SELECT COUNT() vs mysql_num_rows();

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Use COUNT, internally the server will process the request differently.

When doing COUNT, the server will only allocate memory to store the result of the count.

When using mysql_num_rows, the server will process the entire result set, allocate memory for all those results, and put the server in fetching mode, which involves a lot of different details, such as locking.

Think of it like the following pseudo scenarios:

SELECT COUNT(*)

Hey Bob, how many people are in the class room?

mysql_num_rows

Hey Bob, send all the people from the classroom over to me, … I’ll count them to get the number of people myself

In summary, when using mysql_num_rows you are transferring all records to the client, and the client will have to calculate the count itself.

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