yielding subexpr */
    bool        isarray;        /* if true, it yields tid[] not just tid */
    CurrentOfExpr *cexpr;       /* alternatively, we can have CURRENT OF */
} TidExpr;
 
static void TidExprListCreate(TidScanState *tidstate);
static void TidListEval(TidScanState *tidstate);
static int  itemptr_comparator(const void *a, const void *b);
static TupleTableSlot *TidNext(TidScanState *node);
 
 
/*
 * Extract the qual subexpressions that yield TIDs to search for,
 * and compile them into ExprStates if they're ordinary expressions.
 *
 * CURRENT OF is a special case that we can't compile usefully;
 * just drop it into the TidExpr list as-is.
 */
static void
TidExprListCreate(TidScanState *tidstate)
{
    TidScan    *node = (TidScan *) tidstate->ss.ps.plan;
    ListCell   *l;
 
    tidstate->tss_tidexprs = NIL;
    tidstate->tss_isCurrentOf = false;
 
    foreach(l, node->tidquals)
    {
        Expr       *expr = (Expr *) lfirst(l);
        TidExpr    *tidexpr = (TidExpr *) palloc0(sizeof(TidExpr));
 
        if (is_opclause(expr))
        {
            Node       *arg1;
            Node       *arg2;
 
            arg1 = get_leftop(expr);
            arg2 = get_rightop(expr);
            if (IsCTIDVar(arg1))
                tidexpr->exprstate = ExecInitExpr((Expr *) arg2,
                                                  &tidstate->ss.ps);
            else if (IsCTIDVar(arg2))
                tidexpr->exprstate = ExecInitExpr((Expr *) arg1,
                                                  &tidstate->ss.ps);
            else
                elog(ERROR, "could not identify CTID variable");
            tidexpr->isarray = false;
        }
        else if (expr && IsA(expr, ScalarArrayOpExpr))
        {
            ScalarArrayOpExpr *saex = (ScalarArrayOpExpr *) expr;
 
            Assert(IsCTIDVar(linitial(saex->args)));
            tidexpr->exprstate = ExecInitExpr(lsecond(saex->args),
                                              &tidstate->ss.ps);
            tidexpr->isarray = true;
        }
        else if (expr && IsA(expr, CurrentOfExpr))
        {
            CurrentOfExpr *cexpr = (CurrentOfExpr *) expr;
 
            tidexpr->cexpr = cexpr;
            tidstate->tss_isCurrentOf = true;
        }
        else
            elog(ERROR, "could not identify CTID expression");
 
        tidstate->tss_tidexprs = lappend(tidstate->tss_tidexprs, tidexpr);
    }
 
    /* CurrentOfExpr could never appear OR'd with something else */
    Assert(list_length(tidstate->tss_tidexprs) == 1 ||
           !tidstate->tss_isCurrentOf);
}
 
/*
 * 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
TidListEval(TidScanState *tidstate)
{
    ExprContext *econtext = tidstate->ss.ps.ps_ExprContext;
    TableScanDesc scan;
    ItemPointerData *tidList;
    int         numAllocTids;
    int         numTids;
    ListCell   *l;
 
    /*
     * Start scan on-demand - initializing a scan isn't free (e.g. heap stats
     * the size of the table), so it makes sense to delay that until needed -
     * the node might never get executed.
     */
    if (tidstate->ss.ss_currentScanDesc == NULL)
        tidstate->ss.ss_currentScanDesc =
            table_beginscan_tid(tidstate->ss.ss_currentRelation,
                                tidstate->ss.ps.state->es_snapshot);
    scan = tidstate->ss.ss_currentScanDesc;
 
    /*
     * 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(tidstate->tss_tidexprs);
    tidList = (ItemPointerData *)
        palloc(numAllocTids * sizeof(ItemPointerData));
    numTids = 0;
 
    foreach(l, tidstate->tss_tidexprs)
    {
        TidExpr    *tidexpr = (TidExpr *) lfirst(l);
        ItemPointer itemptr;
        bool        isNull;
 
        if (tidexpr->exprstate && !tidexpr->isarray)
        {
            itemptr = (ItemPointer)
                DatumGetPointer(ExecEvalExprSwitchContext(tidexpr->exprstate,
                                                          econtext,
                                                          &isNull));
            if (isNull)
                continue;
 
            /*
             * We silently discard any TIDs that the AM considers invalid
             * (E.g. for heap, they could be 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.).
             */
            if (!table_tuple_tid_valid(scan, itemptr))
                continue;
 
            if (numTids >= numAllocTids)
            {
                numAllocTids *= 2;
                tidList = (ItemPointerData *)
                    repalloc(tidList,
                             numAllocTids * sizeof(ItemPointerData));
            }
            tidList[numTids++] = *itemptr;
        }
        else if (tidexpr->exprstate && tidexpr->isarray)
        {
            Datum       arraydatum;
            ArrayType  *itemarray;
            Datum      *ipdatums;
            bool       *ipnulls;
            int         ndatums;
            int         i;
 
            arraydatum = ExecEvalExprSwitchContext(tidexpr->exprstate,
                                                   econtext,
                                                   &isNull);
            if (isNull)
                continue;
            itemarray = DatumGetArrayTypeP(arraydatum);
            deconstruct_array_builtin(itemarray, TIDOID, &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])
                    continue;
 
                itemptr = (ItemPointer) DatumGetPointer(ipdatums[i]);
 
                if (!table_tuple_tid_valid(scan, itemptr))
                    continue;
 
                tidList[numTids++] = *itemptr;
            }
            pfree(ipdatums);
            pfree(ipnulls);
        }
        else
        {
            ItemPointerData cursor_tid;
 
            Assert(tidexpr->cexpr);
            if (execCurrentOf(tidexpr->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;
            }
        }
    }
 
    /*
     * 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)
    {
        /* CurrentOfExpr could never appear OR'd with something else */
        Assert(!tidstate->tss_isCurrentOf);
 
        qsort(tidList, numTids, sizeof(ItemPointerData),
              itemptr_comparator);
        numTids = qunique(tidList, numTids, sizeof(ItemPointerData),
                          itemptr_comparator);
    }
 
    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;
    TableScanDesc scan;
    Relation    heapRelation;
    TupleTableSlot *slot;
    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)
        TidListEval(node);
 
    scan = node->ss.ss_currentScanDesc;
    tidList = node->tss_TidList;
    numTids = node->tss_NumTids;
 
    /*
     * 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)
    {
        ItemPointerData tid = 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)
            table_tuple_get_latest_tid(scan, &tid);
 
        if (table_tuple_fetch_row_version(heapRelation, &tid, snapshot, slot))
            return slot;
 
        /* Bad TID or failed snapshot qual; try next */
        if (bBackward)
            node->tss_TidPtr--;
        else
            node->tss_TidPtr++;
 
        CHECK_FOR_INTERRUPTS();
    }
 
    /*
     * 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.
 *        -- tss_TidPtr is -1.
 * ----------------------------------------------------------------
 */
static TupleTableSlot *
ExecTidScan(PlanState *pstate)
{
    TidScanState *node = castNode(TidScanState, pstate);
 
    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;
 
    /* not really necessary, but seems good form */
    if (node->ss.ss_currentScanDesc)
        table_rescan(node->ss.ss_currentScanDesc, NULL);
 
    ExecScanReScan(&node->ss);
}
 
/* ----------------------------------------------------------------
 *      ExecEndTidScan
 *
 *      Releases any storage allocated through C routines.
 *      Returns nothing.
 * ----------------------------------------------------------------
 */
void
ExecEndTidScan(TidScanState *node)
{
    if (node->ss.ss_currentScanDesc)
        table_endscan(node->ss.ss_currentScanDesc);
}
 
/* ----------------------------------------------------------------
 *      ExecInitTidScan
 *
 *      Initializes the tid scan's state information, creates
 *      scan keys, and opens the base and tid relations.
 *
 *      Parameters:
 *        node: TidScan 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;
    tidstate->ss.ps.ExecProcNode = ExecTidScan;
 
    /*
     * Miscellaneous initialization
     *
     * create expression context for node
     */
    ExecAssignExprContext(estate, &tidstate->ss.ps);
 
    /*
     * mark tid list as not computed yet
     */
    tidstate->tss_TidList = NULL;
    tidstate->tss_NumTids = 0;
    tidstate->tss_TidPtr = -1;
 
    /*
     * open the scan relation
     */
    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.
     */
    ExecInitScanTupleSlot(estate, &tidstate->ss,
                          RelationGetDescr(currentRelation),
                          table_slot_callbacks(currentRelation));
 
    /*
     * Initialize result type and projection.
     */
    ExecInitResultTypeTL(&tidstate->ss.ps);
    ExecAssignScanProjectionInfo(&tidstate->ss);
 
    /*
     * initialize child expressions
     */
    tidstate->ss.ps.qual =
        ExecInitQual(node->scan.plan.qual, (PlanState *) tidstate);
 
    TidExprListCreate(tidstate);
 
    /*
     * all done.
     */
    return tidstate;
}