Pass InitPlan values to workers via Gather (Merge).

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

1 files changed, 4 insertions, 2 deletions

diff --git a/src/include/executor/execParallel.h b/src/include/executor/execParallel.h
index e1b3e7af1f7..99a13f3b7d1 100644
--- a/src/include/executor/execParallel.h
+++ b/src/include/executor/execParallel.h
@@ -28,6 +28,7 @@ typedef struct ParallelExecutorInfo
 	BufferUsage *buffer_usage;	/* points to bufusage area in DSM */
 	SharedExecutorInstrumentation *instrumentation; /* optional */
 	dsa_area   *area;			/* points to DSA area in DSM */
+	dsa_pointer	param_exec;		/* serialized PARAM_EXEC parameters */
 	bool		finished;		/* set true by ExecParallelFinish */
 	/* These two arrays have pcxt->nworkers_launched entries: */
 	shm_mq_handle **tqueue;		/* tuple queues for worker output */
@@ -35,12 +36,13 @@ typedef struct ParallelExecutorInfo
 } ParallelExecutorInfo;
 
 extern ParallelExecutorInfo *ExecInitParallelPlan(PlanState *planstate,
-					 EState *estate, int nworkers, int64 tuples_needed);
+					 EState *estate, Bitmapset *sendParam, int nworkers,
+					 int64 tuples_needed);
 extern void ExecParallelCreateReaders(ParallelExecutorInfo *pei);
 extern void ExecParallelFinish(ParallelExecutorInfo *pei);
 extern void ExecParallelCleanup(ParallelExecutorInfo *pei);
 extern void ExecParallelReinitialize(PlanState *planstate,
-						 ParallelExecutorInfo *pei);
+						 ParallelExecutorInfo *pei, Bitmapset *sendParam);
 
 extern void ParallelQueryMain(dsm_segment *seg, shm_toc *toc);