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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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5
src/backend/executor/nodeAgg.c
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@ -3277,10 +3277,7 @@ ExecInitAgg(Agg *node, EState *estate, int eflags)
}
if (use_hashing)
{
ExecAssignExprContext(estate, &aggstate->ss.ps);
aggstate->hashcontext = aggstate->ss.ps.ps_ExprContext;
}
aggstate->hashcontext = CreateWorkExprContext(estate);
ExecAssignExprContext(estate, &aggstate->ss.ps);