Hi,
I got somewhat scared when my explain took a few seconds to complete and
used a few gigs of RAM.
To reproduce try the following:
discard temp;
create temp table a as select to_timestamp(generate_series(1, 7000)) i;
analyze a;
set work_mem to '3GB';
explain select distinct a1.i - a2.i from a a1, a a2;
I would appreciate if someone could have a look at the patch attached,
which makes executor skip initializing hash tables when doing explain only.
Best, Alex
diff --git a/src/backend/executor/nodeAgg.c b/src/backend/executor/nodeAgg.c
index d87677d659..196fe09c52 100644
--- a/src/backend/executor/nodeAgg.c
+++ b/src/backend/executor/nodeAgg.c
@@ -3665,7 +3665,11 @@ ExecInitAgg(Agg *node, EState *estate, int eflags)
&aggstate->hash_ngroups_limit,
&aggstate->hash_planned_partitions);
find_hash_columns(aggstate);
- build_hash_tables(aggstate);
+
+ /* Skip massive memory allocation when running only explain */
+ if (!(eflags & EXEC_FLAG_EXPLAIN_ONLY))
+ build_hash_tables(aggstate);
+
aggstate->table_filled = false;
/* Initialize this to 1, meaning nothing spilled, yet */
