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 : 

• <auto_key> 

An automatically generated key for a temporary table. 

