Hi!
Neil Tompkins wrote: > Thanks for your reply. So should we create individual indexes on each > field or a multiple column index ?? This question cannot be answered without checking and measuring your installation. The decision whether to create an index is always an act of balancing: - If there is an index, the database server can use it to find data records by looking up the index, not scanning the base data. This results in load reduction (both CPU and disk IO) and speeds up query execution. - If there is an index, the database server must maintain it whenever data are altered (insert/update/delete), in addition to the base data. This is increased load (both CPU and disk IO) and slows down data changes. So obviously you want to create only those indexes that are helpful for query execution: you will never (voluntarily) create an index on a column which isn't used in search conditions, or whose use is already provided by other indexes. Of the remaining candidate indexes, you will never (voluntarily) create one that provides less gain in searches than it costs in data changes. With MySQL, AFAIK there is the limitation that on one table only one index can be used. As a result, the choice of indexes to create depends on the searches executed by your commands, their relative frequency, and the frequency of data changes. To answer your other question: If you run aggregate functions (like SUM(), MIN(), or MAX()) on all records of a table, their results could be computed by accessing a matching index only. I don't know whether MySQL does this, I propose you check that yourself using EXPLAIN. If you run them on subsets of a table only, an index on that column will not help in general. In database implementations, there is the concept of a "covering index": If you have an index on columns A and B of some table, its contents (without the base data) would suffice to answer SELECT SUM(B) WHERE A = x Again, I don't know whether MySQL does this, and I refer you to EXPLAIN. HTH, Jörg -- Joerg Bruehe, MySQL Build Team, joerg.bru...@oracle.com ORACLE Deutschland B.V. & Co. KG, Komturstrasse 18a, D-12099 Berlin Geschaeftsfuehrer: Juergen Kunz, Marcel v.d. Molen, Alexander v.d. Ven Amtsgericht Muenchen: HRA 95603 -- MySQL General Mailing List For list archives: http://lists.mysql.com/mysql To unsubscribe: http://lists.mysql.com/mysql?unsub=arch...@jab.org
