Question

So I started off with this query:

SELECT * FROM TABLE1 WHERE hash IN (SELECT id FROM temptable);

It took forever, so I ran an explain:

mysql> explain SELECT * FROM TABLE1 WHERE hash IN (SELECT id FROM temptable);
+----+--------------------+-----------------+------+---------------+------+---------+------+------------+-------------+
| id | select_type        | table           | type | possible_keys | key  | key_len | ref  | rows       | Extra       |
+----+--------------------+-----------------+------+---------------+------+---------+------+------------+-------------+
|  1 | PRIMARY            | TABLE1          | ALL  | NULL          | NULL | NULL    | NULL | 2554388553 | Using where | 
|  2 | DEPENDENT SUBQUERY | temptable       | ALL  | NULL          | NULL | NULL    | NULL |       1506 | Using where | 
+----+--------------------+-----------------+------+---------------+------+---------+------+------------+-------------+
2 rows in set (0.01 sec)

It wasn’t using an index. So, my second pass:

mysql> explain SELECT * FROM TABLE1 JOIN temptable ON TABLE1.hash=temptable.hash;
+----+-------------+-----------------+------+---------------+----------+---------+------------------------+------+-------------+
| id | select_type | table           | type | possible_keys | key      | key_len | ref                    | rows | Extra       |
+----+-------------+-----------------+------+---------------+----------+---------+------------------------+------+-------------+
|  1 | SIMPLE      | temptable       | ALL  | hash          | NULL     | NULL    | NULL                   | 1506 |             | 
|  1 | SIMPLE      | TABLE1          | ref  | hash          | hash     | 5       | testdb.temptable.hash  |  527 | Using where | 
+----+-------------+-----------------+------+---------------+----------+---------+------------------------+------+-------------+
2 rows in set (0.00 sec)

Can I do any other optimization?

Answer 1

You can gain some more speed by using a covering index, at the cost of extra space consumption. A covering index is one which can satisfy all requested columns in a query without performing a further lookup into the clustered index.

First of all get rid of the SELECT * and explicitly select the fields that you require. Then you can add all the fields in the SELECT clause to the right hand side of your composite index. For example, if your query will look like this:

SELECT  first_name, last_name, age 
FROM    table1 
JOIN    temptable ON table1.hash = temptable.hash;

Then you can have a covering index that looks like this:

CREATE INDEX ix_index ON table1 (hash, first_name, last_name, age);