Hi

Find attached the first of my queries, I require to be checked over.  Let me know if you need any more information, I've included the query, EXPLAIN output, and the relavant SHOW CREATE TABLE 

Regards
Neil

On Mon, Oct 4, 2010 at 9:50 PM, Neil Tompkins <neil.tompkins@googlemail.com> wrote:
I've got a fair few number of queries to be checked over. Will send them tommorrow


On 4 Oct 2010, at 18:27, Gavin Towey <gtowey@ffn.com> wrote:

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.tompkins@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.bruehe@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.bruehe@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