Question

I am wondering if there is a performance difference between these two queries that check if a record exists?

select count(1) from table where id = 1;

or

select id from table where id = 1;

Answer 1

I don’t think there will be much of a difference between those two queries : the difference being selecting a field (which is part of an index) in the second case, and counting one line in the first… Not that much of a difference.

Still, out of curiosity, I did a very quick benchmark of those kind of queries on a database I have on my computer — note there are only like 7 lines in the post table, so it might not be that close to a real situation, but as there is a PK on id, which means an index….

Here’s what I got :

mysql> select benchmark(10000000000, 'select sql_no_cache id from post where id = 1');
+-------------------------------------------------------------------------+
| benchmark(10000000000, 'select sql_no_cache id from post where id = 1') |
+-------------------------------------------------------------------------+
|                                                                       0 |
+-------------------------------------------------------------------------+
1 row in set (1 min 0,25 sec)

mysql> select benchmark(10000000000, 'select sql_no_cache count(1) from post where id = 1');
+-------------------------------------------------------------------------------+
| benchmark(10000000000, 'select sql_no_cache count(1) from post where id = 1') |
+-------------------------------------------------------------------------------+
|                                                                             0 |
+-------------------------------------------------------------------------------+
1 row in set (1 min 0,23 sec)

So, really not that much of a difference, it seems ^^