MySQL MyISAM sorting query makes it slower - mysql

Question

I am using MySQL 5.1 on a Windows Server 2008 (with 4GB RAM) and have the following configuration:
I have 2 MyISAM tables. One is in 1 database (DB1) and has 14 columns, which are mostly varchar. This table has about 5,000,000 rows and is the DB1.games table below. It has a primary key on GameNumber (int(10)).
The other table is the DB2.gameposition and consists of the columns GameNumber (links to
DB1.games) and PositionCode (int(10)). This table has about 400,000,000 rows and there is an index IX_PositionCode on PositionCode.
These 2 databases are on the same server.
I want to run a query on DB2.gameposition to find a particular PositionCode, and have the results sorted by the linking DB1.games.Yr field (smallint(6) - this represents a Year). This sorting of results I only introduced recently. There is an index on this Yr field in DB1.games.
In my stored procedure, I perform the following:
CREATE TEMPORARY TABLE tblGameNumbers(GameNumber INT UNSIGNED NOT NULL PRIMARY KEY);
INSERT INTO tblGameNumbers(GameNumber)
SELECT DISTINCT gp.GameNumber
FROM DB2.gameposition gp
WHERE PositionCode = var_PositionCode LIMIT 1000;
I just get 1000 to make it quicker
And then join it to the DB1.games table.
In order to generate an EXPLAIN from that, I took out the temporary table (I use in the stored procedure) and refactored it as seen in the inner subquery below:
EXPLAIN
SELECT *
FROM DB1.games g
INNER JOIN (SELECT DISTINCT gp.GameNumber
FROM DB2.gameposition gp
WHERE PositionCode = 669312116 LIMIT 1000
) B ON g.GameNumber = B.GameNumber
ORDER BY g.Yr DESC
LIMIT 0,28
Running the EXPLAIN above, I see the following:
1, 'PRIMARY', '', 'ALL', '', '', '', '', 1000, 'Using temporary; Using filesort'
1, 'PRIMARY', 'g', 'eq_ref', 'PRIMARY', 'PRIMARY', '4', 'B.GameNumber', 1, ''
2, 'DERIVED', 'gp', 'ref', 'IX_PositionCode', 'IX_PositionCode', '4', '', 1889846, 'Using temporary'
The query used to be almost instant before I introduced the ORDER BY clause. Now, sometimes it is quick (depending on different PositionCode), but other times it can take up to 10 seconds to return the rows. Before I introduced the sorting, it was always virtually instantaneous. Unfortunately, I am not too proficient in interpreting the EXPLAIN output. Or how to make the query faster.
Any help would be greatly appreciated!
Thanks in advance,
Tim

Answer 1

Without the order by, your limit means the first 28 results are returned and then the query stops. With order by, all results need to be retrieved so they can be sorted and the first 28 returned.
The explain shows what MySql is doing:
sort 5000000 games records by yr
for each games record from sorted list
get the games record by primary key (to get all the columns)
read gamepositions by position code
if it does not match gamenumber, discard it
when 1000 matches found, stop reading
end read
end for
Try this instead:
select distinct ... from gameposition gp
inner join games g on g.gamenumber = gp.gamenumber
where gp.positioncode = ...
order by g.yr limit ...