Hi,
I’m attempting to optimize a join and am having a difficult time using
multiple columns from a composite index. The second column of the composite
key is being used when tested for equality, but not for IN or BETWEEN
criteria.
As an example, say that I’m searching two tables: portfolio and trades. The
portfolio table contains a list of security IDs. The trades table keeps
tracks of the price and time when I’ve traded securities in my portfolio.
Tables are:
CREATE TABLE portfolio (
sec_id bigint(20) NOT NULL AUTO_INCREMENT,
name char(10) NOT NULL,
PRIMARY KEY (sec_id)
) ENGINE=InnoDB ;
CREATE TABLE trades (
tx_id bigint(20) NOT NULL AUTO_INCREMENT,
sec_id bigint(20) NOT NULL,
trade_time datetime NOT NULL,
price int NOT NULL,
PRIMARY KEY (tx_id),
KEY sec_time (sec_id, trade_time)
) ENGINE=InnoDB ;
If I query the trades table directly both columns of the composite index
"sec_time" will be used when I'm using a range criteria on the trade_time
column:
mysql> explain select price from trades force index(sec_time)
-> where sec_id IN (1, 2) and trade_time BETWEEN '2012-07-01' AND
'2012-07-04';
+----+-------------+--------+-------+---------------+----------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len |
ref | rows | Extra |
+----+-------------+--------+-------+---------------+----------+---------+------+------+-------------+
| 1 | SIMPLE | trades | range | sec_time | sec_time | 16 |
NULL | 2 | Using where |
+----+-------------+--------+-------+---------------+----------+---------+------+------+-------------+
If I introduce a join to retrieve all trades for my portfolio, the entire
index will continue to be used if I make trade_time a constant:
mysql> explain select price from portfolio p inner join trades t force
index(sec_time) on p.sec_id = t.sec_id
-> where trade_time = '2012-07-01';
+----+-------------+-------+-------+---------------+----------+---------+------------------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len |
ref | rows | Extra |
+----+-------------+-------+-------+---------------+----------+---------+------------------------+------+-------------+
| 1 | SIMPLE | p | index | PRIMARY | PRIMARY | 8 |
NULL | 1 | Using index |
| 1 | SIMPLE | t | ref | sec_time | sec_time | 16 |
vantage.p.sec_id,const | 1 | |
+----+-------------+-------+-------+---------------+----------+---------+------------------------+------+-------------+
However, if I expand the trade_time search (either using IN or BETWEEN),
only the sec_id column of the composite query is used:
mysql> explain select price from portfolio p inner join trades t force
index(sec_time) on p.sec_id = t.sec_id
-> where trade_time IN ('2012-07-01', '2012-07-02');
+----+-------------+-------+-------+---------------+----------+---------+------------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len |
ref | rows | Extra |
+----+-------------+-------+-------+---------------+----------+---------+------------------+------+-------------+
| 1 | SIMPLE | p | index | PRIMARY | PRIMARY | 8 |
NULL | 1 | Using index |
| 1 | SIMPLE | t | ref | sec_time | sec_time | 8 |
vantage.p.sec_id | 1 | Using where |
+----+-------------+-------+-------+---------------+----------+---------+------------------+------+-------------+
My expectation is that MySQL would be able to use both columns of the
sec_time index, but I've been unable to find either confirmation of
refutation of that assumption. If MySQL cannot optimize a join in this
case, is there another approach to optimizing this query that I should
pursue? Essentially, my trades table may contain many historical records
and pulling the entire history of trades for each security would produce a
much larger result set than would be retrieved if the trade_time criteria
was applied in the index reference.
I'm using MySQL 5.5.11.
Thanks for any guidance,
Jeff
