Appendix B. Using EXPLAIN

This appendix shows you how to invoke EXPLAIN to get information about the query execution plan, and how to interpret the output. The EXPLAIN command is the main way to find out how the query optimizer decides to execute queries. This feature has many limitations and doesn’t always tell the truth, but its output is the best information available, and it’s worth studying, so you can make an educated guess about how your queries are executed.

Invoking EXPLAIN

To use EXPLAIN, simply add the word EXPLAIN just before the SELECT keyword in your query. MySQL will set a flag on the query. When it executes the query, the flag causes it to return information about each step in the execution plan, instead of executing it. It returns one or more rows, which show each part of the execution plan and the order of execution.

Here’s the simplest possible EXPLAIN result:

*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: NULL
         type: NULL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: NULL
        Extra: No tables used

There’s one row in the output per table in the query. If the query joins two tables, there will be two rows of output. An aliased table counts as a separate table, so if you join a table to itself, there will be two rows in the output. The meaning of “table” is fairly broad here: it can mean a subquery, a UNION result, and so on. You’ll see later why this is so.

There are two important variations ...

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