Very slow query when using ORDER BY and LIMIT? - mysql

Question

The following query takes 10 seconds to finish when having order by. Without order by it finish in 0.0005 seconds. I am already having an index on field "sku", "vid" AND "timestamp". I have more 200,000 record in this table. Please help, what is wrong with the query when using order by.
SELECT i.pn,i.sku,i.title, fl.f_inserted,fl.f_special, fl.f_notinserted
FROM inventory i
LEFT JOIN inventory_flags fl ON fl.sku = i.sku AND fl.vid = i.vid
WHERE i.qty >=2 ORDER BY i.timestamp LIMIT 0,100;
-- --------------------------------------------------------
--
-- Table structure for table `inventory`
--
CREATE TABLE IF NOT EXISTS `inventory` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`pn` varchar(60) DEFAULT NULL,
`sku` varchar(60) DEFAULT NULL,
`title` varchar(60) DEFAULT NULL,
`qty` int(11) DEFAULT NULL,
`vid` int(11) DEFAULT NULL,
`timestamp` timestamp NULL DEFAULT CURRENT_TIMESTAMP,
PRIMARY KEY (`id`),
KEY `vid` (`vid`),
KEY `sku` (`sku`),
KEY `timestamp` (`timestamp`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;
-- --------------------------------------------------------
--
-- Table structure for table `inventory_flags`
--
CREATE TABLE IF NOT EXISTS `inventory_flags` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`f_inserted` tinyint(1) DEFAULT NULL,
`f_notinserted` tinyint(1) DEFAULT NULL,
`f_special` tinyint(1) DEFAULT NULL,
`timestamp` timestamp NULL DEFAULT CURRENT_TIMESTAMP,
`sku` varchar(60) DEFAULT NULL,
`vid` int(11) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `vid` (`vid`),
KEY `sku` (`sku`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;
EXPLANE RESULT:
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE fl system vid,sku NULL NULL NULL 0 const row not found
1 SIMPLE i index NULL timestamp 5 NULL 10 Using where

Answer 1

Instead of adding seprate indexes on columns you need to put multicolumn index on tables as you are using more than one columns from same table in joining condition.
after including columns from WHERE clause also include columns used in ORDER BY clause in composite index.
try adding flowing indexes and test them using EXPLAIN:
ALTER TABLE ADD INDEX ix_if inventory_flags(sku, vid);
ALTER TABLE ADD INDEX ix_i inventory(sku, qty, timestamp);
also try to avoid DISTINCT clause in your query, it is equivalent to GROUP BY clause, if you still need it then consider adding covering index.

Answer 2

If sku is unique to each inventory item then define it as UNIQUE - it'll speed things up. (Or the combination of sku and vid - define a composite index in that case.)
Why are you doing SELECT DISTINCT? The vast majority of the time using DISTINCT is a sign that your query or your table structure is wrong.
Since it's DISTINCT, and sku is not UNIQUE it can't use the index on timestamp to speed things up, so it has to sort a table with 200,000 records - it can't even use an index on qty to speed that part up.
PS. Omesh has some good advice as well.

Answer 3

you can use force index(index_key). try it, and you will see in explain query that mysql now will use the key index when 'order by'