Question

When creating a compound database index, what should influence the ordering of the fields?

For example suppose I am creating a compound index based on DATE, PRICE, VOLUME what should influence whether I create

DATE, PRICE, VOLUME
VOLUME, DATE, PRICE
.
.
.

Presumably one will be more suited to particular types of queries but I don’t know which queries.

Answer 1

First one will be used if you filter(or search) records by DATE, or by DATE and PRICE, or by DATE,PRICE,VOLUME. It won’t be used if you filter records, for instance, by VOLUME. The second one will be used for queries where you search condition includes VOLUME, or VOLUME and DATE, or VOLUME,DATE,and PRICE.

To be more precise, indexes can also be used when you use indexed columns in GROUP BY and ORDER BY.

Some examples:
Index 1 (DATE,PRICE,VOLUME)
Index 2 (VOLUME,DATE,PRICE)

SELECT * FROM table1 WHERE `DATE` = '2011-10-01'; //Index 1 used, Index 2 not used
SELECT * FROM table1 WHERE `VOLUME` = '111'; //Index 1 not used, Index 2 used