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Redesign internal logic of nodeLimit so that it does not need to fetch

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one more row from the subplan than the COUNT would appear to require.
This costs a little more logic but a number of people have complained
about the old implementation.
This commit is contained in:
Tom Lane 2002-11-22 22:10:01 +00:00
parent 95c9c22633
commit e760d22391
2 changed files with 161 additions and 86 deletions
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228
src/backend/executor/nodeLimit.c
View File

@ -8,7 +8,7 @@
*
*
* IDENTIFICATION
* $Header: /cvsroot/pgsql/src/backend/executor/nodeLimit.c,v 1.10 2002/06/20 20:29:28 momjian Exp $
* $Header: /cvsroot/pgsql/src/backend/executor/nodeLimit.c,v 1.11 2002/11/22 22:10:01 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -42,7 +42,6 @@ ExecLimit(Limit *node)
TupleTableSlot *resultTupleSlot;
TupleTableSlot *slot;
Plan *outerPlan;
long netlimit;
/*
* get information from the node
@ -53,93 +52,160 @@ ExecLimit(Limit *node)
resultTupleSlot = limitstate->cstate.cs_ResultTupleSlot;
/*
* If first call for this scan, compute limit/offset. (We can't do
* this any earlier, because parameters from upper nodes may not be
* set until now.)
* The main logic is a simple state machine.
*/
if (!limitstate->parmsSet)
recompute_limits(node);
netlimit = limitstate->offset + limitstate->count;
/*
* now loop, returning only desired tuples.
*/
for (;;)
switch (limitstate->lstate)
{
/*
* If we have reached the subplan EOF or the limit, just quit.
*
* NOTE: when scanning forwards, we must fetch one tuple beyond the
* COUNT limit before we can return NULL, else the subplan won't
* be properly positioned to start going backwards. Hence test
* here is for position > netlimit not position >= netlimit.
*
* Similarly, when scanning backwards, we must re-fetch the last
* tuple in the offset region before we can return NULL. Otherwise
* we won't be correctly aligned to start going forward again. So,
* although you might think we can quit when position equals
* offset + 1, we have to fetch a subplan tuple first, and then
* exit when position = offset.
*/
if (ScanDirectionIsForward(direction))
{
if (limitstate->atEnd)
return NULL;
if (!limitstate->noCount && limitstate->position > netlimit)
return NULL;
}
else
{
if (limitstate->position <= limitstate->offset)
return NULL;
}
/*
* fetch a tuple from the outer subplan
*/
slot = ExecProcNode(outerPlan, (Plan *) node);
if (TupIsNull(slot))
{
case LIMIT_INITIAL:
/*
* We are at start or end of the subplan. Update local state
* appropriately, but always return NULL.
* If backwards scan, just return NULL without changing state.
*/
if (!ScanDirectionIsForward(direction))
return NULL;
/*
* First call for this scan, so compute limit/offset. (We can't do
* this any earlier, because parameters from upper nodes may not
* be set until now.) This also sets position = 0.
*/
recompute_limits(node);
/*
* Check for empty window; if so, treat like empty subplan.
*/
if (limitstate->count <= 0 && !limitstate->noCount)
{
limitstate->lstate = LIMIT_EMPTY;
return NULL;
}
/*
* Fetch rows from subplan until we reach position > offset.
*/
for (;;)
{
slot = ExecProcNode(outerPlan, (Plan *) node);
if (TupIsNull(slot))
{
/*
* The subplan returns too few tuples for us to produce
* any output at all.
*/
limitstate->lstate = LIMIT_EMPTY;
return NULL;
}
limitstate->subSlot = slot;
if (++limitstate->position > limitstate->offset)
break;
}
/*
* Okay, we have the first tuple of the window.
*/
limitstate->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))
{
Assert(!limitstate->atEnd);
/* must bump position to stay in sync for backwards fetch */
/*
* Forwards scan, so check for stepping off end of window.
* If we are at the end of the window, return NULL without
* advancing the subplan or the position variable; but
* change the state machine state to record having done so.
*/
if (!limitstate->noCount &&
limitstate->position >= limitstate->offset + limitstate->count)
{
limitstate->lstate = LIMIT_WINDOWEND;
return NULL;
}
/*
* Get next tuple from subplan, if any.
*/
slot = ExecProcNode(outerPlan, (Plan *) node);
if (TupIsNull(slot))
{
limitstate->lstate = LIMIT_SUBPLANEOF;
return NULL;
}
limitstate->subSlot = slot;
limitstate->position++;
limitstate->atEnd = true;
}
else
{
limitstate->position = 0;
limitstate->atEnd = false;
/*
* Backwards scan, so check for stepping off start of window.
* As above, change only state-machine status if so.
*/
if (limitstate->position <= limitstate->offset + 1)
{
limitstate->lstate = LIMIT_WINDOWSTART;
return NULL;
}
/*
* Get previous tuple from subplan; there should be one!
*/
slot = ExecProcNode(outerPlan, (Plan *) node);
if (TupIsNull(slot))
elog(ERROR, "ExecLimit: subplan failed to run backwards");
limitstate->subSlot = slot;
limitstate->position--;
}
return NULL;
}
break;
/*
* We got the next subplan tuple successfully, so adjust state.
*/
if (ScanDirectionIsForward(direction))
limitstate->position++;
else
{
limitstate->position--;
Assert(limitstate->position > 0);
}
limitstate->atEnd = false;
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, (Plan *) node);
if (TupIsNull(slot))
elog(ERROR, "ExecLimit: subplan failed to run backwards");
limitstate->subSlot = slot;
limitstate->lstate = LIMIT_INWINDOW;
/* position does not change 'cause we didn't advance it before */
break;
/*
* Now, is this a tuple we want? If not, loop around to fetch
* another tuple from the subplan.
*/
if (limitstate->position > limitstate->offset &&
(limitstate->noCount || limitstate->position <= netlimit))
case LIMIT_WINDOWEND:
if (ScanDirectionIsForward(direction))
return NULL;
/*
* Backing up from window end: simply re-return the last
* tuple fetched from the subplan.
*/
slot = limitstate->subSlot;
limitstate->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 = limitstate->subSlot;
limitstate->lstate = LIMIT_INWINDOW;
/* position does not change 'cause we didn't change it before */
break;
default:
elog(ERROR, "ExecLimit: impossible state %d",
(int) limitstate->lstate);
slot = NULL; /* keep compiler quiet */
break;
}
/* Return the current tuple */
Assert(!TupIsNull(slot));
ExecStoreTuple(slot->val,
resultTupleSlot,
InvalidBuffer,
@ -181,6 +247,7 @@ recompute_limits(Limit *node)
if (node->limitCount)
{
limitstate->noCount = false;
limitstate->count =
DatumGetInt32(ExecEvalExprSwitchContext(node->limitCount,
econtext,
@ -199,12 +266,9 @@ recompute_limits(Limit *node)
limitstate->noCount = true;
}
/* Reset position data to start-of-scan */
/* Reset position to start-of-scan */
limitstate->position = 0;
limitstate->atEnd = false;
/* Set flag that params are computed */
limitstate->parmsSet = true;
limitstate->subSlot = NULL;
}
/* ----------------------------------------------------------------
@ -230,7 +294,7 @@ ExecInitLimit(Limit *node, EState *estate, Plan *parent)
*/
limitstate = makeNode(LimitState);
node->limitstate = limitstate;
limitstate->parmsSet = false;
limitstate->lstate = LIMIT_INITIAL;
/*
* Miscellaneous initialization
@ -297,10 +361,10 @@ ExecReScanLimit(Limit *node, ExprContext *exprCtxt, Plan *parent)
{
LimitState *limitstate = node->limitstate;
ExecClearTuple(limitstate->cstate.cs_ResultTupleSlot);
/* resetting lstate will force offset/limit recalculation */
limitstate->lstate = LIMIT_INITIAL;
/* force recalculation of limit expressions on first call */
limitstate->parmsSet = false;
ExecClearTuple(limitstate->cstate.cs_ResultTupleSlot);
/*
* if chgParam of subnode is not null then plan will be re-scanned by

19
src/include/nodes/execnodes.h
View File

@ -7,7 +7,7 @@
* Portions Copyright (c) 1996-2002, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* $Id: execnodes.h,v 1.78 2002/11/15 02:50:10 momjian Exp $
* $Id: execnodes.h,v 1.79 2002/11/22 22:10:01 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -775,17 +775,28 @@ typedef struct SetOpState
* offset is the number of initial tuples to skip (0 does nothing).
* count is the number of tuples to return after skipping the offset tuples.
* If no limit count was specified, count is undefined and noCount is true.
* When lstate == LIMIT_INITIAL, offset/count/noCount haven't been set yet.
* ----------------
*/
typedef enum
{
LIMIT_INITIAL, /* initial state for LIMIT node */
LIMIT_EMPTY, /* there are no returnable rows */
LIMIT_INWINDOW, /* have returned a row in the window */
LIMIT_SUBPLANEOF, /* at EOF of subplan (within window) */
LIMIT_WINDOWEND, /* stepped off end of window */
LIMIT_WINDOWSTART /* stepped off beginning of window */
} LimitStateCond;
typedef struct LimitState
{
CommonState cstate; /* its first field is NodeTag */
long offset; /* current OFFSET value */
long count; /* current COUNT, if any */
long position; /* 1-based index of last tuple fetched */
bool parmsSet; /* have we calculated offset/limit yet? */
bool noCount; /* if true, ignore count */
bool atEnd; /* if true, we've reached EOF of subplan */
LimitStateCond lstate; /* state machine status, as above */
long position; /* 1-based index of last tuple returned */
TupleTableSlot *subSlot; /* tuple last obtained from subplan */
} LimitState;