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EXPLAIN FORMAT=JSON: order_by_subqueries, group_by_subqueries details on subqueries in ORDER BY and GROUP BY

 | December 29, 2015 |  Posted In: EXPLAIN, EXPLAIN FORMAT=JSON is cool!, JSON, MySQL

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EXPLAIN FORMATAnother post in the EXPLAIN FORMAT=JSON is Cool! series! In this post, we’ll discuss how the EXPLAIN FORMAT=JSON provides optimization details for  ORDER BY and   GROUP BY operations in conjunction with  order_by_subqueries and   group_by_subqueries

EXPLAIN FORMAT=JSON can print details on how a subquery in ORDER BY is optimized:

The above code shows member ordering_operation of query_block  (which includes the  order_by_subqueries array) with information on how the subquery in ORDER BY  was optimized.

This is a simple example. In real life you can have larger subqueries in the  ORDER BY  clause. For example, take this more complicated and slightly crazy query:

Run a regular EXPLAIN on it. If we imagine this is a regular subquery, we won’t know if it can be cached or would be executed for each row sorted.

EXPLAIN FORMAT=JSON  provides a completely different picture:

We see that the subquery was optimized away: member optimized_away_subqueries exists, but there is no order_by_subqueries in the ordering_operation object. We can also see that the subquery was cached: "cacheable": true.

EXPLAIN FORMAT=JSON also provides information about subqueries in the  GROUP BY clause. It uses the  group_by_subqueries array in the  grouping_operation  member for this purpose.

Again, this output gives a clear view of query optimization: subquery in GROUP BY itself cannot be optimized, cached or converted into temporary table, but the subquery inside the subquery ( select dept_no, sum(salary) as s, count(emp_no) as c from salaries join dept_emp using (emp_no) group by dept_no) could be materialized into a temporary table and cached.

A regular EXPLAIN command does not provide such details:

Most importantly, we cannot guess from the output if the  DERIVED subquery can be cached.

Conlcusion: EXPLAIN FORMAT=JSON  provides details on how subqueries in ORDER BY and GROUP BY clauses are optimized.

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Sveta Smirnova

Sveta joined Percona in 2015. Her main professional interests are problem solving, working with tricky issues, bugs, finding patterns that can solve typical issues quicker and teaching others how to deal with MySQL issues, bugs and gotchas effectively. Before joining Percona Sveta worked as a Support Engineer in the MySQL Bugs Analysis Support Group in MySQL AB-Sun-Oracle. She is the author of the book "MySQL Troubleshooting" and JSON UDF functions for MySQL.

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