Optimize this Mysql query

select count(xxx) where id='5'

xxx column is INT type
i have an index on id column
doe's help or inimical if i add an index on xxx column?

 

You just need an index on id. The query as it sounds should be fast. If you have a very large table and are running the same query multiple times it may be better to cache the results of have an auto generated table that contains the counts.

 

the database is large and the query is slow.. have any effect if i have or not index on xxx column?

 

An index on xxx will not help, does it make any difference if you do count(*)

 

if its possible, use a cron to save this information every hour or so into a seperate table.
a SELECT statement from the new table will obviously be fast.

if you want the information updated manually, just run the update script manually from an admin panel or something.

 

adding an index on id is the only thing you can do.
count(*) and count(column name) is the same.

 

$sql = select count(8) where id=5;

Will give the o/p..