Published on February 20, 2009
MySQL EXPLAIN Explained Quick and Easy Query Optimisation Adrian Hardy <email@example.com>
Before we begin... What you need to know How and why we add indexes to tables ● The benefits of correct field typing ● Understanding of the ideals of 3NF ● Basic understanding of SQL JOINs ● This presentation Very quick introduction to EXPLAIN ● Improve understanding of MySQL and indexing ● Simplified examples / results ●
Introduction - Using MySQL EXPLAIN Prefix a SELECT query with EXPLAIN MySQL won't actually execute the query, just analyse it ● EXPLAIN helps us understand how and when MySQL ● will use indexes EXPLAIN returns a table of data from which you identify ● potential improvements Optimise queries in three ways ● Modify or create indexes ● Modify query structure ● Modify data structure ● Optimised queries = faster results, lower server load... ●
Introduction - Review of Indexing Fast, compact structure for identifying row locations ● Keep indexes in memory by trimming the fat: ● Can I reduce the characters in that VARCHAR index? ● Can I use a TINYINT instead of a BIGINT? ● Can I use an INTEGER to describe a status or flag (rather ● than a textual description)? Chop down your result set as quickly as possible ● MySQL will only use one index per query/table – it cannot ● combine two separate indexes to make a useful one * Understanding and preparation brings about Indexing Strategy * Not strictly true - look up “Index Merge” operations
Booking application schema attendees attendee_id surname conference_id registration_status INTEGER (PK) VARCHAR INTEGER (FK) TINYINT conferences conference_id location_id topic_id date INTEGER (PK) INTEGER (FK) INTEGER (FK) DATE
EXPLAIN – Worked Example EXPLAIN SELECT * FROM attendees WHERE conference_id = 123 AND registration_status > 0 table possible_keys key rows attendees NULL NULL 14052 The three most important columns returned by EXPLAIN 1)Possible keys All the possible indexes which MySQL could have used ● Based on a series of very quick lookups and ● calculations 2)Chosen key 3)Rows scanned Indication of effort required to identify your result set ●
EXPLAIN – Worked Example EXPLAIN SELECT * FROM attendees WHERE conference_id = 123 AND registration_status > 0 table possible_keys key rows attendees NULL NULL 14052 Interpreting the results No suitable indexes for this query ● MySQL had to do a full table scan ● Full table scans are almost always the slowest query ● Full table scans, while not always bad, are usually an ● indication that an index is required
EXPLAIN – Worked Example ALTER TABLE ADD INDEX conf (conference_id); ALTER TABLE ADD INDEX reg (registration_status); EXPLAIN SELECT * FROM attendees WHERE conference_id = 123 AND registration_status > 1; table possible_keys key rows attendees conf, reg conf 331 MySQL had two indexes to choose from, but discarded “reg” ● “reg” isn't sufficiently unique ● The spread of values can also be a factor (e.g when 99% of ● rows contain the same value) Index “uniqueness” is called cardinality ● There is scope for some performance increase... ● Lower server load, quicker response ●
EXPLAIN – Worked Example ALTER TABLE ADD INDEX reg_conf_index (registration_status, conference_id); EXPLAIN SELECT * FROM attendees WHERE conference_id = 123 AND registration_status > 1; table possible_keys key rows reg, conf, attendees reg_conf_index 204 reg_conf_index reg_conf_index is a much better choice ● Note that the other two keys are still available, just ● not as effective Our query is now served well by the new index ●
EXPLAIN – Worked Example DELETE INDEX conf; DELETE INDEX reg; EXPLAIN SELECT * FROM attendees WHERE conference_id = 123 table possible_keys key rows attendees NULL NULL 14052 Without the “conf” index, we're back to square one ● The order in which fields were defined in a composite index ● affects whether it is available for use in a query ● Remember, we defined our index : (registration_status, conference_id) Potential workaround: EXPLAIN SELECT * FROM attendees WHERE conference_id = 123 AND registration_status >= -1 table possible_keys key rows attendees reg_conf_index reg_conf_index 204
EXPLAIN – Example 2 EXPLAIN SELECT * FROM attendees WHERE surname LIKE 'har%'; table possible_keys key rows attendees surname surname 234 MySQL uses an index on surname – which is good. EXPLAIN SELECT * FROM attendees WHERE surname LIKE '%har%'; table possible_keys key rows attendees NULL NULL 14052 MySQL doesn't even try to use an index!
EXPLAIN – Example 3 EXPLAIN SELECT * FROM conferences WHERE location_id = 2 OR topic_id IN (4,6,1) table possible_keys key rows location_id, conferences NULL 5043 topic_id MySQL doesn't use an index, because of the OR ALTER TABLE ADD INDEX location_topic (location_id, topic_id); EXPLAIN SELECT * FROM conferences WHERE location_id = 2 OR topic_id IN (4,6,1) table possible_keys key rows location_id, conferences location_topic 15 topic_id, location_topic Full table scan avoided – could also use UNION (ALL) trick
EXPLAIN – Example 4 EXPLAIN SELECT * FROM attendees WHERE MD5(conference_id) = MD5(123) table possible_keys key rows attendees NULL NULL 14052 Understandably, MySQL has to do a full table scan A more realistic example? EXPLAIN SELECT * FROM conferences WHERE DATE_FORMAT(date,'%a') = 'Sat' table possible_keys key rows conferences NULL NULL 5043 A good candidate for Optimisation #3 – Modify Data Structure
JOINs JOINing together large data sets (>= 100,000) is really ● where EXPLAIN becomes useful Each JOIN in a query gets its own row in EXPLAIN ● Make sure each JOIN condition is FAST ● Make sure each joined table is getting to its result set ● as quickly as possible ● The benefits compound if each join requires less effort
JOINs – Simple Example EXPLAIN SELECT * FROM conferences INNER JOIN attendees USING (conference_id) WHERE conferences.location_id = 2 AND conferences.topic_id IN (4,6,1) AND attendees.registration_status > 1 table type possible_keys key rows conferences ref conference_topic conference_topic 15 attendees ALL NULL NULL 14052 Looks like I need an index on attendees.conference_id There are 13 different values for “type” ● Another indication of effort, aside from rows scanned ● Here, “ALL” is bad – we should be aiming for “ref” ● Common values are “const”, “ref”, and “all” ● http://dev.mysql.com/doc/refman/5.0/en/using-explain.html
The “extra” column With every EXPLAIN, you get an “extra” column, which shows additional operations invoked to get your result set. table possible_keys key rows extra Using where attendees conf conf 331 Using filesort Some example “extra” values: Using where ● Using temporary table ● Using filesort ● Using index ● There are many more “extra” values which are discussed in the MySQL manual.
“Using filesort” Avoid, because: ● Doesn't use an index ● Involves a full scan of your result set ● Employs a generic (i.e. one size fits all) algorithm ● Uses the filesystem (eeek) ● Will get slower with more data It's not all bad... Perfectly acceptable provided you get to your ● result set as quickly as possible, and keep it predictably small Sometimes unavoidable - ORDER BY RAND() ● ORDER BY operations can use indexes to do the ● sorting!
“Using filesort” – Example EXPLAIN SELECT * FROM attendees WHERE conference_id = 123 ORDER BY surname table possible_keys key rows Extra attendees conference_id conference_id 331 Using filesort MySQL is using an index, but it's sorting the results slowly ALTER TABLE attendees ADD INDEX conf_surname (conference_id, surname); EXPLAIN SELECT * FROM attendees WHERE conference_id = 123 ORDER BY surname table possible_keys key rows Extra conference_id, attendees conf_surname 331 conf_surname We've avoided a filesort
“Using index” Celebrate, because: MySQL got your results just by consulting the index, ● ● Which could well have been sat in memory ●MySQL didn't need to even look at the table to get you your results ● Opening a table can be an expensive operation. ●MySQL can answer the next query more quickly ●The fastest way for you to get your data? Particularly useful... When you're just interested in a single date or an id ● ●Or the COUNT(), SUM(), AVG() etc. of a field
“Using index” – Example EXPLAIN SELECT AVG(age) FROM attendees WHERE conference_id = 123 table possible_keys key rows Extra attendees conference_id conference_id 331 Nothing is actually wrong with this query – it could just be quicker! ALTER TABLE attendees ADD INDEX conf_age (conference_id, age); EXPLAIN SELECT AVG(age) FROM attendees WHERE conference_id = 123 table possible_keys key rows Extra conference_id, attendees conf_surname 331 Using index conf_surname Outside of caching, the fastest way to get your data * *Not a guarantee
Moving forward... Just because your queries are fast now, doesn't mean that they will stay that way forever Enable MySQL's Slow Query Log ● --log-slow-queries=/var/lib/mysql/slow-query.log ● Defaults to logging queries which take more than 10 seconds ● --long_query_time=1 ● Use Percona's “microslow” patch for values < 1 second ● Find the query in the log, EXPLAIN it, improve it, rinse and repeat
Moving forward... Use the command line to identify more general problems ● mysqladmin -u dbuser -p -r -i 10 extended-status ● Figures are relative, updated every 10 seconds ● Slow_queries = number of slow queries in last period ● Select_Scan = full table scans ● Select_full_join = full scans to complete join operations ● Created_tmp_disk_tables = filesorts ● Key_read_requests/Key_write_requests ● Determine write/read weighting of our application and alter your indexes accordingly
MySQL Resources http://dev.mysql.com/doc/refman/5.0/en/using-explain.html ● High Performance MySQL - Baron Schwartz ● ISBN 0596101716 – £20 (Money well spent) – http://www.mysqlperformanceblog.com ● Regular posts –
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