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+/*-------------------------------------------------------------------------
+ *
+ * nodeLimit.c
+ * Routines to handle limiting of query results where appropriate
+ *
+ * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ *
+ * IDENTIFICATION
+ * src/backend/executor/nodeLimit.c
+ *
+ *-------------------------------------------------------------------------
+ */
+/*
+ * INTERFACE ROUTINES
+ * ExecLimit - extract a limited range of tuples
+ * ExecInitLimit - initialize node and subnodes..
+ * ExecEndLimit - shutdown node and subnodes
+ */
+
+#include "postgres.h"
+
+#include "executor/executor.h"
+#include "executor/nodeLimit.h"
+#include "miscadmin.h"
+#include "nodes/nodeFuncs.h"
+
+static void recompute_limits(LimitState *node);
+static int64 compute_tuples_needed(LimitState *node);
+
+
+/* ----------------------------------------------------------------
+ * ExecLimit
+ *
+ * This is a very simple node which just performs LIMIT/OFFSET
+ * filtering on the stream of tuples returned by a subplan.
+ * ----------------------------------------------------------------
+ */
+static TupleTableSlot * /* return: a tuple or NULL */
+ExecLimit(PlanState *pstate)
+{
+ LimitState *node = castNode(LimitState, pstate);
+ ExprContext *econtext = node->ps.ps_ExprContext;
+ ScanDirection direction;
+ TupleTableSlot *slot;
+ PlanState *outerPlan;
+
+ CHECK_FOR_INTERRUPTS();
+
+ /*
+ * get information from the node
+ */
+ direction = node->ps.state->es_direction;
+ outerPlan = outerPlanState(node);
+
+ /*
+ * The main logic is a simple state machine.
+ */
+ switch (node->lstate)
+ {
+ case LIMIT_INITIAL:
+
+ /*
+ * First call for this node, so compute limit/offset. (We can't do
+ * this any earlier, because parameters from upper nodes will not
+ * be set during ExecInitLimit.) This also sets position = 0 and
+ * changes the state to LIMIT_RESCAN.
+ */
+ recompute_limits(node);
+
+ /* FALL THRU */
+
+ case LIMIT_RESCAN:
+
+ /*
+ * If backwards scan, just return NULL without changing state.
+ */
+ if (!ScanDirectionIsForward(direction))
+ return NULL;
+
+ /*
+ * Check for empty window; if so, treat like empty subplan.
+ */
+ if (node->count <= 0 && !node->noCount)
+ {
+ node->lstate = LIMIT_EMPTY;
+ return NULL;
+ }
+
+ /*
+ * Fetch rows from subplan until we reach position > offset.
+ */
+ for (;;)
+ {
+ slot = ExecProcNode(outerPlan);
+ if (TupIsNull(slot))
+ {
+ /*
+ * The subplan returns too few tuples for us to produce
+ * any output at all.
+ */
+ node->lstate = LIMIT_EMPTY;
+ return NULL;
+ }
+
+ /*
+ * Tuple at limit is needed for comparison in subsequent
+ * execution to detect ties.
+ */
+ if (node->limitOption == LIMIT_OPTION_WITH_TIES &&
+ node->position - node->offset == node->count - 1)
+ {
+ ExecCopySlot(node->last_slot, slot);
+ }
+ node->subSlot = slot;
+ if (++node->position > node->offset)
+ break;
+ }
+
+ /*
+ * Okay, we have the first tuple of the window.
+ */
+ node->lstate = LIMIT_INWINDOW;
+ break;
+
+ case LIMIT_EMPTY:
+
+ /*
+ * The subplan is known to return no tuples (or not more than
+ * OFFSET tuples, in general). So we return no tuples.
+ */
+ return NULL;
+
+ case LIMIT_INWINDOW:
+ if (ScanDirectionIsForward(direction))
+ {
+ /*
+ * Forwards scan, so check for stepping off end of window. At
+ * the end of the window, the behavior depends on whether WITH
+ * TIES was specified: if so, we need to change the state
+ * machine to WINDOWEND_TIES, and fall through to the code for
+ * that case. If not (nothing was specified, or ONLY was)
+ * return NULL without advancing the subplan or the position
+ * variable, but change the state machine to record having
+ * done so.
+ *
+ * Once at the end, ideally, we would shut down parallel
+ * resources; but that would destroy the parallel context
+ * which might be required for rescans. To do that, we'll
+ * need to find a way to pass down more information about
+ * whether rescans are possible.
+ */
+ if (!node->noCount &&
+ node->position - node->offset >= node->count)
+ {
+ if (node->limitOption == LIMIT_OPTION_COUNT)
+ {
+ node->lstate = LIMIT_WINDOWEND;
+ return NULL;
+ }
+ else
+ {
+ node->lstate = LIMIT_WINDOWEND_TIES;
+ /* we'll fall through to the next case */
+ }
+ }
+ else
+ {
+ /*
+ * Get next tuple from subplan, if any.
+ */
+ slot = ExecProcNode(outerPlan);
+ if (TupIsNull(slot))
+ {
+ node->lstate = LIMIT_SUBPLANEOF;
+ return NULL;
+ }
+
+ /*
+ * If WITH TIES is active, and this is the last in-window
+ * tuple, save it to be used in subsequent WINDOWEND_TIES
+ * processing.
+ */
+ if (node->limitOption == LIMIT_OPTION_WITH_TIES &&
+ node->position - node->offset == node->count - 1)
+ {
+ ExecCopySlot(node->last_slot, slot);
+ }
+ node->subSlot = slot;
+ node->position++;
+ break;
+ }
+ }
+ else
+ {
+ /*
+ * Backwards scan, so check for stepping off start of window.
+ * As above, only change state-machine status if so.
+ */
+ if (node->position <= node->offset + 1)
+ {
+ node->lstate = LIMIT_WINDOWSTART;
+ return NULL;
+ }
+
+ /*
+ * Get previous tuple from subplan; there should be one!
+ */
+ slot = ExecProcNode(outerPlan);
+ if (TupIsNull(slot))
+ elog(ERROR, "LIMIT subplan failed to run backwards");
+ node->subSlot = slot;
+ node->position--;
+ break;
+ }
+
+ Assert(node->lstate == LIMIT_WINDOWEND_TIES);
+ /* FALL THRU */
+
+ case LIMIT_WINDOWEND_TIES:
+ if (ScanDirectionIsForward(direction))
+ {
+ /*
+ * Advance the subplan until we find the first row with
+ * different ORDER BY pathkeys.
+ */
+ slot = ExecProcNode(outerPlan);
+ if (TupIsNull(slot))
+ {
+ node->lstate = LIMIT_SUBPLANEOF;
+ return NULL;
+ }
+
+ /*
+ * Test if the new tuple and the last tuple match. If so we
+ * return the tuple.
+ */
+ econtext->ecxt_innertuple = slot;
+ econtext->ecxt_outertuple = node->last_slot;
+ if (ExecQualAndReset(node->eqfunction, econtext))
+ {
+ node->subSlot = slot;
+ node->position++;
+ }
+ else
+ {
+ node->lstate = LIMIT_WINDOWEND;
+ return NULL;
+ }
+ }
+ else
+ {
+ /*
+ * Backwards scan, so check for stepping off start of window.
+ * Change only state-machine status if so.
+ */
+ if (node->position <= node->offset + 1)
+ {
+ node->lstate = LIMIT_WINDOWSTART;
+ return NULL;
+ }
+
+ /*
+ * Get previous tuple from subplan; there should be one! And
+ * change state-machine status.
+ */
+ slot = ExecProcNode(outerPlan);
+ if (TupIsNull(slot))
+ elog(ERROR, "LIMIT subplan failed to run backwards");
+ node->subSlot = slot;
+ node->position--;
+ node->lstate = LIMIT_INWINDOW;
+ }
+ break;
+
+ case LIMIT_SUBPLANEOF:
+ if (ScanDirectionIsForward(direction))
+ return NULL;
+
+ /*
+ * Backing up from subplan EOF, so re-fetch previous tuple; there
+ * should be one! Note previous tuple must be in window.
+ */
+ slot = ExecProcNode(outerPlan);
+ if (TupIsNull(slot))
+ elog(ERROR, "LIMIT subplan failed to run backwards");
+ node->subSlot = slot;
+ node->lstate = LIMIT_INWINDOW;
+ /* position does not change 'cause we didn't advance it before */
+ break;
+
+ case LIMIT_WINDOWEND:
+ if (ScanDirectionIsForward(direction))
+ return NULL;
+
+ /*
+ * We already past one position to detect ties so re-fetch
+ * previous tuple; there should be one! Note previous tuple must
+ * be in window.
+ */
+ if (node->limitOption == LIMIT_OPTION_WITH_TIES)
+ {
+ slot = ExecProcNode(outerPlan);
+ if (TupIsNull(slot))
+ elog(ERROR, "LIMIT subplan failed to run backwards");
+ node->subSlot = slot;
+ node->lstate = LIMIT_INWINDOW;
+ }
+ else
+ {
+ /*
+ * Backing up from window end: simply re-return the last tuple
+ * fetched from the subplan.
+ */
+ slot = node->subSlot;
+ node->lstate = LIMIT_INWINDOW;
+ /* position does not change 'cause we didn't advance it before */
+ }
+ break;
+
+ case LIMIT_WINDOWSTART:
+ if (!ScanDirectionIsForward(direction))
+ return NULL;
+
+ /*
+ * Advancing after having backed off window start: simply
+ * re-return the last tuple fetched from the subplan.
+ */
+ slot = node->subSlot;
+ node->lstate = LIMIT_INWINDOW;
+ /* position does not change 'cause we didn't change it before */
+ break;
+
+ default:
+ elog(ERROR, "impossible LIMIT state: %d",
+ (int) node->lstate);
+ slot = NULL; /* keep compiler quiet */
+ break;
+ }
+
+ /* Return the current tuple */
+ Assert(!TupIsNull(slot));
+
+ return slot;
+}
+
+/*
+ * Evaluate the limit/offset expressions --- done at startup or rescan.
+ *
+ * This is also a handy place to reset the current-position state info.
+ */
+static void
+recompute_limits(LimitState *node)
+{
+ ExprContext *econtext = node->ps.ps_ExprContext;
+ Datum val;
+ bool isNull;
+
+ if (node->limitOffset)
+ {
+ val = ExecEvalExprSwitchContext(node->limitOffset,
+ econtext,
+ &isNull);
+ /* Interpret NULL offset as no offset */
+ if (isNull)
+ node->offset = 0;
+ else
+ {
+ node->offset = DatumGetInt64(val);
+ if (node->offset < 0)
+ ereport(ERROR,
+ (errcode(ERRCODE_INVALID_ROW_COUNT_IN_RESULT_OFFSET_CLAUSE),
+ errmsg("OFFSET must not be negative")));
+ }
+ }
+ else
+ {
+ /* No OFFSET supplied */
+ node->offset = 0;
+ }
+
+ if (node->limitCount)
+ {
+ val = ExecEvalExprSwitchContext(node->limitCount,
+ econtext,
+ &isNull);
+ /* Interpret NULL count as no count (LIMIT ALL) */
+ if (isNull)
+ {
+ node->count = 0;
+ node->noCount = true;
+ }
+ else
+ {
+ node->count = DatumGetInt64(val);
+ if (node->count < 0)
+ ereport(ERROR,
+ (errcode(ERRCODE_INVALID_ROW_COUNT_IN_LIMIT_CLAUSE),
+ errmsg("LIMIT must not be negative")));
+ node->noCount = false;
+ }
+ }
+ else
+ {
+ /* No COUNT supplied */
+ node->count = 0;
+ node->noCount = true;
+ }
+
+ /* Reset position to start-of-scan */
+ node->position = 0;
+ node->subSlot = NULL;
+
+ /* Set state-machine state */
+ node->lstate = LIMIT_RESCAN;
+
+ /*
+ * Notify child node about limit. Note: think not to "optimize" by
+ * skipping ExecSetTupleBound if compute_tuples_needed returns < 0. We
+ * must update the child node anyway, in case this is a rescan and the
+ * previous time we got a different result.
+ */
+ ExecSetTupleBound(compute_tuples_needed(node), outerPlanState(node));
+}
+
+/*
+ * Compute the maximum number of tuples needed to satisfy this Limit node.
+ * Return a negative value if there is not a determinable limit.
+ */
+static int64
+compute_tuples_needed(LimitState *node)
+{
+ if ((node->noCount) || (node->limitOption == LIMIT_OPTION_WITH_TIES))
+ return -1;
+ /* Note: if this overflows, we'll return a negative value, which is OK */
+ return node->count + node->offset;
+}
+
+/* ----------------------------------------------------------------
+ * ExecInitLimit
+ *
+ * This initializes the limit node state structures and
+ * the node's subplan.
+ * ----------------------------------------------------------------
+ */
+LimitState *
+ExecInitLimit(Limit *node, EState *estate, int eflags)
+{
+ LimitState *limitstate;
+ Plan *outerPlan;
+
+ /* check for unsupported flags */
+ Assert(!(eflags & EXEC_FLAG_MARK));
+
+ /*
+ * create state structure
+ */
+ limitstate = makeNode(LimitState);
+ limitstate->ps.plan = (Plan *) node;
+ limitstate->ps.state = estate;
+ limitstate->ps.ExecProcNode = ExecLimit;
+
+ limitstate->lstate = LIMIT_INITIAL;
+
+ /*
+ * Miscellaneous initialization
+ *
+ * Limit nodes never call ExecQual or ExecProject, but they need an
+ * exprcontext anyway to evaluate the limit/offset parameters in.
+ */
+ ExecAssignExprContext(estate, &limitstate->ps);
+
+ /*
+ * initialize outer plan
+ */
+ outerPlan = outerPlan(node);
+ outerPlanState(limitstate) = ExecInitNode(outerPlan, estate, eflags);
+
+ /*
+ * initialize child expressions
+ */
+ limitstate->limitOffset = ExecInitExpr((Expr *) node->limitOffset,
+ (PlanState *) limitstate);
+ limitstate->limitCount = ExecInitExpr((Expr *) node->limitCount,
+ (PlanState *) limitstate);
+ limitstate->limitOption = node->limitOption;
+
+ /*
+ * Initialize result type.
+ */
+ ExecInitResultTypeTL(&limitstate->ps);
+
+ limitstate->ps.resultopsset = true;
+ limitstate->ps.resultops = ExecGetResultSlotOps(outerPlanState(limitstate),
+ &limitstate->ps.resultopsfixed);
+
+ /*
+ * limit nodes do no projections, so initialize projection info for this
+ * node appropriately
+ */
+ limitstate->ps.ps_ProjInfo = NULL;
+
+ /*
+ * Initialize the equality evaluation, to detect ties.
+ */
+ if (node->limitOption == LIMIT_OPTION_WITH_TIES)
+ {
+ TupleDesc desc;
+ const TupleTableSlotOps *ops;
+
+ desc = ExecGetResultType(outerPlanState(limitstate));
+ ops = ExecGetResultSlotOps(outerPlanState(limitstate), NULL);
+
+ limitstate->last_slot = ExecInitExtraTupleSlot(estate, desc, ops);
+ limitstate->eqfunction = execTuplesMatchPrepare(desc,
+ node->uniqNumCols,
+ node->uniqColIdx,
+ node->uniqOperators,
+ node->uniqCollations,
+ &limitstate->ps);
+ }
+
+ return limitstate;
+}
+
+/* ----------------------------------------------------------------
+ * ExecEndLimit
+ *
+ * This shuts down the subplan and frees resources allocated
+ * to this node.
+ * ----------------------------------------------------------------
+ */
+void
+ExecEndLimit(LimitState *node)
+{
+ ExecFreeExprContext(&node->ps);
+ ExecEndNode(outerPlanState(node));
+}
+
+
+void
+ExecReScanLimit(LimitState *node)
+{
+ /*
+ * Recompute limit/offset in case parameters changed, and reset the state
+ * machine. We must do this before rescanning our child node, in case
+ * it's a Sort that we are passing the parameters down to.
+ */
+ recompute_limits(node);
+
+ /*
+ * if chgParam of subnode is not null then plan will be re-scanned by
+ * first ExecProcNode.
+ */
+ if (node->ps.lefttree->chgParam == NULL)
+ ExecReScan(node->ps.lefttree);
+}
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