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If a PARAM_EXEC parameter is used below a Gather (Merge) but the InitPlan that computes it is attached to or above the Gather (Merge), force the value to be computed before starting parallelism and pass it down to all workers. This allows us to use parallelism in cases where it previously would have had to be rejected as unsafe. We do - in this case - lose the optimization that the value is only computed if it's actually used. An alternative strategy would be to have the first worker that needs the value compute it, but one downside of that approach is that we'd then need to select a parallel-safe path to compute the parameter value; it couldn't for example contain a Gather (Merge) node. At some point in the future, we might want to consider both approaches. Independent of that consideration, there is a great deal more work that could be done to make more kinds of PARAM_EXEC parameters parallel-safe. This infrastructure could be used to allow a Gather (Merge) on the inner side of a nested loop (although that's not a very appealing plan) and cases where the InitPlan is attached below the Gather (Merge) could be addressed as well using various techniques. But this is a good start. Amit Kapila, reviewed and revised by me. Reviewing and testing from Kuntal Ghosh, Haribabu Kommi, and Tushar Ahuja. Discussion: http://postgr.es/m/CAA4eK1LV0Y1AUV4cUCdC+sYOx0Z0-8NAJ2Pd9=UKsbQ5Sr7+JQ@mail.gmail.com
50 lines
1.9 KiB
C
50 lines
1.9 KiB
C
/*--------------------------------------------------------------------
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* execParallel.h
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* POSTGRES parallel execution interface
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*
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* Portions Copyright (c) 1996-2017, PostgreSQL Global Development Group
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* Portions Copyright (c) 1994, Regents of the University of California
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*
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* IDENTIFICATION
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* src/include/executor/execParallel.h
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*--------------------------------------------------------------------
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*/
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#ifndef EXECPARALLEL_H
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#define EXECPARALLEL_H
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#include "access/parallel.h"
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#include "nodes/execnodes.h"
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#include "nodes/parsenodes.h"
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#include "nodes/plannodes.h"
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#include "utils/dsa.h"
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typedef struct SharedExecutorInstrumentation SharedExecutorInstrumentation;
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typedef struct ParallelExecutorInfo
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{
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PlanState *planstate; /* plan subtree we're running in parallel */
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ParallelContext *pcxt; /* parallel context we're using */
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BufferUsage *buffer_usage; /* points to bufusage area in DSM */
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SharedExecutorInstrumentation *instrumentation; /* optional */
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dsa_area *area; /* points to DSA area in DSM */
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dsa_pointer param_exec; /* serialized PARAM_EXEC parameters */
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bool finished; /* set true by ExecParallelFinish */
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/* These two arrays have pcxt->nworkers_launched entries: */
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shm_mq_handle **tqueue; /* tuple queues for worker output */
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struct TupleQueueReader **reader; /* tuple reader/writer support */
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} ParallelExecutorInfo;
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extern ParallelExecutorInfo *ExecInitParallelPlan(PlanState *planstate,
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EState *estate, Bitmapset *sendParam, int nworkers,
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int64 tuples_needed);
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extern void ExecParallelCreateReaders(ParallelExecutorInfo *pei);
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extern void ExecParallelFinish(ParallelExecutorInfo *pei);
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extern void ExecParallelCleanup(ParallelExecutorInfo *pei);
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extern void ExecParallelReinitialize(PlanState *planstate,
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ParallelExecutorInfo *pei, Bitmapset *sendParam);
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extern void ParallelQueryMain(dsm_segment *seg, shm_toc *toc);
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#endif /* EXECPARALLEL_H */
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