Include the query, EXPLAIN output, and the relavant SHOW CREATE TABLE table \G output. Someone should be able to offer suggestions.
-----Original Message----- From: Tompkins Neil [mailto:neil.tompk...@googlemail.com] Sent: Monday, October 04, 2010 8:54 AM To: Joerg Bruehe Cc: [MySQL] Subject: Re: Indexing question Jörg Thanks for the useful reply. Maybe I can EXPLAIN my select queries for you to advise if any changes need to be made ? Regards Neil On Mon, Oct 4, 2010 at 3:38 PM, Joerg Bruehe <joerg.bru...@oracle.com>wrote: > 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 > > This message contains confidential information and is intended only for the individual named. If you are not the named addressee, you are notified that reviewing, disseminating, disclosing, copying or distributing this e-mail is strictly prohibited. Please notify the sender immediately by e-mail if you have received this e-mail by mistake and delete this e-mail from your system. E-mail transmission cannot be guaranteed to be secure or error-free as information could be intercepted, corrupted, lost, destroyed, arrive late or incomplete, or contain viruses. The sender therefore does not accept liability for any loss or damage caused by viruses or errors or omissions in the contents of this message, which arise as a result of e-mail transmission. [FriendFinder Networks, Inc., 220 Humbolt court, Sunnyvale, CA 94089, USA, FriendFinder.com -- MySQL General Mailing List For list archives: http://lists.mysql.com/mysql To unsubscribe: http://lists.mysql.com/mysql?unsub=arch...@jab.org
