Is there any way to trace optimizer's work in MySQL?

Question

Just like the MEMO structure in SQL Server which is kind of a "paper trail" of steps the optimizer takes in optimizing the query. Is there anything in MySQL through which I can get the information like, which plans the optimizer considers, the cost of each plan?

Answer 1

The recent 5.6 version has added this feature.

See: MySQL Internals Manual ::chapter 9. Tracing the Optimizer

Typical Usage:

# Turn tracing on (it's off by default):
SET optimizer_trace="enabled=on";

SELECT ...; # your query here

SELECT * FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE;

# possibly more queries...

# When done with tracing, disable it:
SET optimizer_trace="enabled=off";

You can see it in action in SQL-Fiddle. Not sure how the output can be beautified.

Answer 2

The best I can immediately mention is to simply run EXPLAIN on a query.

There is a graphical way to do this

• pt-visual-explain
• mk-visual-explain (Source Code)

As for steps to optimizing a query, a parse tree is created. Rather than explaining the parser here, please read my post from Mar 11, 2013 (Is there an execution difference between a JOIN condition and a WHERE condition?) on what the parser does.