Re: Index selection problem
Maybe I'm wrong :) On Tuesday, July 21, 2009, John Daisley john.dais...@mypostoffice.co.uk wrote: On Tue, 2009-07-21 at 19:42 +0200, Morten Primdahl wrote: On Jul 21, 2009, at 3:27 PM, Johnny Withers wrote: MySQL is unable to use your index when you use IN and/or OR on yoru column. Is this really true? No its not true! Try running OPTIMIZE TABLE on the affected table, then run the query again and see if the other index is used! I'm reading High Performance MySQL 2nd ed. these days and specifically got the impression that using IN will allow usage of the index. The below quote is from the book, and the multiple equality condition refers to an IN (...) expression. ... we draw a distinction between ranges of values and multiple equality conditions.The second query is a multiple equality condition, in our terminology. We’re not just being picky: these two kinds of index accesses perform differently. The range condition makes MySQL ignore any further columns in the index, but the multiple equality condition doesn’t have that limitation. John Daisley Email: john.dais...@butterflysystems.co.uk Mobile: +44 (0)7812 451238 MySQL Certified Database Administrator (CMDBA) MySQL Certified Developer (CMDEV) MySQL Certified Associate (CMA) Comptia A+ Certified Professional IT Technician --- Life's journey is not to arrive at the grave safely in a well preserved body, but rather to slide in sideways, thoroughly used up, totally worn out and screaming Wow! what a ride! -- - Johnny Withers 601.209.4985 joh...@pixelated.net -- MySQL General Mailing List For list archives: http://lists.mysql.com/mysql To unsubscribe:http://lists.mysql.com/mysql?unsub=arch...@jab.org
Re: Index selection problem
MySQL is unable to use your index when you use IN and/or OR on yoru column. If the query is slow, you should switch to a union: SELECT * FROM orders WHERE item_id = 9602 AND customer_id = 5531 AND status_id =1 UNION SELECT * FROM orders WHERE item_id = 9602 AND customer_id = 5531 AND status_id =2 On Tue, Jul 21, 2009 at 4:52 AM, Morten my.li...@mac.com wrote: Hi, I have a table orders with the columns item_id INT FK items(id) customer_id INT FK customers(id) status_id TINYINT -- Between 1 and 4 always ordered_at DATETIME delivered_at DATETIME There are indexes: index_a: (item_id, customer_id, status_id) index_b: (item_id, status_id, ordered_at, delivered_at) Given this query: SELECT * FROM orders WHERE item_id = 9602 AND customer_id = 5531 AND status_id IN (1,2) Then the key chosen is index_b. Same happens if I use (status_id = 1 OR status_id = 2). If I only check against one status_id, then the correct index_a gets picked with ref const,const,const. I'm not even doing a range scan on status_id and even if I were, it's the last column in index_a. Since ordered_at and delivered_at are both dates then index_b will have a very high selectivity. In reality, index_b may make little sense, but I still don't understand why MySQL would ever pick that when 3 columns in the query can use the covering index_a Can anyone give me some input on how to make sense of this? Thanks, Morten select count(*) from orders where item_id = 9602 - 4534 records select count(*) from orders where item_id = 9602 and status_id IN (1,2) - 4181 records select count(*) from orders where item_id = 9602 and customer_id = 5531 - 1226 records select count(*) from orders where item_id = 9602 and customer_id = 5531 and status_id IN (1,2) - 1174 records -- MySQL General Mailing List For list archives: http://lists.mysql.com/mysql To unsubscribe:http://lists.mysql.com/mysql?unsub=joh...@pixelated.net -- - Johnny Withers 601.209.4985 joh...@pixelated.net
Re: Index selection problem
Try doing a SHOW INDEX FROM orders and look at the cardinality column. These are the stats MySQL uses to determine which index to use. Sometimes they aren't always update properly and you may need to run ANALYZE on the table. But, you can also tell MySQL to use the index you want. SELECT * FROM orders USE INDEX (index_a) WHERE ... Brent Baisley On Tue, Jul 21, 2009 at 5:52 AM, Mortenmy.li...@mac.com wrote: Hi, I have a table orders with the columns item_id INT FK items(id) customer_id INT FK customers(id) status_id TINYINT -- Between 1 and 4 always ordered_at DATETIME delivered_at DATETIME There are indexes: index_a: (item_id, customer_id, status_id) index_b: (item_id, status_id, ordered_at, delivered_at) Given this query: SELECT * FROM orders WHERE item_id = 9602 AND customer_id = 5531 AND status_id IN (1,2) Then the key chosen is index_b. Same happens if I use (status_id = 1 OR status_id = 2). If I only check against one status_id, then the correct index_a gets picked with ref const,const,const. I'm not even doing a range scan on status_id and even if I were, it's the last column in index_a. Since ordered_at and delivered_at are both dates then index_b will have a very high selectivity. In reality, index_b may make little sense, but I still don't understand why MySQL would ever pick that when 3 columns in the query can use the covering index_a Can anyone give me some input on how to make sense of this? Thanks, Morten select count(*) from orders where item_id = 9602 - 4534 records select count(*) from orders where item_id = 9602 and status_id IN (1,2) - 4181 records select count(*) from orders where item_id = 9602 and customer_id = 5531 - 1226 records select count(*) from orders where item_id = 9602 and customer_id = 5531 and status_id IN (1,2) - 1174 records -- MySQL General Mailing List For list archives: http://lists.mysql.com/mysql To unsubscribe: http://lists.mysql.com/mysql?unsub=brentt...@gmail.com -- MySQL General Mailing List For list archives: http://lists.mysql.com/mysql To unsubscribe:http://lists.mysql.com/mysql?unsub=arch...@jab.org
Re: Index selection problem
On Jul 21, 2009, at 3:27 PM, Johnny Withers wrote: MySQL is unable to use your index when you use IN and/or OR on yoru column. Is this really true? I'm reading High Performance MySQL 2nd ed. these days and specifically got the impression that using IN will allow usage of the index. The below quote is from the book, and the multiple equality condition refers to an IN (...) expression. ... we draw a distinction between ranges of values and multiple equality conditions.The second query is a multiple equality condition, in our terminology. We’re not just being picky: these two kinds of index accesses perform differently. The range condition makes MySQL ignore any further columns in the index, but the multiple equality condition doesn’t have that limitation. -- MySQL General Mailing List For list archives: http://lists.mysql.com/mysql To unsubscribe:http://lists.mysql.com/mysql?unsub=arch...@jab.org
Re: Index selection problem
The other index does have a way higher cardinality, but the query is for 3 columns all of which are in the first index. I guess this is just one of the situations where MySQL makes a wrong assessment. On Jul 21, 2009, at 3:54 PM, Brent Baisley wrote: Try doing a SHOW INDEX FROM orders and look at the cardinality column. These are the stats MySQL uses to determine which index to use. Sometimes they aren't always update properly and you may need to run ANALYZE on the table. But, you can also tell MySQL to use the index you want. SELECT * FROM orders USE INDEX (index_a) WHERE ... Brent Baisley On Tue, Jul 21, 2009 at 5:52 AM, Mortenmy.li...@mac.com wrote: Hi, I have a table orders with the columns item_id INT FK items(id) customer_id INT FK customers(id) status_id TINYINT -- Between 1 and 4 always ordered_at DATETIME delivered_at DATETIME There are indexes: index_a: (item_id, customer_id, status_id) index_b: (item_id, status_id, ordered_at, delivered_at) Given this query: SELECT * FROM orders WHERE item_id = 9602 AND customer_id = 5531 AND status_id IN (1,2) Then the key chosen is index_b. Same happens if I use (status_id = 1 OR status_id = 2). If I only check against one status_id, then the correct index_a gets picked with ref const,const,const. I'm not even doing a range scan on status_id and even if I were, it's the last column in index_a. Since ordered_at and delivered_at are both dates then index_b will have a very high selectivity. In reality, index_b may make little sense, but I still don't understand why MySQL would ever pick that when 3 columns in the query can use the covering index_a Can anyone give me some input on how to make sense of this? Thanks, Morten select count(*) from orders where item_id = 9602 - 4534 records select count(*) from orders where item_id = 9602 and status_id IN (1,2) - 4181 records select count(*) from orders where item_id = 9602 and customer_id = 5531 - 1226 records select count(*) from orders where item_id = 9602 and customer_id = 5531 and status_id IN (1,2) - 1174 records -- MySQL General Mailing List For list archives: http://lists.mysql.com/mysql To unsubscribe:http://lists.mysql.com/mysql?unsub=brentt...@gmail.com -- MySQL General Mailing List For list archives: http://lists.mysql.com/mysql To unsubscribe:http://lists.mysql.com/mysql?unsub=arch...@jab.org
Re: Index selection problem
On Tue, 2009-07-21 at 19:42 +0200, Morten Primdahl wrote: On Jul 21, 2009, at 3:27 PM, Johnny Withers wrote: MySQL is unable to use your index when you use IN and/or OR on yoru column. Is this really true? No its not true! Try running OPTIMIZE TABLE on the affected table, then run the query again and see if the other index is used! I'm reading High Performance MySQL 2nd ed. these days and specifically got the impression that using IN will allow usage of the index. The below quote is from the book, and the multiple equality condition refers to an IN (...) expression. ... we draw a distinction between ranges of values and multiple equality conditions.The second query is a multiple equality condition, in our terminology. We’re not just being picky: these two kinds of index accesses perform differently. The range condition makes MySQL ignore any further columns in the index, but the multiple equality condition doesn’t have that limitation. John Daisley Email: john.dais...@butterflysystems.co.uk Mobile: +44 (0)7812 451238 MySQL Certified Database Administrator (CMDBA) MySQL Certified Developer (CMDEV) MySQL Certified Associate (CMA) Comptia A+ Certified Professional IT Technician --- Life's journey is not to arrive at the grave safely in a well preserved body, but rather to slide in sideways, thoroughly used up, totally worn out and screaming Wow! what a ride!
