Question

Why does this code take a long time to run?

SELECT
   concat((select Sname from member order by rand() limit 1),
   Ssurname) as tee
FROM member

But This code is very fast to run

SELECT
   concat(Sname,
   Ssurname) as tee
FROM member

Answer 1

For every result row returned by your first example, MySQL must produce another row from which to CONCAT() Sname, with a custom (random) order. Because order by rand() is used, the whole table must be reordered randomly for every row in your table. That is likely to be a very expensive operation, since the result of the subquery cannot be cached.

In the second example, a simple rowset is returned. Sname and Ssurname are concatenated from columns in the same row.

I ran an EXPLAIN on a similar query, having one indexed column concatenated against a non-indexed subquery. MySQL is using a temporary table to compute the subqueries.

mysql> EXPLAIN SELECT CONCAT(g_userName, (SELECT g_fullName FROM g2_User ORDER BY RAND() LIMIT 1)) FROM g2_User;
+----+----------------------+---------+-------+---------------+------------+---------+------+------+---------------------------------+
| id | select_type          | table   | type  | possible_keys | key        | key_len | ref  | rows | Extra                           |
+----+----------------------+---------+-------+---------------+------------+---------+------+------+---------------------------------+
|  1 | PRIMARY              | g2_User | index | NULL          | g_userName | 98      | NULL |    5 | Using index                     |
|  2 | UNCACHEABLE SUBQUERY | g2_User | ALL   | NULL          | NULL       | NULL    | NULL |    5 | Using temporary; Using filesort |
+----+----------------------+---------+-------+---------------+------------+---------+------+------+---------------------------------+