Is using COUNT(*) or SELECT * a good idea?

Question

I\'ve heard several times that you shouldn\'t perform COUNT(*) or SELECT * for performance reasons, but wasn\'t able to dig up some further informa

Answer 1

COUNT(*) is different from COUNT(column1) !
COUNT(*) returns the number of records, and does NOT use more resources, while COUNT(column1) counts the number of records where column1 is non null.

For SELECT, it is different. SELECT * will of course request more data.

Answer 2

It's absolutely true that "*" is "all columns". And you're right in the point of if you've a table with an incredible number of columns (say 100+), these kind of queries can be bad in terms of efficiency.

I believe that the best solution is creating database views previously filtering the amount of records evolved in the count operation, so, the performance impact isn't a big problem, because views can be cached.

In the other hand, it seems that "*" operator should be avoided when returning records, and it's brutally better to select the fields you really need to use in some business.