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EXPLAIN in the slow query log



After recent changes in the [SHOW] EXPLAIN architecture in MariaDB, it became 
really easy to add an option to print EXPLAIN(*) output into the slow query log, 
so I went ahead and implemented it.

@@log_slow_verbosity has got a 'explain' flag, and if it is on, the slow query
log entries look like this:

# Time: 131017 21:26:02
# User@Host: root[root] @ localhost []
# Thread_id: 2  Schema: dbt3sf1  QC_hit: No
# Query_time: 5.535819  Lock_time: 0.000350  Rows_sent: 1  Rows_examined: 65633
## <explain>
##   id select_type     table   type    possible_keys   key     key_len ref     rows    Extra
##   1  SIMPLE  nation  ref     PRIMARY,n_name  n_name  26      const   1       Using where; Using index
##   1  SIMPLE  customer        ref     PRIMARY,i_c_nationkey   i_c_nationkey   5       dbt3sf1.nation.n_nationkey      25124   Using index
##   1  SIMPLE  orders  ref     i_o_custkey     i_o_custkey     5       dbt3sf1.customer.c_custkey      7       Using index
## </explain>
SET timestamp=1382030762;
select count(*) from customer, orders, nation where c_custkey=o_custkey and c_nationkey=n_nationkey and n_name='GERMANY';

It seems, pt-slow-query-log has no problem parsing this.

Open questions:

- Is this syntax OK?

- Currently, some statements (like SHOW KEYS) print a dumb explain that
shows that full table scan is used. There is a suggestion to not print
EXPLAIN for any SHOW statements. Is this ok?

(*) something like EXPLAIN ANALYZE, with numbers from actual execution, would 
be better but we are not there yet.


Sergei Petrunia, Software Developer
MariaDB | Skype: sergefp | Blog: http://s.petrunia.net/blog

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