Does Order of Fields of Multi-Column Index in MySQL Matter

Question

I know the importance of indexes and how order of joins can change performance. I\'ve done a bunch of reading related to multi-column indexes and haven\'t found the answer t

Answer 1

The two indexes are different. This is true in MySQL and in other databases. MySQL does a pretty good job of explaining the different in the documentation.

Consider the two indexes:

create index idx_lf on name(last_name, first_name);
create index idx_fl on name(first_name, last_name);

Both of these should work equally well on:

where last_name = XXX and first_name = YYY

idx_lf will be optimal for the following conditions:

where last_name = XXX
where last_name like 'X%'
where last_name = XXX and first_name like 'Y%'
where last_name = XXX order by first_name

idx_fl will be optimal for the following:

where first_name = YYY
where first_name like 'Y%'
where first_name = YYY and last_name like 'X%'
where first_name = XXX order by last_name

For many of these cases, both indexes could possibly be used, but one is optimal. For instance, consider idx_lf with the query:

where first_name = XXX order by last_name

MySQL could read the entire table using idx_lf and then do the filtering after the order by. I don't think this is an optimization option in practice (for MySQL), but that can happen in other databases.