Question

In the case when I want to check, if a certain entry in the database exists I have two options.

I can create an sql query using COUNT() and then check, if the result is >0…

…or I can just retrieve the record(s) and then count the number of rows in the returned rowset. For example with $result->num_rows;

What’s better/faster? in mysql? in general?

Answer 1

SELECT 1    FROM (SELECT 1) t   WHERE EXISTS( SELECT * FROM foo WHERE id = 42 ) 

Just tested, works fine on MySQL v5

COUNT(*) is generally less efficient if:

1. you can have duplicates (because the DBMS will have to exhaustively search all of the records/indexes to give you the exact answer) or
2. have NULL entries (for the same reason)

If you are COUNT’ing based on a WHERE clause that is guaranteed to produce a single record (or 0) and the DBMS knows this (based upon UNIQUE indexes), then it ought to be just as efficient. But, it is unlikely that you will always have this condition. Also, the DBMS may not always pick up on this depending on the version and DBMS.

Counting in the application (when you don’t need the row) is almost always guaranteed to be slower/worse because:

1. You have to send data to the client, the client has to buffer it and do some work
2. You may bump out things in the DBMS MRU/LRU data cache that are more important
3. Your DBMS will (generally) have to do more disk I/O to fetch record data that you will never use
4. You have more network activity

Of course, if you want to DO something with the row if it exists, then it is definitely faster/best to simply try and fetch the row to begin with!