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10.8.3 Extended EXPLAIN Output Format

The EXPLAIN statement produces extra (“extended”) information that is not part of EXPLAIN output but can be viewed by issuing a SHOW WARNINGS statement following EXPLAIN. Extended information is available for SELECT, DELETE, INSERT, REPLACE, and UPDATE statements.

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.
<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 <temporary table> is shown 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 semijoin operation. inner_tables shows the tables that were not pulled out. See Optimizing IN and EXISTS Subquery Predicates with Semijoin Transformations.
<temporary table>

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

When some tables are of const or system type, expressions involving columns from these tables are evaluated early by the optimizer and are not part of the displayed statement. However, with FORMAT=JSON, some const table accesses are displayed as a ref access that uses a const value.

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