Get the execution plan for a running query. This statement displays EXPLAIN output for a statement currently executing in another thread.
The SHOW EXPLAIN command allows one to get an (that is, a description of a query plan) of a query running in a certain connection.
will produce an EXPLAIN output for the query that connection number connection_id is running. The connection id can be obtained with .
The output is always accompanied with a warning which shows the query the target connection is running (this shows what the EXPLAIN is for):
EXPLAIN FOR CONNECTION
The EXPLAIN FOR CONNECTION syntax was added for MySQL compatibility.
FORMAT=JSON
SHOW EXPLAIN [FORMAT=JSON] FOR <connection_id> extends SHOW EXPLAIN to return more detailed JSON output.
EXPLAIN FOR CONNECTION and FORMAT=JSON are not available.
The output can be only produced if the target connection is currently running a query, which has a ready query plan. If this is not the case, the output will be:
You will get this error when:
The target connection is not running a command for which one can run EXPLAIN;
The target connection 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).
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 , , and . 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.
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
Running SHOW EXPLAIN requires the same permissions as running would.
, which will perform a query and outputs enhanced EXPLAIN results.
It is also possible to .
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SHOW EXPLAIN [FORMAT=JSON] FOR <connection_id>;
EXPLAIN [FORMAT=JSON] FOR CONNECTION <connection_id>;SHOW EXPLAIN FOR <connection_id>;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)SHOW WARNINGS;
+-------+------+------------------------+
| Level | Code | Message |
+-------+------+------------------------+
| Note | 1003 | select sum(a) from tbl |
+-------+------+------------------------+
1 row in set (0.00 sec)SHOW EXPLAIN FOR 2;
ERROR 1932 (HY000): Target is not running an EXPLAINable command