I have two tables named table_1 (1GB) and reference (250Mb).
When I query a cross join on reference it takes 16hours to update table_1.. We changed the system files EXT3 for XFS but still it's taking 16hrs.. WHAT AM I DOING WRONG??
Here is the update/cross join query:
mysql> UPDATE table_1 CROSS JOIN reference ON
-> (table_1.start >= reference.txStart AND table_1.end <= reference.txEnd)
-> SET table_1.name = reference.name;
Query OK, 17311434 rows affected (16 hours 36 min 48.62 sec)
Rows matched: 17311434 Changed: 17311434 Warnings: 0
Here is a show create table of table_1 and reference:
CREATE TABLE `table_1` (
`strand` char(1) DEFAULT NULL,
`chr` varchar(10) DEFAULT NULL,
`start` int(11) DEFAULT NULL,
`end` int(11) DEFAULT NULL,
`name` varchar(255) DEFAULT NULL,
`name2` varchar(255) DEFAULT NULL,
KEY `annot` (`start`,`end`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1 ;
CREATE TABLE `reference` (
`bin` smallint(5) unsigned NOT NULL,
`name` varchar(255) NOT NULL,
`chrom` varchar(255) NOT NULL,
`strand` char(1) NOT NULL,
`txStart` int(10) unsigned NOT NULL,
`txEnd` int(10) unsigned NOT NULL,
`cdsStart` int(10) unsigned NOT NULL,
`cdsEnd` int(10) unsigned NOT NULL,
`exonCount` int(10) unsigned NOT NULL,
`exonStarts` longblob NOT NULL,
`exonEnds` longblob NOT NULL,
`score` int(11) DEFAULT NULL,
`name2` varchar(255) NOT NULL,
`cdsStartStat` enum('none','unk','incmpl','cmpl') NOT NULL,
`cdsEndStat` enum('none','unk','incmpl','cmpl') NOT NULL,
`exonFrames` longblob NOT NULL,
KEY `chrom` (`chrom`,`bin`),
KEY `name` (`name`),
KEY `name2` (`name2`),
KEY `annot` (`txStart`,`txEnd`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1 ;
You should index table_1.start
, reference.txStart
, table_1.end
and reference.txEnd
table fields:
ALTER TABLE `table_1` ADD INDEX ( `start` ) ;
ALTER TABLE `table_1` ADD INDEX ( `end` ) ;
ALTER TABLE `reference` ADD INDEX ( `txStart` ) ;
ALTER TABLE `reference` ADD INDEX ( `txEnd` ) ;
Cross joins are Cartesian Products, which are probably one of the most computationally expensive things to compute (they don't scale well).
For each table T_i for i = 1 to n, the number of rows generated by crossing tables T_1 to T_n is the size of each table multiplied by the size of each other table, ie
|T_1| * |T_2| *... * |T_n|
Assuming each table has M rows, the resulting cost of computing the cross join is then
M_1 * M_2 ... M_n = O(M^n)
which is exponential in the number of tables involved in the join.
Try this:
UPDATE table_1 SET
table_1.name = (
select reference.name
from reference
where table_1.start >= reference.txStart
and table_1.end <= reference.txEnd)
I see 2 problems with the UPDATE
statement.
There is no index for the End
fields. The compound indexes ( annot
) you have will be used only for the start
fields in this query. You should add them as suggested by Emre:
ALTER TABLE `table_1` ADD INDEX ( `end` ) ;
ALTER TABLE `reference` ADD INDEX ( `txEnd` ) ;
Second, the JOIN
may (and probably does) find many rows of table reference
that are related to a row of table_1
. So some (or all) rows of table_1
that are updated, are updated many times. Check the result of this query, to see if it is the same as your updated rows count ( 17311434
):
SELECT COUNT(*)
FROM table_1
WHERE EXISTS
( SELECT *
FROM reference
WHERE table_1.start >= reference.txStart
AND table_1.`end` <= reference.txEnd
)
There can be other ways to write this query but the lack of a PRIMARY KEY
on both tables makes it harder. If you define a primary key on table_1
, try this, replacing id
with the primary key.
Update : No, do not try it on a table with 34M rows. Check the execution plan and try with smaller tables first.
UPDATE table_1 AS t1
JOIN
( SELECT t2.id
, r.name
FROM table_1 AS t2
JOIN
( SELECT name, txStart, txEnd
FROM reference
GROUP BY txStart, txEnd
) AS r
ON t2.start >= r.txStart
AND t2.`end` <= r.txEnd
GROUP BY t2.id
) AS good
ON good.id = t1.id
SET t1.name = good.name;
You can check the query plan by running EXPLAIN on the equivalent SELECT:
EXPLAIN
SELECT t1.id, t1.name, good.name
FROM table_1 AS t1
JOIN
( SELECT t2.id
, r.name
FROM table_1 AS t2
JOIN
( SELECT name, txStart, txEnd
FROM reference
GROUP BY txStart, txEnd
) AS r
ON t2.start >= r.txStart
AND t2.`end` <= r.txEnd
GROUP BY t2.id
) AS good
ON good.id = t1.id ;
Somebody already offered you to add some indexes. But I think the best performance you may get with these two indexes:
ALTER TABLE `test`.`time`
ADD INDEX `reference_start_end` (`txStart` ASC, `txEnd` ASC),
ADD INDEX `table_1_star_end` (`start` ASC, `end` ASC);
Only one of them will be used by MySQL query, but MySQL will decide which is more useful automatically.
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