MySQL supports invisible indexes; that is, indexes that are not used by the optimizer. The feature applies to indexes other than primary keys (either explicit or implicit).

Indexes are visible by default. To control visibility explicitly for a new index, use a VISIBLE or INVISIBLE keyword as part of the index definition for CREATE TABLE, CREATE INDEX, or ALTER TABLE:

CREATE TABLE t1 (
  i INT,
  j INT,
  k INT,
  INDEX i_idx (i) INVISIBLE
) ENGINE = InnoDB;
CREATE INDEX j_idx ON t1 (j) INVISIBLE;
ALTER TABLE t1 ADD INDEX k_idx (k) INVISIBLE;

To alter the visibility of an existing index, use a VISIBLE or INVISIBLE keyword with the ALTER TABLE ... ALTER INDEX operation:

ALTER TABLE t1 ALTER INDEX i_idx INVISIBLE;
ALTER TABLE t1 ALTER INDEX i_idx VISIBLE;

Information about whether an index is visible or invisible is available from the Information Schema STATISTICS table or SHOW INDEX output. For example:

mysql> SELECT INDEX_NAME, IS_VISIBLE
       FROM INFORMATION_SCHEMA.STATISTICS
       WHERE TABLE_SCHEMA = 'db1' AND TABLE_NAME = 't1';
+------------+------------+
| INDEX_NAME | IS_VISIBLE |
+------------+------------+
| i_idx      | YES        |
| j_idx      | NO         |
| k_idx      | NO         |
+------------+------------+

Invisible indexes make it possible to test the effect of removing an index on query performance, without making a destructive change that must be undone should the index turn out to be required. Dropping and re-adding an index can be expensive for a large table, whereas making it invisible and visible are fast, in-place operations.

If an index made invisible actually is needed or used by the optimizer, there are several ways to notice the effect of its absence on queries for the table:

The use_invisible_indexes flag of the optimizer_switch system variable controls whether the optimizer uses invisible indexes for query execution plan construction. If the flag is off (the default), the optimizer ignores invisible indexes (the same behavior as prior to the introduction of this flag). If the flag is on, invisible indexes remain invisible but the optimizer takes them into account for execution plan construction.

Using the SET_VAR optimizer hint to update the value of optimizer_switch temporarily, you can enable invisible indexes for the duration of a single query only, like this:

mysql> EXPLAIN SELECT /*+ SET_VAR(optimizer_switch = 'use_invisible_indexes=on') */
     >     i, j FROM t1 WHERE j >= 50\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: t1
   partitions: NULL
         type: range
possible_keys: j_idx
          key: j_idx
      key_len: 5
          ref: NULL
         rows: 2
     filtered: 100.00
        Extra: Using index condition

mysql> EXPLAIN SELECT i, j FROM t1 WHERE j >= 50\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: t1
   partitions: NULL
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 5
     filtered: 33.33
        Extra: Using where

Index visibility does not affect index maintenance. For example, an index continues to be updated per changes to table rows, and a unique index prevents insertion of duplicates into a column, regardless of whether the index is visible or invisible.

A table with no explicit primary key may still have an effective implicit primary key if it has any UNIQUE indexes on NOT NULL columns. In this case, the first such index places the same constraint on table rows as an explicit primary key and that index cannot be made invisible. Consider the following table definition:

CREATE TABLE t2 (
  i INT NOT NULL,
  j INT NOT NULL,
  UNIQUE j_idx (j)
) ENGINE = InnoDB;

The definition includes no explicit primary key, but the index on NOT NULL column j places the same constraint on rows as a primary key and cannot be made invisible:

mysql> ALTER TABLE t2 ALTER INDEX j_idx INVISIBLE;
ERROR 3522 (HY000): A primary key index cannot be invisible.

Now suppose that an explicit primary key is added to the table:

ALTER TABLE t2 ADD PRIMARY KEY (i);

The explicit primary key cannot be made invisible. In addition, the unique index on j no longer acts as an implicit primary key and as a result can be made invisible:

mysql> ALTER TABLE t2 ALTER INDEX j_idx INVISIBLE;
Query OK, 0 rows affected (0.03 sec)