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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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71
src/backend/executor/execUtils.c
View File

@ -53,6 +53,7 @@
#include "executor/executor.h"
#include "jit/jit.h"
#include "mb/pg_wchar.h"
#include "miscadmin.h"
#include "nodes/nodeFuncs.h"
#include "parser/parsetree.h"
#include "partitioning/partdesc.h"
@ -227,21 +228,13 @@ FreeExecutorState(EState *estate)
MemoryContextDelete(estate->es_query_cxt);
}
/* ----------------
* CreateExprContext
*
* Create a context for expression evaluation within an EState.
*
* An executor run may require multiple ExprContexts (we usually make one
* for each Plan node, and a separate one for per-output-tuple processing
* such as constraint checking). Each ExprContext has its own "per-tuple"
* memory context.
*
* Note we make no assumption about the caller's memory context.
* ----------------
/*
* Internal implementation for CreateExprContext() and CreateWorkExprContext()
* that allows control over the AllocSet parameters.
*/
ExprContext *
CreateExprContext(EState *estate)
static ExprContext *
CreateExprContextInternal(EState *estate, Size minContextSize,
Size initBlockSize, Size maxBlockSize)
{
ExprContext *econtext;
MemoryContext oldcontext;
@ -264,7 +257,9 @@ CreateExprContext(EState *estate)
econtext->ecxt_per_tuple_memory =
AllocSetContextCreate(estate->es_query_cxt,
"ExprContext",
ALLOCSET_DEFAULT_SIZES);
minContextSize,
initBlockSize,
maxBlockSize);
econtext->ecxt_param_exec_vals = estate->es_param_exec_vals;
econtext->ecxt_param_list_info = estate->es_param_list_info;
@ -294,6 +289,52 @@ CreateExprContext(EState *estate)
return econtext;
}
/* ----------------
* CreateExprContext
*
* Create a context for expression evaluation within an EState.
*
* An executor run may require multiple ExprContexts (we usually make one
* for each Plan node, and a separate one for per-output-tuple processing
* such as constraint checking). Each ExprContext has its own "per-tuple"
* memory context.
*
* Note we make no assumption about the caller's memory context.
* ----------------
*/
ExprContext *
CreateExprContext(EState *estate)
{
return CreateExprContextInternal(estate, ALLOCSET_DEFAULT_SIZES);
}
/* ----------------
* CreateWorkExprContext
*
* Like CreateExprContext, but specifies the AllocSet sizes to be reasonable
* in proportion to work_mem. If the maximum block allocation size is too
* large, it's easy to skip right past work_mem with a single allocation.
* ----------------
*/
ExprContext *
CreateWorkExprContext(EState *estate)
{
Size minContextSize = ALLOCSET_DEFAULT_MINSIZE;
Size initBlockSize = ALLOCSET_DEFAULT_INITSIZE;
Size maxBlockSize = ALLOCSET_DEFAULT_MAXSIZE;
/* choose the maxBlockSize to be no larger than 1/16 of work_mem */
while (16 * maxBlockSize > work_mem * 1024L)
maxBlockSize >>= 1;
if (maxBlockSize < ALLOCSET_DEFAULT_INITSIZE)
maxBlockSize = ALLOCSET_DEFAULT_INITSIZE;
return CreateExprContextInternal(estate, minContextSize,
initBlockSize, maxBlockSize);
}
/* ----------------
* CreateStandaloneExprContext
*

5
src/backend/executor/nodeAgg.c
View File

@ -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);

1
src/include/executor/executor.h
View File

@ -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);