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Commit301eb3e

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Disable WindowAgg inverse transitions when subplans are present
When an aggregate function is used as a WindowFunc and a tuple transitionsout of the window frame, we ordinarily try to make use of the aggregatefunction's inverse transition function to "unaggregate" the exiting tuple.This optimization is disabled for various cases, including when theaggregate contains a volatile function. In such a case we'd be unable toensure that the transition value was calculated to the same value duringtransitions and inverse transitions. Unfortunately, we did this check bycalling contain_volatile_functions() which does not recursively searchSubPlans for volatile functions. If the aggregate function's arguments orits FILTER clause contained a subplan with volatile functions then we'dfail to notice this.Here we fix this by just disabling the optimization when the WindowFunccontains any subplans. Volatile functions are not the only reason that asubplan may have nonrepeatable results.Bug: #17777Reported-by: Anban CompanyDiscussion:https://postgr.es/m/17777-860b739b6efde977%40postgresql.orgReviewed-by: Tom LaneBackpatch-through: 11
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‎src/backend/executor/nodeWindowAgg.c

Lines changed: 10 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -41,6 +41,7 @@
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#include"executor/nodeWindowAgg.h"
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#include"miscadmin.h"
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#include"nodes/nodeFuncs.h"
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#include"optimizer/clauses.h"
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#include"optimizer/optimizer.h"
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#include"parser/parse_agg.h"
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#include"parser/parse_coerce.h"
@@ -2662,16 +2663,24 @@ initialize_peragg(WindowAggState *winstate, WindowFunc *wfunc,
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* aggregate's arguments (and FILTER clause if any) contain any calls to
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* volatile functions. Otherwise, the difference between restarting and
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* not restarting the aggregation would be user-visible.
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*
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* We also don't risk using moving aggregates when there are subplans in
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* the arguments or FILTER clause. This is partly because
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* contain_volatile_functions() doesn't look inside subplans; but there
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* are other reasons why a subplan's output might be volatile. For
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* example, syncscan mode can render the results nonrepeatable.
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*/
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if (!OidIsValid(aggform->aggminvtransfn))
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use_ma_code= false;/* sine qua non */
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elseif (aggform->aggmfinalmodify==AGGMODIFY_READ_ONLY&&
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aggform->aggfinalmodify!=AGGMODIFY_READ_ONLY)
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aggform->aggfinalmodify!=AGGMODIFY_READ_ONLY)
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use_ma_code= true;/* decision forced by safety */
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elseif (winstate->frameOptions&FRAMEOPTION_START_UNBOUNDED_PRECEDING)
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use_ma_code= false;/* non-moving frame head */
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elseif (contain_volatile_functions((Node*)wfunc))
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use_ma_code= false;/* avoid possible behavioral change */
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elseif (contain_subplans((Node*)wfunc))
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use_ma_code= false;/* subplans might contain volatile functions */
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else
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use_ma_code= true;/* yes, let's use it */
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if (use_ma_code)

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