EXPLAIN FORMAT=JSON provides insights on optimizer_switch effectiveness

EXPLAIN FORMAT=JSONThe previous post in the EXPLAIN FORMAT=JSON is Cool! series showed an example of the query select dept_name from departments where dept_no in (select dept_no from dept_manager where to_date is not null), where the subquery was materialized into a temporary table and then joined with the outer query. This is known as a semi-join optimization. But what happens if we turn off this optimization?

EXPLAIN FORMAT=JSON can help us with this investigation too.

First lets look at the original output again:

To repeat what happened here: the subquery was materialized into a  temporary table, then  joined with the departments table. Semi-join optimization is ON by default (as would be most likely without intervention).

What happens if we temporarily turn semi-join optimization OFF?

And then execute EXPLAIN one more time:

Now the picture is completely different. There is no nested_loop member, and instead there is an  attached_subqueries array containing a single member: the temporary table materialized from the subquery select dept_no from dept_manager where to_date is not null (including all the details of this materialization).

Conclusion: We can experiment with the value of optimizer_switch and use EXPLAIN FORMAT=JSON to examine how a particular optimization affects our queries.

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