Extended EXPLAIN Output Format The  Message  value in  SHOW WARNINGS  output displays how the optimizer qualifies table and column names in the  SELECT  statement, what the  SELECT  looks like after the application of rewriting and optimization rules, and possibly other notes about the optimization process. The extended information displayable with a  SHOW WARNINGS  statement following  EXPLAIN  is produced only for  SELECT  statements.  SHOW WARNINGS  displays an empty result for other explainable statements ( DELETE ,  INSERT ,  REPLACE , and  UPDATE ). Here is an example of extended  EXPLAIN  output: mysql>  EXPLAIN SELECT t1.a, t1.a IN (SELECT t2.a FROM t2) FROM t1\G *************************** 1. row *************************** id: 1 select_type: PRIMARY table: t1 type: index possible_keys: NULL key: PRIMARY key_len: 4 ref: NULL rows: 4 filtered: 100.00 Extra: Using index *************************** 2. row *************************** id: 2 select_type: SUBQUERY table: t2 type: index possible_keys: a key: a key_len: 5 ref: NULL rows: 3 filtered: 100.00 Extra: Using index 2 rows in set, 1 warning (0.00 sec) mysql>  SHOW WARNINGS\G *************************** 1. row *************************** Level: Note Code: 1003 Message: /* select#1 */ select ‘test‘.‘t1‘.‘a‘ AS ‘a‘, <in_optimizer>(‘test‘.‘t1‘.‘a‘,‘test‘.‘t1‘.‘a‘ in ( <materialize> (/* select#2 */ select ‘test‘.‘t2‘.‘a‘ from ‘test‘.‘t2‘ where 1 having 1 ), <primary_index_lookup>(‘test‘.‘t1‘.‘a‘ in <temporary table> on <auto_key> where ((‘test‘.‘t1‘.‘a‘ = ‘materialized-subquery‘.‘a‘))))) AS ‘t1.a IN (SELECT t2.a FROM t2)‘ from ‘test‘.‘t1‘ 1 row in set (0.00 sec) Because the statement displayed by  SHOW WARNINGS  may contain special markers to provide information about query rewriting or optimizer actions, the statement is not necessarily valid SQL and is not intended to be executed. The output may also include rows with  Message  values that provide additional non-SQL explanatory notes about actions taken by the optimizer. The following list describes special markers that can appear in the extended output displayed by  SHOW WARNINGS : An automatically generated key for a temporary table. • <auto_key> 1777