At 9:10 +1000 4/21/05, Daniel Kasak wrote:
>Daevid Vincent wrote:
>>If I have a table with a primary key like this:
>>CREATE TABLE `answers` (
>>`qid` INT UNSIGNED NOT NULL ,
>>`userid` INT UNSIGNED NOT NULL ,
>>`groupid` INT UNSIGNED NOT NULL ,
>>`comments` TEXT NOT NULL ,
>> PRIMARY ( `qid` , `userid` , `groupid` )
>>But I will also be searching in various pages, for all 'answers' from a
>>certain userid or say that belong to a certain groupid, do I still need to
>>create separate INDEXes for those fields, or is it enough to have them in
>>that combination PRIMARY key?
>You need to have separate indexes. If you have a composite index and
>just have one field in the where clause ( or in a join ), the index
>won't be used at all. It says this somewhere in the documentation.
Not quite. The index could be used if were searching for a column or
columns that form a leftmost prefix of the index. That means it could
be used if you were searching for qid values, or qid+userid values.
But just groupid values are not leftmost prefixes of the index, so
you do need a separate index for groupid.
>Also, while the above table definition is legal, I would have an
>auto_increment primary key, and then if you want to make ( `qid` ,
>`userid` , `groupid` ) unique, define a unique index across them.
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