如何在MySQL 4.1上加快此SQL查询的速度？

Question

I have a SQL query that takes a very long time to run on MySQL (it takes several minutes). The query is run against a table that has over 100 million rows, so I'm not surprised it's slow. In theory, though, it should be possible to speed it up as I really only want to get back the rows from the large table (let's call it A) that have a reference in another table, B.

So my query is:

SELECT id FROM A, B where A.ref = B.ref;

(A has over 100 million rows; B has just a few thousand).

I've added INDEXes:

alter table A add index(ref);
alter table B add index(ref);

But it's still very slow (several minutes -- I'd be happy with one minute).

Unfortunately, I'm stuck with MySQL 4.1.22, so I can't use views.

I'd rather not copy all of the relevant rows from A into a separate, smaller table, as the rows that I need will change from time to time. On the other hand, at the moment that's the only solution I can think of.

Any suggestions welcome!

EDIT: Here's the output of running EXPLAIN on my query:

+----+-------------+------------------------+------+------------------------------------------+-------------------------+---------+------------------------------------------------+-------+-------------+
| id | select_type | table                  | type | possible_keys                            | key                     | key_len | ref                                            | rows  | Extra       |
+----+-------------+------------------------+------+------------------------------------------+-------------------------+---------+------------------------------------------------+-------+-------------+
|  1 | SIMPLE      | B                      | ALL  | B_ref,ref                                | NULL                    |    NULL | NULL                                           | 16718 | Using where |
|  1 | SIMPLE      | A                      | ref  | A_REF,ref                                | A_ref                   |       4 | DATABASE.B.ref                                 |  5655 |             |
+----+-------------+------------------------+------+------------------------------------------+-------------------------+---------+------------------------------------------------+-------+-------------+

(In redacting my original query example, I chose to use "ref" as my column name, which happens to be the same as one of the types, but hopefully that's not too confusing...)

Answer 1

查询优化器可能已经在尽力而为，但是在不太可能的情况下，它首先读取了巨表（A），您可以使用STRAIGHT_JOIN语法明确地告诉它首先读取B：

SELECT STRAIGHT_JOIN id FROM B, A where B.ref = A.ref;

Answer 2

From the answers, it seems like you're doing the most efficient thing you can with the SQL. The A table seems to be the big problem, how about splitting it into three individual tables, kind of like a local version of sharding? Alternatively, is it worth denormalising the B table into the A table, assuming B doesn't have too many columns?

Finally, you could just have to buy a faster box to run it on - there's no substitute for horsepower!

Good luck.

Answer 3

SELECT id FROM A JOIN B ON A.ref = B.ref

You may be able to optimize further by using an appropriate type of join eg LEFT JOIN

http://en.wikipedia.org/wiki/Join_(SQL)