Need some clarification on indexes (WHERE, JOIN)

Question

We are facing some performance issues in some reports that work on millions of rows. I tried optimizing sql queries, but it only reduces the time of execution to half.

The next step is to analyse and modify or add some indexes, therefore i have some questions:

1- the sql queries contain a lot of joins: do i have to create an index for each foreignkey?

2- Imagine the request SELECT * FROM A LEFT JOIN B on a.b_id = b.id where a.attribute2 = 'someValue' , and we have an index on the table A based on b_id and attribute2: does my request use this index for the where part ( i know if the two conditions were on the where clause the index will be used).

3- If an index is based on columns C1, C2 and C3, and I decided to add an index based on C2, do i need to remove the C2 from the first index?

Thanks for your time

Answer 1

You can use EXPLAIN query to see what MySQL will do when executing it. This helps a LOT when trying to figure out why its slow.

JOIN-ing happens one table at a time, and the order is determined by MySQL analyzing the query and trying to find the fastest order. You will see it in the EXPLAIN result.

1. Only one index can be used per JOIN and it has to be on the table being joined. In your example the index used will be the id (primary key) on table B. Creating an index on every FK will give MySQL more options for the query plan, which may help in some cases.

2. There is only a difference between WHERE and JOIN conditions when there are NULL (missing rows) for the joined table (there is no difference at all for INNER JOIN). For your example the index on b_id does nothing. If you change it to an INNER JOIN (eg by adding b.something = 42 in the where clause), then it might be used if MySQL determines that it should do the query in reverse (first b, then a).

3. No.. It is 100% OK to have a column in multiple indexes. If you have an index on (A,B,C) and you add another one on (A) that will be redundant and pointless (because it is a prefix of another index). An index on B is perfectly fine.