From 5bcf389ecfd40daf92238e1abbff4fc4d3f18b33 Mon Sep 17 00:00:00 2001
From: Andres Freund
Date: Tue, 5 Dec 2017 10:55:56 -0800
Subject: 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
---
 src/test/regress/expected/join.out | 15 +++++++++++++++
 src/test/regress/sql/join.sql      | 11 +++++++++++
 2 files changed, 26 insertions(+)

(limited to 'src/test')

diff --git a/src/test/regress/expected/join.out b/src/test/regress/expected/join.out
index b7d17900978..001d96dc2d8 100644
--- a/src/test/regress/expected/join.out
+++ b/src/test/regress/expected/join.out
@@ -6173,6 +6173,21 @@ $$);
 
 rollback to settings;
 -- A couple of other hash join tests unrelated to work_mem management.
+-- Check that EXPLAIN ANALYZE has data even if the leader doesn't participate
+savepoint settings;
+set local max_parallel_workers_per_gather = 2;
+set local work_mem = '4MB';
+set local parallel_leader_participation = off;
+select * from hash_join_batches(
+$$
+  select count(*) from simple r join simple s using (id);
+$$);
+ original | final 
+----------+-------
+        1 |     1
+(1 row)
+
+rollback to settings;
 -- A full outer join where every record is matched.
 -- non-parallel
 savepoint settings;
diff --git a/src/test/regress/sql/join.sql b/src/test/regress/sql/join.sql
index c6d4a513e86..882601b3388 100644
--- a/src/test/regress/sql/join.sql
+++ b/src/test/regress/sql/join.sql
@@ -2159,6 +2159,17 @@ rollback to settings;
 
 -- A couple of other hash join tests unrelated to work_mem management.
 
+-- Check that EXPLAIN ANALYZE has data even if the leader doesn't participate
+savepoint settings;
+set local max_parallel_workers_per_gather = 2;
+set local work_mem = '4MB';
+set local parallel_leader_participation = off;
+select * from hash_join_batches(
+$$
+  select count(*) from simple r join simple s using (id);
+$$);
+rollback to settings;
+
 -- A full outer join where every record is matched.
 
 -- non-parallel
-- 
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