EXPLAIN lists a subquery that constant folding removed from WHERE

EXPLAIN reports a subquery that the optimizer removed and that can never run; ANALYZE FORMAT=JSON shows r_loops 0 for the subquery's table.

create table t1 (t text character set utf8mb4);

insert into t1 values ('a'),('b');

explain select * from t1 where t = 'b' and (1 = 1 or t = (select t from t1 limit 1));

explain select * from t1 where t = 'b' and 1 = 0 and t = (select t from t1 limit 1);

drop table t1;

id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra

1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	2	Using where

2	SUBQUERY	t1	ALL	NULL	NULL	NULL	NULL	2

id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra

1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE

2	SUBQUERY	t1	ALL	NULL	NULL	NULL	NULL	2

The first WHERE reduces to t = 'b' and the second to Impossible WHERE, yet both plans list select#2. A constant IN shows the same thing, for example 'b' in ('b', (select t from t1 limit 1)) and t = 'b'.

The column is utf8mb4, the default character set in 11.8, so no multiple equality is built for t = (select ...). With latin1, the default in 11.4, the multiple equality evaluates the subquery once during optimization (Item_equal::add_const()), before the WHERE is folded. ANALYZE then shows r_loops 1 for the first two queries, on 11.4 and on 11.8 alike. EXPLAIN lists select#2 regardless of the character set. For a table created without a character set, 11.4 lists a subquery that ran once yet is no longer part of the plan, while 11.8 lists a subquery that never runs (r_loops 0). The constant IN example never runs the subquery under either character set.