Poor query performance in MySQL I have a 10 million record table in a MySQL 5.5 InnoDB. It's running in a 16GB RAM server with good CPU and fast HD. When I run the following query

SELECT DISTINCT knowledge_id, 
                tag_order, 
                tag_weight, 
                total_title_direct_words, 
                total_title_parenthesis_words, 
                tag_level 
FROM   knowledge_tags 
WHERE  ( tag_text = 'washington' ) 
       AND ( tag_level &lt; 10 ); 


It will take around 9 seconds (first run) locally (localhost). The result count is 640. There is a Btree index for the fields 'tag_text, tag_level'. This query time isn't acceptable for what i want. What can I do?

Here is the EXPLAIN result:

1   SIMPLE  knowledge_tags  range   tags_by_word_text   tags_by_word_text   308 [null]  344 Using index condition; Using temporary


And the CREATE TABLE statement:

CREATE TABLE `knowledge_tags` (
  `tag_id` int(11) NOT NULL AUTO_INCREMENT,
  `knowledge_id` int(11) DEFAULT NULL,
  `dictionary_word_id` int(11) DEFAULT NULL,
  `tag_text` varchar(100) DEFAULT NULL,
  `tag_order` int(11) DEFAULT NULL,
  `tag_level` int(11) DEFAULT NULL,
  `knowledge_family_id` int(11) DEFAULT NULL,
  `tag_weight` double DEFAULT '1',
  `total_title_direct_words` int(11) DEFAULT NULL,
  `total_title_parenthesis_words` int(11) DEFAULT NULL,
  PRIMARY KEY (`tag_id`),
  KEY `tags_by_knowledge_id` (`knowledge_id`),
  KEY `tags_by_word_text` (`tag_text`,`tag_level`) USING BTREE,
  KEY `tags_by_family_dictionary` (`knowledge_family_id`,`dictionary_word_id`,`tag_level`) USING BTREE,
  KEY `tags_by_family_word_text` (`knowledge_family_id`,`tag_text`,`tag_level`) USING BTREE,
  KEY `tags_by_dictionary` (`dictionary_word_id`,`tag_level`) USING BTREE
) ENGINE=InnoDB AUTO_INCREMENT=3316 DEFAULT CHARSET=utf8;
I suggest you add an index on all the columns used in the query, with this order (actually the order matters for the first 2 columns only, the rest 5 can be in any order within the index):

 (tag_text, 
  tag_level, 
  knowledge_id, tag_order, tag_weight, 
  total_title_direct_words, 
  total_title_parenthesis_words)


The query should then use this index for both the WHERE and the DISTINCT (or group by) part. You probably need to change the query slightly, removing the DISTINCT and using GROUP BY (this acts like a hint to the MySQL optimizer.) 

SELECT   knowledge_id,                      -- remove DISTINCT
         tag_order, 
         tag_weight, 
         total_title_direct_words, 
         total_title_parenthesis_words, 
         tag_level 
FROM   knowledge_tags 
WHERE  ( tag_text = 'washington' ) 
  AND  ( tag_level &lt; 10 )
GROUP BY   tag_text,                        -- use the same order
           tag_level,                       -- as in
           knowledge_id,                    -- the index,          
           tag_order,                       -- all
           tag_weight,                      -- the
           total_title_direct_words,        -- way
           total_title_parenthesis_words ;  -- down


The EXPLAIN should then show:  


  Using where; Using index for group-by


If the result are good enough for your expectations, you can then drop the tags_by_word_text index.