Question

My question is about indexes and primary keys.

Let’s suppose we have a table like this:

demo_table

| id                   | int(11)      | NO   | PRI |         |
| subject              | varchar(255) | YES  |     | NULL    |
| body                 | text         | YES  |     | NULL    |
| to                   | varchar(255) | YES  | MUL | NULL    |
| from                 | varchar(255) | YES  | MUL | NULL    |
| media                | varchar(255) | YES  |     | NULL    |

I have a query that looks for the last row of a specific ‘to’ / ‘media’ couple:

select * from demo_table where to="" and media="" order by id desc limit 0,1;

and I would like to create an index for my table. Should I create the index like this:

CREATE INDEX to_media on demo_table(media,to);

or like this:

CREATE INDEX to_media on demo_table(id,media,to);

seeing that the ID is sorted?

Answer 1

CREATE INDEX to_media on demo_table(id,media,to);