Getting max value from many tables - mysql

Question

There are two ways, that I can think of, to obtain similar results from multiple tables. One is UNION and the other is JOIN. The similar questions on SO have all been answered with a UNION. Here's the coder I just found:
SELECT max(up.id) AS up, max(sc.id) AS sc, max(cl.id) AS cl
FROM updates up, chat_staff sc, change_log cl
explain:
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Select tables optimized away |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
My question is -- Is this better than the following?
SELECT "up.id" AS K, max(id) AS V FROM updates
UNION
SELECT "sc.id" AS K, max(id) AS V FROM chat_staff
UNION
SELECT "cl.id" AS K, max(id) AS V FROM change_log
explain:
+----+--------------+--------------+------+---------------+------+---------+------+-------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------+--------------+------+---------------+------+---------+------+------+------------------------------+
| 1 | PRIMARY | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Select tables optimized away |
| 2 | UNION | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Select tables optimized away |
| 3 | UNION | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Select tables optimized away |
| NULL | UNION RESULT | <union1,2,3> | ALL | NULL | NULL | NULL | NULL | NULL | |
+----+--------------+--------------+------+---------------+------+---------+------+------+------------------------------+

Answer 1

Both of those methods are just fine. In fact, I have another method:
SELECT
IFNULL(maxidup,0) max_id_up,
IFNULL(maxscup,0) max_sc_up,
IFNULL(maxclup,0) max_cl_up
FROM
(SELECT max(id) maxidup FROM updates) up,
(SELECT max(id) maxidsc FROM chat_staff) sc,
(SELECT max(id) maxidcl FROM change_log) cl
;
This method presents the three values side by side like your first example. It also shows 0 in the event one of the tables are empty.
mysql> DROP DATABASE IF EXISTS junk;
Query OK, 3 rows affected (0.11 sec)
mysql> CREATE DATABASE junk;
Query OK, 1 row affected (0.00 sec)
mysql> use junk
Database changed
mysql> CREATE TABLE updates (id int not null auto_increment primary key,x int);
Query OK, 0 rows affected (0.07 sec)
mysql> CREATE TABLE chat_staff LIKE updates;
Query OK, 0 rows affected (0.07 sec)
mysql> CREATE TABLE change_log LIKE updates;
Query OK, 0 rows affected (0.06 sec)
mysql> INSERT INTO updates (x) VALUES (37),(84),(12);
Query OK, 3 rows affected (0.06 sec)
Records: 3 Duplicates: 0 Warnings: 0
mysql> INSERT INTO change_log (x) VALUES (37),(84),(12),(14),(35);
Query OK, 5 rows affected (0.09 sec)
Records: 5 Duplicates: 0 Warnings: 0
mysql> SELECT
-> IFNULL(maxidup,0) max_id_up,
-> IFNULL(maxidsc,0) max_sc_up,
-> IFNULL(maxidcl,0) max_cl_up
-> FROM
-> (SELECT max(id) maxidup FROM updates) up,
-> (SELECT max(id) maxidsc FROM chat_staff) sc,
-> (SELECT max(id) maxidcl FROM change_log) cl
-> ;
+-----------+-----------+-----------+
| max_id_up | max_sc_up | max_cl_up |
+-----------+-----------+-----------+
| 3 | 0 | 5 |
+-----------+-----------+-----------+
1 row in set (0.00 sec)
mysql> explain SELECT IFNULL(maxidup,0) max_id_up, IFNULL(maxidsc,0) max_sc_up, IFNULL(maxidcl,0) max_cl_up FROM (SELECT max(id) maxidup FROM updates) up, (SELECT max(id) maxidsc FROM chat_staff) sc, (SELECT max(id) maxidcl FROM change_log) cl;
+----+-------------+------------+--------+---------------+------+---------+------+------+------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+--------+---------------+------+---------+------+------+------------------------------+
| 1 | PRIMARY | <derived2> | system | NULL | NULL | NULL | NULL | 1 | |
| 1 | PRIMARY | <derived3> | system | NULL | NULL | NULL | NULL | 1 | |
| 1 | PRIMARY | <derived4> | system | NULL | NULL | NULL | NULL | 1 | |
| 4 | DERIVED | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Select tables optimized away |
| 3 | DERIVED | NULL | NULL | NULL | NULL | NULL | NULL | NULL | No matching min/max row |
| 2 | DERIVED | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Select tables optimized away |
+----+-------------+------------+--------+---------------+------+---------+------+------+------------------------------+
6 rows in set (0.02 sec)
In my EXPLAIN plan, it has Select tables optimized away just like yours. Why ?
Since id is indexed in all the tables, the index is used to retrieve the max(id) rather than the table. Thus, Select tables optimized away is the correct response.
Six of one, half dozen of the other. How you present data from there is strictly your personal preference.
UPDATE 2011-10-20 15:32 EDT
You commented : Do you know how table locking would compromise this? Let's say one of the tables in question is locked. Would this query lock the other two and keep 'em locked until the first one was freed up?
This would depend on the storage engine. If all tables in question are MyISAM, definite possibility since MyISAM performs a full table lock on INSERT, UPDATE, DELETE. If the three tables are InnoDB, you have the benefit of MVCC to provide transaction isolation. This would allow everyone their view of the data in a point-in-time. Aside from DDL and an explcit LOCK TABLES against InnoDB, your query should not be blocked.

Answer 2

Actually, while they're similar, there's a subtle difference. The first gives you a one-row, three-column table (with the values going "across") and the second gives you a three-row, two-column table (with the values going "down").
Provided you're happy processing or viewing that data in either form, it's probably going to come down to performance.
In my experience (and this is nothing to do specifically with MySQL), the latter query will probably be better. That's because the DBMS' I work with are able to run queries like that in parallel for efficiency, combining them at completion of all. The fact that they're on different tables means that lock contention between them will be zero.
It may be that the query analysis engine of a DBMS could do a similar optimisation for the first query but it would require a lot more intelligence than I've seen from most of them.
One quick point, if you use union all instead of just union, you tell the database not to remove duplicate rows. You won't get any duplicates in this case due to the K column being different for all three sub-queries.
But, as with all optimisations, measure, don't guess! Certainly don't take as gospel the rants of random internet roamers (yes, even me).
Put together various candidate tables with the properties you're likely to have in production, and compare the performance of each.