It would seem that the planner does not take into account whether the
datatypes involved have the capability to use hash aggregates or not.
sdb=# explain select query_start, current_query from pg_locks join
stat_activity on pid = procpid where granted = true and transaction in (select
transaction from pg_locks where granted = false);
ERROR: could not find hash function for hash operator 716373
ssdb=# set enable_hashagg = off;
SET
ssdb=# explain select query_start, current_query from pg_locks join
stat_activity on pid = procpid where granted = true and transaction in (select
transaction from pg_locks where granted = false);
QUERY PLAN
------------------------------------------------------------------------------------------------------------
Hash Join (cost=60.79..159.12 rows=1244 width=40)
Hash Cond: ("outer".procpid = "inner".pid)
-> Function Scan on stat_activity (cost=0.00..15.00 rows=995 width=44)
Filter: (current_query <> '<IDLE>'::text)
-> Hash (cost=60.16..60.16 rows=250 width=4)
-> Hash Join (cost=40.16..60.16 rows=250 width=4)
Hash Cond: ("outer"."transaction" = "inner"."transaction")
-> Function Scan on pg_lock_status l (cost=0.00..15.00
rows=500 width=8)
Filter: (granted = true)
-> Hash (cost=39.91..39.91 rows=100 width=4)
-> Unique (cost=37.41..39.91 rows=100 width=4)
-> Sort (cost=37.41..38.66 rows=500 width=4)
Sort Key: l."transaction"
-> Function Scan on pg_lock_status l
(cost=0.00..15.00 rows=500 width=4)
Filter: (granted = false)
(15 rows)
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