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Create memory context for HashAgg with a reasonable maxBlockSize.

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If the memory context's maxBlockSize is too big, a single block
allocation can suddenly exceed work_mem. For Hash Aggregation, this
can mean spilling to disk too early or reporting a confusing memory
usage number for EXPLAN ANALYZE.

Introduce CreateWorkExprContext(), which is like CreateExprContext(),
except that it creates the AllocSet with a maxBlockSize that is
reasonable in proportion to work_mem.

Right now, CreateWorkExprContext() is only used by Hash Aggregation,
but it may be generally useful in the future.

Discussion: https://postgr.es/m/412a3fbf306f84d8d78c4009e11791867e62b87c.camel@j-davis.com
This commit is contained in:
Jeff Davis
2020-04-07 20:42:04 -07:00
parent f0705bb628
commit 50a38f6517
3 changed files with 58 additions and 19 deletions
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src/include/executor/executor.h
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@ -493,6 +493,7 @@ extern void end_tup_output(TupOutputState *tstate);
extern EState *CreateExecutorState(void);
extern void FreeExecutorState(EState *estate);
extern ExprContext *CreateExprContext(EState *estate);
extern ExprContext *CreateWorkExprContext(EState *estate);
extern ExprContext *CreateStandaloneExprContext(void);
extern void FreeExprContext(ExprContext *econtext, bool isCommit);
extern void ReScanExprContext(ExprContext *econtext);