Fix EXPLAIN ANALYZE of hash join when the leader doesn't participate.

If a hash join appears in a parallel query, there may be no hash table
available for explain.c to inspect even though a hash table may have
been built in other processes.  This could happen either because
parallel_leader_participation was set to off or because the leader
happened to hit the end of the outer relation immediately (even though
the complete relation is not empty) and decided not to build the hash
table.

Commit bf11e7ee introduced a way for workers to exchange
instrumentation via the DSM segment for Sort nodes even though they
are not parallel-aware.  This commit does the same for Hash nodes, so
that explain.c has a way to find instrumentation data from an
arbitrary participant that actually built the hash table.

Author: Thomas Munro
Reviewed-By: Andres Freund
Discussion: https://postgr.es/m/CAEepm%3D3DUQC2-z252N55eOcZBer6DPdM%3DFzrxH9dZc5vYLsjaA%40mail.gmail.com

2 files changed, 35 insertions, 0 deletions

diff --git a/src/include/executor/nodeHash.h b/src/include/executor/nodeHash.h
index 3ae556fb6c5..75d4c70f6f6 100644
--- a/src/include/executor/nodeHash.h
+++ b/src/include/executor/nodeHash.h
@@ -14,6 +14,7 @@
 #ifndef NODEHASH_H
 #define NODEHASH_H
 
+#include "access/parallel.h"
 #include "nodes/execnodes.h"
 
 extern HashState *ExecInitHash(Hash *node, EState *estate, int eflags);
@@ -48,5 +49,13 @@ extern void ExecChooseHashTableSize(double ntuples, int tupwidth, bool useskew,
 						int *numbatches,
 						int *num_skew_mcvs);
 extern int	ExecHashGetSkewBucket(HashJoinTable hashtable, uint32 hashvalue);
+extern void ExecHashEstimate(HashState *node, ParallelContext *pcxt);
+extern void ExecHashInitializeDSM(HashState *node, ParallelContext *pcxt);
+extern void ExecHashInitializeWorker(HashState *node, ParallelWorkerContext *pwcxt);
+extern void ExecHashReInitializeDSM(HashState *node, ParallelContext *pcxt);
+extern void ExecHashRetrieveInstrumentation(HashState *node);
+extern void ExecShutdownHash(HashState *node);
+extern void ExecHashGetInstrumentation(HashInstrumentation *instrument,
+									   HashJoinTable hashtable);
 
 #endif							/* NODEHASH_H */
diff --git a/src/include/nodes/execnodes.h b/src/include/nodes/execnodes.h
index e05bc04f525..084d59ef834 100644
--- a/src/include/nodes/execnodes.h
+++ b/src/include/nodes/execnodes.h
@@ -1981,6 +1981,29 @@ typedef struct GatherMergeState
 } GatherMergeState;
 
 /* ----------------
+ *	 Values displayed by EXPLAIN ANALYZE
+ * ----------------
+ */
+typedef struct HashInstrumentation
+{
+	int			nbuckets;		/* number of buckets at end of execution */
+	int			nbuckets_original;	/* planned number of buckets */
+	int			nbatch;			/* number of batches at end of execution */
+	int			nbatch_original;	/* planned number of batches */
+	size_t		space_peak;		/* speak memory usage in bytes */
+} HashInstrumentation;
+
+/* ----------------
+ *	 Shared memory container for per-worker hash information
+ * ----------------
+ */
+typedef struct SharedHashInfo
+{
+	int			num_workers;
+	HashInstrumentation hinstrument[FLEXIBLE_ARRAY_MEMBER];
+} SharedHashInfo;
+
+/* ----------------
  *	 HashState information
  * ----------------
  */
@@ -1990,6 +2013,9 @@ typedef struct HashState
 	HashJoinTable hashtable;	/* hash table for the hashjoin */
 	List	   *hashkeys;		/* list of ExprState nodes */
 	/* hashkeys is same as parent's hj_InnerHashKeys */
+
+	SharedHashInfo *shared_info;	/* one entry per worker */
+	HashInstrumentation *hinstrument;	/* this worker's entry */
 } HashState;
 
 /* ----------------