MySQL 8.0 Reference Manual Including MySQL NDB Cluster 8.0

8.3.11 Optimizer Use of Generated Column Indexes

MySQL supports indexes on generated columns. For example:

CREATE TABLE t1 (f1 INT, gc INT AS (f1 + 1) STORED, INDEX (gc));

The generated column, gc, is defined as the expression f1 + 1. The column is also indexed and the optimizer can take that index into account during execution plan construction. In the following query, the WHERE clause refers to gc and the optimizer considers whether the index on that column yields a more efficient plan:

SELECT * FROM t1 WHERE gc > 9;

The optimizer can use indexes on generated columns to generate execution plans, even in the absence of direct references in queries to those columns by name. This occurs if the WHERE, ORDER BY, or GROUP BY clause refers to an expression that matches the definition of some indexed generated column. The following query does not refer directly to gc but does use an expression that matches the definition of gc:

SELECT * FROM t1 WHERE f1 + 1 > 9;

The optimizer recognizes that the expression f1 + 1 matches the definition of gc and that gc is indexed, so it considers that index during execution plan construction. You can see this using EXPLAIN:

mysql> EXPLAIN SELECT * FROM t1 WHERE f1 + 1 > 9\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: t1
   partitions: NULL
         type: range
possible_keys: gc
          key: gc
      key_len: 5
          ref: NULL
         rows: 1
     filtered: 100.00
        Extra: Using index condition

In effect, the optimizer has replaced the expression f1 + 1 with the name of the generated column that matches the expression. That is also apparent in the rewritten query available in the extended EXPLAIN information displayed by SHOW WARNINGS:

*************************** 1. row ***************************
  Level: Note
   Code: 1003
Message: /* select#1 */ select `test`.`t1`.`f1` AS `f1`,`test`.`t1`.`gc`
         AS `gc` from `test`.`t1` where (`test`.`t1`.`gc` > 9)

The following restrictions and conditions apply to the optimizer's use of generated column indexes: