-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Iain Lang wrote:
.From http://www.mysql.com/doc/en/Where_optimisations.html :
But that is my question! *When* do they provide a performance difference?
Obviously my (fairly simple) question ("...and beyond what number of records might indices provide faster extraction/presentation?... ) has not been understood. Can anyone else help, please?
At 11:22 22/12/02 +1100, Alan McDonald wrote:
You need indexes as soon as (or rather just before) they provide a performance difference.
"Each table index is queried, and the best index that spans fewer than 30% of the rows is used. If no such index can be found, a quick table scan is used.".
So if you can't create an index that will have values that will cause MySQL to look at less that 30% of the rows, it will not be used. For example if you have a column that contains the value 'foo' 90% of the time, queries looking for 'foo' in that column will not use an index.
Indexes almost _always_ slow down data _modification_. You can tell when/if they speed-up data retrieval by using the 'EXPLAIN' command in MySQL, see:
http://www.mysql.com/doc/en/EXPLAIN.html
-Mark
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