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;

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);

  "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

              }

            }

          }

        }

      }

    }