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.
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: