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Fix unnecessary use of moving-aggregate mode with non-moving frame. 

When a plain aggregate is used as a window function, and the window
frame start is specified as UNBOUNDED PRECEDING, the frame's head
cannot move so we do not need to use moving-aggregate mode.  The check
for that was put into initialize_peragg(), failing to notice that
ExecInitWindowAgg() calls that function before it's filled in
winstate->frameOptions.  Since makeNode() would have zeroed the field,
this didn't provoke uninitialized-value complaints, nor would the
erroneous decision have resulted in more than a little inefficiency.
Still, it's wrong, so move the initialization of
winstate->frameOptions earlier to make it work properly.

While here, also fix a thinko in a comment.  Both errors crept in in
commit a9d9acbf2 which introduced the moving-aggregate mode.

Spotted by Vallimaharajan G.  Back-patch to all supported branches.

Discussion: https://postgr.es/m/18e7f2a5167.fe36253866818.977923893562469143@zohocorp.com
This commit is contained in:
Tom Lane 2024-03-27 13:39:03 -04:00
parent 44a4cca991
commit a767cdc84c
1 changed files with 4 additions and 4 deletions
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8
src/backend/executor/nodeWindowAgg.c
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@ -2399,6 +2399,9 @@ ExecInitWindowAgg(WindowAgg *node, EState *estate, int eflags)
winstate->ss.ps.state = estate;
winstate->ss.ps.ExecProcNode = ExecWindowAgg;
/* copy frame options to state node for easy access */
winstate->frameOptions = frameOptions;
/*
* Create expression contexts. We need two, one for per-input-tuple
* processing and one for per-output-tuple processing. We cheat a little
@ -2649,9 +2652,6 @@ ExecInitWindowAgg(WindowAgg *node, EState *estate, int eflags)
/* Set the status to running */
winstate->status = WINDOWAGG_RUN;
/* copy frame options to state node for easy access */
winstate->frameOptions = frameOptions;
/* initialize frame bound offset expressions */
winstate->startOffset = ExecInitExpr((Expr *) node->startOffset,
(PlanState *) winstate);
@ -2785,7 +2785,7 @@ initialize_peragg(WindowAggState *winstate, WindowFunc *wfunc,
/*
* Figure out whether we want to use the moving-aggregate implementation,
* and collect the right set of fields from the pg_attribute entry.
* and collect the right set of fields from the pg_aggregate entry.
*
* It's possible that an aggregate would supply a safe moving-aggregate
* implementation and an unsafe normal one, in which case our hand is