8.8.3. `EXPLAIN EXTENDED` Output Format

When EXPLAIN is used with the EXTENDED keyword, the output includes a filtered column not otherwise displayed. This column indicates the estimated percentage of table rows that will be filtered by the table condition. In addition, the statement produces extra information that can be viewed by issuing a SHOW WARNINGS statement following the EXPLAIN statement. 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. Here is an example:

mysql> EXPLAIN
        EXTENDED    -> SELECT t1.a, t1.a IN (SELECT t2.a FROM t2)
        FROM t1\G*************************** 1. row ***************************           id: 1  select_type: PRIMARY        table: t1         type: indexpossible_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: indexpossible_keys: a          key: a      key_len: 5          ref: NULL         rows: 3     filtered: 100.00        Extra: Using index2 rows in set, 1 warning (0.00 sec)mysql> SHOW WARNINGS\G*************************** 1. row ***************************  Level: Note   Code: 1003Message: /* 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)

EXPLAIN EXTENDED can be used with SELECT, DELETE, INSERT, REPLACE, and UPDATE statements. However, the following SHOW WARNINGS statement displays a nonempty result only for SELECT statements.

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 EXTENDED output displayed by SHOW WARNINGS:

<auto_key>

An automatically generated key for a temporary table.
<cache>(expr)

The expression (such as a scalar subquery) is executed once and the resulting value is saved in memory for later use. For results consisting of multiple values, a temporary table may be created and you will see <temporary table> instead.
<exists>(query fragment)

The subquery predicate is converted to an EXISTS predicate and the subquery is transformed so that it can be used together with the EXISTS predicate.
<in_optimizer>(query fragment)

This is an internal optimizer object with no user significance.
<index_lookup>(query fragment)

The query fragment is processed using an index lookup to find qualifying rows.
<if>(condition, expr1, expr2)

If the condition is true, evaluate to expr1, otherwise expr2.
<is_not_null_test>(expr)

A test to verify that the expression does not evaluate to NULL.
<materialize>(query fragment)

Subquery materialization is used.
`materialized-subquery`.col_name

A reference to the column col_name in an internal temporary table materialized to hold the result from evaluating a subquery.
<primary_index_lookup>(query fragment)

The query fragment is processed using a primary key lookup to find qualifying rows.
<ref_null_helper>(expr)

This is an internal optimizer object with no user significance.
/* select#N */ select_stmt

The SELECT is associated with the row in non-EXTENDED EXPLAIN output that has an id value of N.
outer_tables semi join (inner_tables)

A semi-join operation. inner_tables shows the tables that were not pulled out. See Section 8.13.16.1, "Optimizing Subqueries with Semi-Join Transformations".
<temporary table>

This represents an internal temporary table created to cache an intermediate result.

8.8.3. EXPLAIN EXTENDED Output Format

8.8.3. `EXPLAIN EXTENDED` Output Format