MariaDB starting with 10.0.0

The SHOW EXPLAIN command is a new feature in MariaDB 10.0.0.

Command description

The SHOW EXPLAIN command allows one to get an EXPLAIN (that is, a description of a query plan) of a query running in a certain thread.

The syntax is:

SHOW EXPLAIN FOR <thread_id>;

which will produce an EXPLAIN output for the query that thread number thread_id is running:

SHOW EXPLAIN FOR 1;
+------+-------------+-------+-------+---------------+------+---------+------+---------+-------------+
| id   | select_type | table | type  | possible_keys | key  | key_len | ref  | rows    | Extra       |
+------+-------------+-------+-------+---------------+------+---------+------+---------+-------------+
|    1 | SIMPLE      | tbl   | index | NULL          | a    | 5       | NULL | 1000107 | Using index |
+------+-------------+-------+-------+---------------+------+---------+------+---------+-------------+
1 row in set, 1 warning (0.00 sec)

The output is always accompanied with a warning which shows the query the target thread is running (this shows what the EXPLAIN is for):

SHOW WARNINGS;
+-------+------+------------------------+
| Level | Code | Message                |
+-------+------+------------------------+
| Note  | 1003 | select sum(a) from tbl |
+-------+------+------------------------+
1 row in set (0.00 sec)

Possible errors

The output can be only produced if the target thread is currently running a query, which has a ready query plan. If this is not the case, the output will be:

SHOW EXPLAIN FOR 2;
ERROR 1932 (HY000): Target is not running an EXPLAINable command

You will get this error when:

  • the target thread is not running a command for which one can run EXPLAIN
  • the target thread is running a command for which one can run EXPLAIN, but
    • there is no query plan yet (for example, tables are open and locks are acquired before the query plan is produced)

Differences between SHOW EXPLAIN and EXPLAIN outputs

Background

In MySQL, EXPLAIN execution takes a slightly different route from the way the real query (typically the SELECT) is optimized. This is unfortunate, and has caused a number of bugs in EXPLAIN. (For example, see MDEV-326, MDEV-410, and lp:1013343. lp:992942 is not directly about EXPLAIN, but it also would not have existed if MySQL didn't try to delete parts of a query plan in the middle of the query)

SHOW EXPLAIN examines a running SELECT, and hence its output may be slightly different from what EXPLAIN SELECT would produce. We did our best to make sure that either the difference is negligible, or SHOW EXPLAIN's output is closer to reality than EXPLAIN's output.

List of recorded differences

  • SHOW EXPLAIN may have Extra='no matching row in const table', where EXPLAIN would produce Extra='Impossible WHERE ...'
  • For queries with subqueries, SHOW EXPLAIN may print select_type==PRIMARY where regular EXPLAIN used to print select_type==SIMPLE, or vice versa.

Required permissions

Running SHOW EXPLAIN requires the same permissions as running SHOW PROCESSLIST would.

See also

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