EXPLAIN FORMAT=JSON
The regular EXPLAIN command already provides some information about subquery optimization. For example, you can find out if the subquery is dependent or not, and (since version 5.6) if it was materialized:
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mysql> explain select dept_name from departments where dept_no in (select dept_no from dept_manager where to_date is not null)G *************************** 1. row *************************** id: 1 select_type: SIMPLE table: departments partitions: NULL type: index possible_keys: PRIMARY key: dept_name key_len: 42 ref: NULL rows: 9 filtered: 100.00 Extra: Using where; Using index *************************** 2. row *************************** id: 1 select_type: SIMPLE table: <subquery2> partitions: NULL type: eq_ref possible_keys: <auto_key> key: <auto_key> key_len: 4 ref: employees.departments.dept_no rows: 1 filtered: 100.00 Extra: NULL *************************** 3. row *************************** id: 2 select_type: MATERIALIZED table: dept_manager partitions: NULL type: ALL possible_keys: dept_no key: NULL key_len: NULL ref: NULL rows: 24 filtered: 90.00 Extra: Using where 3 rows in set, 1 warning (0.00 sec) Note (Code 1003): /* select#1 */ select `employees`.`departments`.`dept_name` AS `dept_name` from `employees`.`departments` semi join (`employees`.`dept_manager`) where ((`<subquery2>`.`dept_no` = `employees`.`departments`.`dept_no`) and (`employees`.`dept_manager`.`to_date` is not null)) |
However, you can’t find details on exactly how this subquery was materialized. To find out more, use EXPLAIN FORMAT=JSON:
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mysql> explain format=json select dept_name from departments where dept_no in (select dept_no from dept_manager where to_date is not null)G *************************** 1. row *************************** EXPLAIN: { "query_block": { "select_id": 1, "cost_info": { "query_cost": "16.72" }, "nested_loop": [ { "table": { "table_name": "departments", <skipped> }, { "table": { "table_name": "<subquery2>", "access_type": "eq_ref", "key": "<auto_key>", "key_length": "4", "ref": [ "employees.departments.dept_no" ], "rows_examined_per_scan": 1, "materialized_from_subquery": { "using_temporary_table": true, "query_block": { "table": { "table_name": "dept_manager", "access_type": "ALL", "possible_keys": [ "dept_no" ], "rows_examined_per_scan": 24, "rows_produced_per_join": 21, "filtered": "90.00", "cost_info": { "read_cost": "1.48", "eval_cost": "4.32", "prefix_cost": "5.80", "data_read_per_join": "345" }, "used_columns": [ "dept_no", "to_date" ], "attached_condition": "(`employees`.`dept_manager`.`to_date` is not null)" } } } } } ] } } 1 row in set, 1 warning (0.00 sec) Note (Code 1003): /* select#1 */ select `employees`.`departments`.`dept_name` AS `dept_name` from `employees`.`departments` semi join (`employees`.`dept_manager`) where ((`<subquery2>`.`dept_no` = `employees`.`departments`.`dept_no`) and (`employees`.`dept_manager`.`to_date` is not null)) |
We can see here that the new temporary table ( "using_temporary_table": true) with internal name <subquery2> was materilized from the subquery ( materialized_from_subquery). We also see how this new temporary table was accessed.
We also can see that the member that contains information about the materialized subquery is part of the nested_loop array. Nested loop optimization applies to a regular JOIN command. In this case, it means that the MySQL Server would join the temporary table with an outer query.
Another interesting case is optimized_away_subqueries: this command shows subqueries that were executed only once and were replaced by their result. A regular EXPLAIN command provides no information about such optimization:
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mysql> explain select emp_no, salary from salaries order by (select max(salary) from salaries)G *************************** 1. row *************************** id: 1 select_type: PRIMARY table: salaries partitions: NULL type: ALL possible_keys: NULL key: NULL key_len: NULL ref: NULL rows: 2838525 filtered: 100.00 Extra: NULL *************************** 2. row *************************** id: 2 select_type: SUBQUERY table: salaries partitions: NULL type: ALL possible_keys: NULL key: NULL key_len: NULL ref: NULL rows: 2838525 filtered: 100.00 Extra: NULL 2 rows in set, 1 warning (0.00 sec) Note (Code 1003): /* select#1 */ select `employees`.`salaries`.`emp_no` AS `emp_no`,`employees`.`salaries`.`salary` AS `salary` from `employees`.`salaries` order by (/* select#2 */ select max(`employees`.`salaries`.`salary`) from `employees`.`salaries`) |
We can see that in select_type: SUBQUERY, the output of SHOW WARNINGS doesn’t give us much insight either.
However,