Question

If I have a MyISAM table with a 3-column index, something like

create table t (
  a int,
  b int,
  c int,
  index abc (a, b, c)
) engine=MyISAM;

the question is, can the following query fully utilize the index:

select * from t where a=1 and c=2;

in other words, considering that an index is a b-tree, can MySQL skip the column in the middle and still do a quick search on first and last columns?

EXPLAIN does seem to be showing that the index will be used, however, the Extra says: Using where; Using index and I have no idea what this really means.

Answer 1

The answer is “no”.

The MySQL documentation is quite clear on how indexes are used:

If the table has a multiple-column index, any leftmost prefix of the index can be used by the optimizer to find rows. For example, if you have a three-column index on (col1, col2, col3), you have indexed search capabilities on (col1), (col1, col2), and (col1, col2, col3). (http://dev.mysql.com/doc/refman/5.0/en/mysql-indexes.html.)

What happens is that the index gets used for “a=1”. All records that match are loaded, to see if “c=2” is true. The filter ends up using a combination of indexes and explicit record filtering.

By the way, if you want to handle all combinations of two columns, you need several indexes:

• (a, b, c)
• (b, a, c)
• (c, b, a)