EXPLAIN currently supports SELECT queries that target information_schema tables. For example, these queries are valid:

EXPLAIN SELECT * FROM information_schema.TABLES 

   WHERE TABLE_SCHEMA='db1';

EXPLAIN SELECT * FROM information_schema.TRIGGERS 

   WHERE EVENT_OBJECT_SCHEMA='db1';

EXPLAIN SELECT * FROM information_schema.GLOBAL_VARIABLES 

   WHERE VARIABLE_NAME='character_set_server';

Sometimes this functionality is very important, because innocent-looking SELECT queries that target information_schema tables can actually cause serious performance issues. For example, see here:

https://dev.mysql.com/doc/refman/5.7/en/information-schema-optimization.html

However, one problem is that if we convert the above SELECT queries to their equivalent SHOW queries, then EXPLAIN fails. For example:

EXPLAIN SHOW TABLE STATUS 

   FROM db1;

EXPLAIN SHOW TRIGGERS 

   FROM db1;

EXPLAIN SHOW GLOBAL VARIABLES 

   WHERE VARIABLE_NAME='character_set_server';

Some of these SHOW queries can probably have similar problems as SELECT queries that target information_schema tables.

If so, then wouldn't it make sense to implement EXPLAIN for them as well?