Switching it On

EXPLAIN output can be switched on by specifying the "explain" keyword in the log_slow_verbosity system variable. Alternatively, you can set with the log-slow-verbosity command line argument.

[mysqld]
log-slow-verbosity=query_plan,explain

EXPLAIN output will only be recorded if the slow query log is written to a file (and not to a table - see Writing logs into tables). This limitation also applies to other extended statistics that are written into the slow query log.

What it Looks Like

When explain recording is on, slow query log entries look like this:

# Time: 131112 17:03:32
# User@Host: root[root] @ localhost []
# Thread_id: 2  Schema: dbt3sf1  QC_hit: No
# Query_time: 5.524103  Lock_time: 0.000337  Rows_sent: 1  Rows_examined: 65633
#
# explain: id   select_type     table   type    possible_keys   key     key_len ref     rows    Extra
# explain: 1    SIMPLE  nation  ref     PRIMARY,n_name  n_name  26      const   1       Using where; Using index
# explain: 1    SIMPLE  customer        ref     PRIMARY,i_c_nationkey   i_c_nationkey   5       dbt3sf1.nation.n_nationkey      3145    Using index
# explain: 1    SIMPLE  orders  ref     i_o_custkey     i_o_custkey     5       dbt3sf1.customer.c_custkey      7       Using index
#
SET timestamp=1384261412;
select count(*) from customer, orders, nation where c_custkey=o_custkey and c_nationkey=n_nationkey and n_name='GERMANY';

EXPLAIN lines start with # explain:.

See Also

• MDEV-407