InnoDB automatically extends each secondary index by appending the primary key columns to it. Consider this table definition:

CREATE TABLE t1 (
  i1 INT NOT NULL DEFAULT 0,
  i2 INT NOT NULL DEFAULT 0,
  d DATE DEFAULT NULL,
  PRIMARY KEY (i1, i2),
  INDEX k_d (d)
) ENGINE = InnoDB;

This table defines the primary key on columns (i1, i2). It also defines a secondary index k_d on column (d), but internally InnoDB extends this index and treats it as columns (d, i1, i2).

The optimizer takes into account the primary key columns of the extended secondary index when determining how and whether to use that index. This can result in more efficient query execution plans and better performance.

The optimizer can use extended secondary indexes for ref, range, and index_merge index access, for Loose Index Scan access, for join and sorting optimization, and for MIN()/MAX() optimization.

The following example shows how execution plans are affected by whether the optimizer uses extended secondary indexes. Suppose that t1 is populated with these rows:

INSERT INTO t1 VALUES
(1, 1, '1998-01-01'), (1, 2, '1999-01-01'),
(1, 3, '2000-01-01'), (1, 4, '2001-01-01'),
(1, 5, '2002-01-01'), (2, 1, '1998-01-01'),
(2, 2, '1999-01-01'), (2, 3, '2000-01-01'),
(2, 4, '2001-01-01'), (2, 5, '2002-01-01'),
(3, 1, '1998-01-01'), (3, 2, '1999-01-01'),
(3, 3, '2000-01-01'), (3, 4, '2001-01-01'),
(3, 5, '2002-01-01'), (4, 1, '1998-01-01'),
(4, 2, '1999-01-01'), (4, 3, '2000-01-01'),
(4, 4, '2001-01-01'), (4, 5, '2002-01-01'),
(5, 1, '1998-01-01'), (5, 2, '1999-01-01'),
(5, 3, '2000-01-01'), (5, 4, '2001-01-01'),
(5, 5, '2002-01-01');

Now consider this query:

EXPLAIN SELECT COUNT(*) FROM t1 WHERE i1 = 3 AND d = '2000-01-01'

The execution plan depends on whether the extended index is used.

When the optimizer does not consider index extensions, it treats the index k_d as only (d). EXPLAIN for the query produces this result:

mysql> EXPLAIN SELECT COUNT(*) FROM t1 WHERE i1 = 3 AND d = '2000-01-01'\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: t1
         type: ref
possible_keys: PRIMARY,k_d
          key: k_d
      key_len: 4
          ref: const
         rows: 5
        Extra: Using where; Using index

When the optimizer takes index extensions into account, it treats k_d as (d, i1, i2). In this case, it can use the leftmost index prefix (d, i1) to produce a better execution plan:

mysql> EXPLAIN SELECT COUNT(*) FROM t1 WHERE i1 = 3 AND d = '2000-01-01'\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: t1
         type: ref
possible_keys: PRIMARY,k_d
          key: k_d
      key_len: 8
          ref: const,const
         rows: 1
        Extra: Using index

In both cases, key indicates that the optimizer uses secondary index k_d but the EXPLAIN output shows these improvements from using the extended index:

Differences in optimizer behavior for use of extended indexes can also be seen with SHOW STATUS:

FLUSH TABLE t1;
FLUSH STATUS;
SELECT COUNT(*) FROM t1 WHERE i1 = 3 AND d = '2000-01-01';
SHOW STATUS LIKE 'handler_read%'

The preceding statements include FLUSH TABLES and FLUSH STATUS to flush the table cache and clear the status counters.

Without index extensions, SHOW STATUS produces this result:

+-----------------------+-------+
| Variable_name         | Value |
+-----------------------+-------+
| Handler_read_first    | 0     |
| Handler_read_key      | 1     |
| Handler_read_last     | 0     |
| Handler_read_next     | 5     |
| Handler_read_prev     | 0     |
| Handler_read_rnd      | 0     |
| Handler_read_rnd_next | 0     |
+-----------------------+-------+

With index extensions, SHOW STATUS produces this result. The Handler_read_next value decreases from 5 to 1, indicating more efficient use of the index:

+-----------------------+-------+
| Variable_name         | Value |
+-----------------------+-------+
| Handler_read_first    | 0     |
| Handler_read_key      | 1     |
| Handler_read_last     | 0     |
| Handler_read_next     | 1     |
| Handler_read_prev     | 0     |
| Handler_read_rnd      | 0     |
| Handler_read_rnd_next | 0     |
+-----------------------+-------+

The use_index_extensions flag of the optimizer_switch system variable permits control over whether the optimizer takes the primary key columns into account when determining how to use an InnoDB table's secondary indexes. By default, use_index_extensions is enabled. To check whether disabling use of index extensions can improve performance, use this statement:

SET optimizer_switch = 'use_index_extensions=off';

Use of index extensions by the optimizer is subject to the usual limits on the number of key parts in an index (16) and the maximum key length (3072 bytes).