MySQL performance: Single query using GROUP_CONCAT, or two separate queries?

Question

I have a MySQL database in which each user has an account, and each account can have multiple permissions.

My ultimate goal is to end up with the account's username, and a comma-delimited list of permissions ids. There are two ways I can accomplish this:

SELECT a.username, GROUP_CONCAT(rp.permission_id) as permission_ids
FROM account AS a
JOIN role_permission AS rp
ON rp.role_id = a.role_id
WHERE a.id = 1902

... or ...

SELECT username
FROM account
WHERE id = 1902;

SELECT permission_id
FROM account_permission
WHERE account_id = 1902

With the single query, I get my results exactly as I want them. With two queries, I have to create the comma-delimited list in the app (PHP) using the second result set.

Are there any performance reasons to NOT choose the first option? I have never used GROUP_CONCAT before, so I don't know the implications of it, performance-wise.

Answer 1

The performance should be OK - better than two queries. You need to be aware that the length is limited though:

The result is truncated to the maximum length that is given by the group_concat_max_len system variable, which has a default value of 1024. The value can be set higher, although the effective maximum length of the return value is constrained by the value of max_allowed_packet. The syntax to change the value of group_concat_max_len at runtime is as follows, where val is an unsigned integer:

SET [GLOBAL | SESSION] group_concat_max_len = val;