Hello Igor, While reviewing MDEV-12845, I've found this piece in sql_select.cc: while ((tbl= li++)) { /* Do not push conditions from where into materialized inner tables of outer joins: this is not valid. */ if (tbl->is_materialized_derived()) { /* Do not push conditions from where into materialized inner tables of outer joins: this is not valid. */ if (!tbl->is_inner_table_of_outer_join()) Q1: Is int meaningful to have two identical comments? Q2: It is not valid push parts of WHERE, but it is valid to push the parts of ON expression. An example: explain format=json select * from t21 LEFT JOIN (select a,b, COUNT(*) as CNT from t22 group by a,b) TBL ON (t21.a=TBL.a AND TBL.a<5); here, "TBL.a<5" can be pushed down into the subquery, but it is not. EXPLAIN EXTENDED confirms it: "query_block": { "select_id": 1, "const_condition": "1", "table": { "table_name": "t21", "access_type": "ALL", "rows": 10, "filtered": 100 }, "table": { "table_name": "<derived2>", "access_type": "ref", "possible_keys": ["key0"], "key": "key0", "key_length": "5", "used_key_parts": ["a"], "ref": ["test.t21.a"], "rows": 2, "filtered": 100, "attached_condition": "trigcond(trigcond(t21.a < 5 and t21.a is not null))", "materialized": { "query_block": { "select_id": 2, "filesort": { "sort_key": "t22.a, t22.b", "temporary_table": { "table": { "table_name": "t22", "access_type": "ALL", "rows": 10, "filtered": 100 } } } } } } Just in case, the commands to create the dataset were: create table ten(a int); insert into ten values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9); create table t21 (a int, b int, c int); insert into t21 select a,a,a from ten; create table t22 (a int, b int, c int); insert into t22 select a,a,a from ten; BR Sergei -- Sergei Petrunia, Software Developer MariaDB Corporation | Skype: sergefp | Blog: http://s.petrunia.net/blog