How to use MySQL index columns?

Question

When do you use each MySQL index type?

• PRIMARY - Primary key columns?
• UNIQUE - Foreign keys?
• INDEX - ??

For really large table

Answer 1

I'm not that familiar with MySQL, however I believe the following to be true across most database servers. An index is a balanced tree which is used to allow the database to scan the table for given data. For example say you have the following table.

CREATE TABLE person (
    id    SERIAL,
    name  VARCHAR(20),
    dob   VARCHAR(20)
);

If you created an index on the 'name' field this would create in a balanced tree for that data in the table for the name column. Balanced tree data structures allow for faster searching of results (see http://www.solutionhacker.com/tag/balanced-tree/).

You should note however indexing a column only allows you to search on the data as it is stored in the database. For example:

This would not be able to search on the index and would instead do a sequential scan on the table, calling UPPER() on each of the column:name rows in the table.

select *
from person
where UPPER(name) = "BOB";

This would also have the following effect, because the index will be sorted starting with the first letter. Replacing the search term with "%B" would however use the index.

select *
from person
where name like "%B"