Explaining the MySQL EXPLAIN
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1 Explaining the MySQL EXPLAIN Ronald Bradford OTN South America Tour July 2011
2 Agenda EXPLAIN syntax options How to read QEP QEP examples MySQL optimizer limitations
3 PURPOSE EXPLAIN is used for Determine Query Execution Plan (QEP) Understand MySQL optimizer Identify information needed for tuning Not providing tuning recommendations
4 ABOUT THE AUTHOR Ronald Bradford All time top blog contributor to Planet MySQL Published Author of Expert PHP & MySQL Oracle ACE Director (first in MySQL) MySQL community member of the year 22 years of RDBMS experience,12 years with MySQL MySQL Inc ( ) Oracle Corporation ( ) Provide independent consulting - Available NOW
5 MySQL QEP Cost Based Optimizer No pinning Few hints Calculated every SQL execution Except Query Cache
6 SYNTAX EXPLAIN SELECT... EXPLAIN PARTITIONS... EXPLAIN EXTENDED... Does not support UPDATE,DELETE
7 Support Commands SHOW CREATE TABLE SHOW INDEXES SHOW TABLE STATUS INFORMATION_SCHEMA SHOW VARIABLES SHOW STATUS
8 EXPLAIN USAGE Does not execute query BUT May execute portions WARNING!!!
9 EXPLAIN Example mysql> EXPLAIN SELECT * FROM proposal WHERE post_id= ; id select_type table type possible_keys key key_len ref rows Extra SIMPLE proposal ALL NULL NULL NULL NULL Using where
10 MYSQL CLIENT Statement terminator ; \G
11 EXPLAIN Example mysql> EXPLAIN SELECT * FROM inventory -> WHERE item_id = \G *************************** 1. row ********************** id: 1 select_type: SIMPLE table: inventory type: ref possible_keys: item_id key: item_id key_len: 4 ref: const rows: 1 Extra:
12 Attributes
13 Attributes id select_type table type possible_keys key key_len ref rows Extra mysql> EXPLAIN SELECT * FROM invento -> WHERE item_id = \G ************************** 1. row ** id: 1 select_type: SIMPLE table: inventory type: ref possible_keys: item_id key: item_id key_len: 4 ref: const rows: 1 Extra:
14 Essential Example id select_type table type possible_keys key key_len ref rows Extra mysql> EXPLAIN SELECT * FROM invento -> WHERE item_id = \G ************************* 1. row *** id: 1 select_type: SIMPLE table: inventory type: ref possible_keys: item_id key: item_id key_len: 4 ref: const rows: 1 Extra:
15 Essential Example id select_type table type possible_keys key key_len ref rows Extra mysql> EXPLAIN SELECT * FROM invento -> WHERE item_id = \G ************************* 1. row *** id: 1 select_type: SIMPLE table: inventory type: ref possible_keys: item_id key: item_id key_len: 4 ref: const rows: 1 Extra:
16 Essential Example id select_type table type possible_keys key key_len ref rows Extra mysql> EXPLAIN SELECT * FROM invento -> WHERE item_id = \G ************************** 1. row *** id: 1 select_type: SIMPLE table: inventory type: ALL possible_keys: NULL key: NULL key_len: NULL ref: NULL rows: Extra: Using where
17 Essential Example id select_type table type possible_keys key key_len ref rows Extra mysql> EXPLAIN SELECT * FROM invento -> WHERE item_id = \G *************************** row row *** * id: 1 select_type: SIMPLE table: inventory type: ref ALL possible_keys: item_id NULL key: item_id NULL key_len: 4NULL ref: const NULL rows: Extra: Using where
18 key Identify index to be used Generally only one per table (*) Associated elements possible_keys key_len id: 1 select_type: SIMPLE table: invento type: ALL possible_keys: NULL key: NULL key_len: NULL ref: NULL rows: Extra: Using w
19 key Example mysql> EXPLAIN SELECT * FROM inventory -> WHERE item_id = \G *************************** 1. row ********************** id: 1 select_type: SIMPLE table: inventory type: ref possible_keys: item_id key: item_id key_len: 4 ref: const rows: 1 Extra:
20 key MERGE INDEX Example mysql> EXPLAIN SELECT id FROM users -> WHERE first = 'west' OR last='west'\g ************************ 1. row ********************** id: 1 select_type: SIMPLE table: users type: index_merge possible_keys: first,last key: first,last key_len: 22,22 ref: NULL rows: 2 Extra: Using union(first,last); Using where CREATE TABLE `users` ( `id` int(10) unsigned NOT NULL AU `first` varchar(20) NOT NULL, `last` varchar(20) NOT NULL, `username` varchar(20) NOT NULL, `last_login` timestamp NOT NULL D PRIMARY KEY (`id`), KEY `first` (`first`), KEY `last` (`last`), KEY `username` (`username`)...
21 possible_keys Indexes the optimizer considered Why was no index used? Too many is not good Associated elements key id: 1 select_type: SIMPLE table: invento type: ALL possible_keys: NULL key: NULL key_len: NULL ref: NULL rows: Extra: Using w
22 possible_keys Example id select_type table type possible_keys key key_len ref rows Extra SIMPLE c const PRIMARY PRIMARY 4 const 1 Using filesort 1 SIMPLE i ALL customer_id NULL NULL NULL 7 Using where id select_type table type possible_keys key key_len ref rows Extra SIMPLE c const PRIMARY PRIMARY 4 const 1 1 SIMPLE i ref customer_id customer_id 4 const 5 Using where
23 possible_keys Example id select_type table type possible_keys key key_len ref rows Extra SIMPLE c const PRIMARY PRIMARY 4 const 1 Using filesort 1 SIMPLE i ALL customer_id NULL NULL NULL 7 Using where id select_type table type possible_keys key key_len ref rows Extra SIMPLE c const PRIMARY PRIMARY 4 const 1 1 SIMPLE i ref customer_id customer_id 4 const 5 Using where
24 rows Estimated number of table rows (*) Associated elements key id: 1 select_type: SIMPLE table: invento type: ALL possible_keys: NULL key: NULL key_len: NULL ref: NULL rows: Extra: Using w
25 key_len Amount of index used (*) Associated elements Extra = Using Index id: 1 select_type: SIMPLE table: invento type: ALL possible_keys: NULL key: NULL key_len: NULL ref: NULL rows: Extra: Using w
26 key_len Example mysql> EXPLAIN SELECT user_id,balance,created -> FROM accounts -> WHERE id = 42\G *************************** 1. row ********************** id: 1 select_type: SIMPLE table: accounts type: const possible_keys: PRIMARY key: PRIMARY key_len: 8 ref: const rows: 1 Extra: CREATE TABLE `accounts` ( `id` BIGINT NOT NULL AUTO_INCREMEN... PRIMARY KEY (id)...
27 key_len with varchar Example mysql> EXPLAIN SELECT * -> FROM categories -> WHERE name LIKE 'NCAA%'; *************************** 1. row ********************* id: 1 select_type: SIMPLE table: categories type: range possible_keys: name key: name key_len: 32 ref: NULL rows: 6 Extra: Using where CREATE TABLE categories ( id int NOT NULL AUTO_INCREMENT, name VARCHAR(30) NOT NULL,... INDEX (name)...
28 key_len with utf8 Example mysql> EXPLAIN SELECT * -> FROM categories -> WHERE name LIKE 'NCAA%'; *************************** 1. row ********************* id: 1 select_type: SIMPLE table: categories type: range possible_keys: name key: name key_len: 92 ref: NULL rows: 6 Extra: Using where CREATE TABLE `categories` ( `id` int(4) NOT NULL AUTO_INCREMEN `name` VARCHAR(30) NOT NULL,... INDEX (name) )... DEFAULT CHARSET=utf8
29 key_len Calculations TINYINT - 1 byte SMALLINT - 2 bytes INT - 4 bytes BIGINT - 8 bytes DATE - 3 bytes TIMESTAMP - 4 bytes DATETIME - 8 bytes CHAR(n) - n bytes VARCHAR(n) - n bytes NULL + 1 byte VARCHAR + 2 bytes Character set x [1-3] bytes
30 key_len (1) (3) Example mysql> EXPLAIN SELECT ID, post_title FROM wp_posts WHERE post_type='post' and post_date > ' '; id select_type table type possible_keys key key_len ref rows Extra SIMPLE wp_posts ref type_status_date type_status_date 62 const 1132 Using where CREATE TABLE `wp_posts` (... `post_date` datetime NOT NULL DEFAULT ' :00:00', `post_status` varchar(20) NOT NULL DEFAULT 'publish', `post_type` varchar(20) NOT NULL DEFAULT 'post',... PRIMARY KEY (`ID`), (1) (2) (3) (4) KEY `type_status_date` (`post_type`,`post_status`,`post_date`,`id`), ) DEFAULT CHARSET=utf
31 key_len mysql> EXPLAIN SELECT ID, post_title FROM wp_posts WHERE post_type='post' and post_date > ' '; id select_type table type possible_keys key key_len ref rows Extra SIMPLE wp_posts ref type_status_date type_status_date 62 const 1132 Using where mysql> EXPLAIN SELECT ID, post_title FROM wp_posts WHERE post_type='post' AND post_status='publish' AND post_date > ' '; id select_type table type possible_keys key key_len ref rows Extra SIMPLE wp_posts range type_status_date type_status_date 132 NULL 1 Using where CREATE TABLE `wp_posts` (... `post_date` datetime NOT NULL DEFAULT ' :00:00', `post_status` varchar(20) NOT NULL DEFAULT 'publish', `post_type` varchar(20) NOT NULL DEFAULT 'post',... PRIMARY KEY (`ID`), (1) (3) Example (1) (2) (3) (1) (2) (3) (4) KEY `type_status_date` (`post_type`,`post_status`,`post_date`,`id`), ) DEFAULT CHARSET=utf
32 select_type SIMPLE PRIMARY SUBQUERY DERIVED UNION id: 1 select_type: SIMPLE table: invento type: ALL possible_keys: NULL key: NULL key_len: NULL ref: NULL rows: Extra: Using w
33 select_type SIMPLE PRIMARY SUBQUERY DEPENDENT UNION UNION RESULT UNCACHEABLE QUERY DERIVED UNION UNCACHEABLE UNION id: 1 select_type: SIMPLE table: invento type: ALL possible_keys: NULL key: NULL key_len: NULL ref: NULL rows: Extra: Using w
34 select_type Example mysql> EXPLAIN SELECT MAX(id) FROM (SELECT id FROM users WHERE first = 'west') c; id select_type table type possible_keys key key_len ref rows Extra PRIMARY <derived2> ALL NULL NULL NULL NULL 2 2 DERIVED users ref first first 22 1 Using where mysql> EXPLAIN SELECT p.* FROM parent p WHERE p.val LIKE 'a%' -> UNION -> SELECT p.* FROM parent p WHERE p.id > 5; id select_type table type possible_keys key key_len ref rows Extra PRIMARY p range val val 12 NULL 1 Using where 2 UNION p ALL PRIMARY NULL NULL NULL 8 Using where NULL UNION RESULT <union1,2> ALL NULL NULL NULL NULL NULL
35 select_type Example mysql> EXPLAIN SELECT MAX(id) FROM (SELECT id FROM users WHERE first = 'west') c; id select_type table type possible_keys key key_len ref rows Extra PRIMARY <derived2> ALL NULL NULL NULL NULL 2 2 DERIVED users ref first first 22 1 Using where mysql> EXPLAIN SELECT p.* FROM parent p WHERE p.val LIKE 'a%' -> UNION 2 represents id -> SELECT p.* FROM parent p WHERE p.id > 5; id select_type table type possible_keys key key_len ref rows Extra PRIMARY p range val val 12 NULL 1 Using where 2 UNION p ALL PRIMARY NULL NULL NULL 8 Using where NULL UNION RESULT <union1,2> ALL NULL NULL NULL NULL NULL ,2 represents id
36 select_type Example mysql> EXPLAIN SELECT p.* FROM parent p WHERE p.id NOT IN (SELECT c.parent_id FROM child c); id select_type table type possible_keys key key_len ref rows Extra PRIMARY p ALL NULL NULL NULL NULL 7 Using where 2 DEPENDENT SUBQUERY c index_subquery parent_id parent_id 4 func 2 Using index mysql> EXPLAIN SELECT p.* FROM parent p LEFT JOIN child c ON p.id = c.parent_id WHERE c.child_id IS NULL; id select_type table type possible_keys key key_len ref rows Extra SIMPLE p ALL NULL NULL NULL NULL 7 1 SIMPLE c ref parent_id parent_id 4 p.id 2 Using where; Not exists mysql> EXPLAIN SELECT p.* FROM parent p WHERE NOT EXISTS (SELECT parent_id FROM child c WHERE c.parent_id = p.id); id select_type table type possible_keys key key_len ref rows Extra PRIMARY p ALL NULL NULL NULL NULL 7 Using where 2 DEPENDENT SUBQUERY c ref parent_id parent_id 4 p.id 2 Using index
37 select_type 3 ways to get same query results Example mysql> EXPLAIN SELECT p.* FROM parent p WHERE p.id NOT IN (SELECT c.parent_id FROM child c); id select_type table type possible_keys key key_len ref rows Extra PRIMARY p ALL NULL NULL NULL NULL 7 Using where 2 DEPENDENT SUBQUERY c index_subquery parent_id parent_id 4 func 2 Using index mysql> EXPLAIN SELECT p.* FROM parent p LEFT JOIN child c ON p.id = c.parent_id WHERE c.child_id IS NULL; id select_type table type possible_keys key key_len ref rows Extra SIMPLE p ALL NULL NULL NULL NULL 7 1 SIMPLE c ref parent_id parent_id 4 p.id 2 Using where; Not exists mysql> EXPLAIN SELECT p.* FROM parent p WHERE NOT EXISTS (SELECT parent_id FROM child c WHERE c.parent_id = p.id); id select_type table type possible_keys key key_len ref rows Extra PRIMARY p ALL NULL NULL NULL NULL 7 Using where 2 DEPENDENT SUBQUERY c ref parent_id parent_id 4 p.id 2 Using index Which query is best?
38 Extra Most Common Using where Using temporary Using filesort Using index Using join buffer **GOOD** id: 1 select_type: SIMPLE table: invento type: ALL possible_keys: NULL key: NULL key_len: NULL ref: NULL rows: Extra: Using w
39 Extra - Using temporary Internal Table (MEMORY based) Can have multiple per query To Disk Impact TEXT/BLOB Size
40 Extra - Using filesort ORDER BY Can be CPU intensive Is order via DB necessary? Can you leverage an index?
41 Extra - Using filesort Example EXPLAIN SELECT i.invoice_date, i.customer_id, i.invoice_total, c.company, c.state FROM invoice i INNER JOIN customer c USING (customer_id) WHERE i.customer_id = 42 ORDER BY i.invoice_date; table type possible_keys key key_len ref rows Extra c const PRIMARY PRIMARY 4 const 1 Using filesort i ref customer_id customer_id 4 const 5 Using where CREATE TABLE invoice( invoice_id INT UNSIGNED NOT NULL AUTO_INCREMENT, invoice_date DATE NOT NULL, customer_id INT UNSIGNED NOT NULL, invoice_total DECIMAL(10,2) NOT NULL, PRIMARY KEY(invoice_id), KEY (customer_id) ) ENGINE=InnoDB;
42 Extra - Using filesort Example EXPLAIN SELECT i.invoice_date, i.customer_id, i.invoice_total, c.company, c.state FROM invoice i INNER JOIN customer c USING (customer_id) WHERE i.customer_id = 42 ORDER BY i.invoice_date; table type possible_keys key key_len ref rows Extra c const PRIMARY PRIMARY 4 const 1 Using filesort i ref customer_id customer_id 4 const 5 Using where CREATE TABLE invoice( invoice_id INT UNSIGNED NOT NULL AUTO_INCREMENT, invoice_date DATE NOT NULL, customer_id INT UNSIGNED NOT NULL, invoice_total DECIMAL(10,2) NOT NULL, PRIMARY KEY(invoice_id), KEY (customer_id, invoice_date) ) ENGINE=InnoDB;
43 Extra - Using index Does not mean using index Means using ONLY THE INDEX Additional Presentation Improving Performance with Better Indexes
44 Extra - Using index Example id select_t tab key key_ id rows select_t Extra tab key key_ Extra PRIMARY c statu PRIMARY Using where c adver 4 Using where 1 PRIMARY e PRIMA PRIMARY Using where e statu 1 Using where; Using index 1 PRIMARY g campa PRIMARY Using where g campa 4 Using where 10 DEPENDEN crb id_ca DEPENDEN Using where crb id_ca 66 Using where 9 DEPENDEN csb pub_s DEPENDEN Using where csb pub_s 98 Using where 8 DEPENDEN arb id_ad DEPENDEN Using where arb id_ad 26 Using where 7 DEPENDEN asb pub_s DEPENDEN Using where asb id_ad 40 Using where; Using index 6 DEPENDEN pm id_adr DEPENDEN Using where pm id_adr 12 Using index 5 DEPENDEN tgv searc DEPENDEN Using where tgv searc 10 Using where; Using index 4 DEPENDEN st id_sc DEPENDEN Using where st id_sc 4 Using where; Using index 4 DEPENDEN t PRIMA DEPENDEN Using where t PRIMA 4 Using where 3 DEPENDEN k2 keywo DEPENDEN Using where k2 keywo 302 Using where; Using index 3 DEPENDEN gk2 PRIMA DEPENDEN Using where gk2 PRIMA 100 Using where 2 DEPENDEN k1 keywo DEPENDEN Using where k1 keywo 302 Using where; Using index 2 DEPENDEN gk1 PRIMA DEPENDEN Using where gk1 PRIMA 100 Using where
45 Extra - Using index Executed thousand queries per second Example id select_t tab key key_ id rows select_t Extra tab key key_ Extra PRIMARY c statu PRIMARY Using where c adver 4 Using where 1 PRIMARY e PRIMA PRIMARY Using where e statu 1 Using where; Using index 1 PRIMARY g campa PRIMARY Using where g campa 4 Using where 10 DEPENDEN crb id_ca DEPENDEN Using where crb id_ca 66 Using where 9 DEPENDEN csb pub_s DEPENDEN Using where csb pub_s 98 Using where 8 DEPENDEN arb id_ad DEPENDEN Using where arb id_ad 26 Using where 7 DEPENDEN asb pub_s DEPENDEN Using where asb id_ad 40 Using where; Using index 6 DEPENDEN pm id_adr DEPENDEN Using where pm id_adr 12 Using index 5 DEPENDEN tgv searc DEPENDEN Using where tgv searc 10 Using where; Using index 4 DEPENDEN st id_sc DEPENDEN Using where st id_sc 4 Using where; Using index 4 DEPENDEN t PRIMA DEPENDEN Using where t PRIMA 4 Using where 3 DEPENDEN k2 keywo DEPENDEN Using where k2 keywo 302 Using where; Using index 3 DEPENDEN gk2 PRIMA DEPENDEN Using where gk2 PRIMA 100 Using where 2 DEPENDEN k1 keywo DEPENDEN Using where k1 keywo 302 Using where; Using index 2 DEPENDEN gk1 PRIMA DEPENDEN Using where gk1 PRIMA 100 Using where Before was 175ms After was 5ms
46 Extra - Using join buffer Example id select_type table type possible_keys key key_len ref rows Extra 1 SIMPLE fs ref PRIMARY,sts sts 768 const 21 Using where; Using temporary; U 1 SIMPLE fsu ref PRIMARY,d,sid sid 4 fs.sid 21 Using where 1 SIMPLE fes ref sasi,eid,sid,sues,suid sues 8 fsi.suid,fs.sid 26 1 SIMPLE ma ALL masi NULL NULL NULL 200 Using where; Using join buffer 1 SIMPLE fas eq_ref PRIMARY PRIMARY 4 ma.faid 1 Using where; Using index 1 SIMPLE las eq_ref PRIMARY,asai PRIMARY 4 ma.laid 1 1 SIMPLE la eq_ref PRIMARY PRIMARY 4 las.aid 1 1 SIMPLE fp eq_ref PRIMARY PRIMARY 4 ses.eid 1
47 Extra - Using join buffer Example id select_type table type possible_keys key key_len ref rows Extra 1 SIMPLE fs ref PRIMARY,sts sts 768 const 21 Using where; Using temporary; U 1 SIMPLE fsu ref PRIMARY,d,sid sid 4 fs.sid 21 Using where 1 SIMPLE fes ref sasi,eid,sid,sues,suid sues 8 fsi.suid,fs.sid 26 1 SIMPLE ma ALL masi NULL NULL NULL 200 Using where; Using join buffer 1 SIMPLE fas eq_ref PRIMARY PRIMARY 4 ma.faid 1 Using where; Using index 1 SIMPLE las eq_ref PRIMARY,asai PRIMARY 4 ma.laid 1 1 SIMPLE la eq_ref PRIMARY PRIMARY 4 las.aid 1 1 SIMPLE fp eq_ref PRIMARY PRIMARY 4 ses.eid 1 No index can be satisfied for join condition. i.e. full table scan
48 Extra cont. Less common Impossible WHERE... Distinct Not exists Select tables optimized away
49 Extra Example mysql> EXPLAIN SELECT COUNT(*) FROM (SELECT id FROM users WHERE first = 'west') c *************************** 1. row *************************** id: 1 select_type: PRIMARY table: NULL type: NULL possible_keys: NULL key: NULL key_len: NULL ref: NULL rows: NULL Extra: Select tables optimized away *************************** 2. row *************************** id: 2 select_type: DERIVED table: users type: ref possible_keys: first key: first key_len: 22 ref: rows: 1
50 Extra cont. Merge Indexes Using sort_union(...) Using union(...) Using intersect(...)
51 SYNTAX EXPLAIN SELECT... EXPLAIN PARTITIONS... EXPLAIN EXTENDED... Does not support UPDATE,DELETE
52 EXPLAIN PARTITIONS Example mysql> EXPLAIN PARTITIONS SELECT * from audit_log WHERE yr in (2011,2012)\G *************************** 1. row *************************** id: 1 select_type: SIMPLE table: audit_log partitions: p2,p3 type: ALL possible_keys: NULL key: NULL key_len: NULL ref: NULL rows: 2 Extra: Using where
53 EXPLAIN EXTENDED Example mysql> EXPLAIN EXTENDED select t1.name from test1 t1 INNER JOIN test2 t2 USING(uid)\G *************************** 1. row *************************** id: 1 select_type: SIMPLE table: t1 type: ALL possible_keys: NULL key: NULL key_len: NULL ref: NULL rows: 1 filtered: Extra: *************************** 2. row *************************** id: 1 select_type: SIMPLE table: t2 type: eq_ref possible_keys: PRIMARY key: PRIMARY key_len: 98 ref: func rows: 1 filtered: Extra: Using where; Using index 2 rows in set, 1 warning (0.00 sec)
54 EXPLAIN EXTENDED Example mysql> EXPLAIN EXTENDED select t1.name from test1 t1 INNER JOIN test2 t2 USING(uid)\G mysql> SHOW WARNINGS\G *************************** 1. row *************************** Level: Note Code: 1003 Message: select `book`.`t1`.`name` AS `name` from `book`.`test1` `t1` join `book`.`test2` `t2` where (convert(`book`.`t1`.`uid` using utf8) = `book`.`t2`.`uid`)
55 INDEX HINTS USE INDEX IGNORE INDEX FORCE INDEX FOR JOIN ORDER BY GROUP BY
56 INDEX HINTS USE INDEX IGNORE INDEX Can specify multiple indexes FORCE INDEX FOR JOIN ORDER BY GROUP BY
57 SELECT HINTS STRAIGHT_JOIN Defines order of tables in QEP HIGH_PRIORITY Certain engines (*)
58 SELECT HINTS SQL_CACHE SQL_NO_CACHE SQL_CALC_FOUND_ROWS SQL_BIG_RESULT SQL_SMALL_RESULT SQL_BUFFER_RESULT
59 SQL_CALC_FOUND_ROWS Example SELECT id,username FROM users WHERE last LIKE 'w%' LIMIT 10; rows in set (0.00 sec) SELECT SQL_CALC_FOUND_ROWS id,username FROM users WHERE last LIKE 'w%' LIMIT 10; SELECT FOUND_ROWS(); rows in set (8.81 sec) mysql> SELECT FOUND_ROWS(); FOUND_ROWS() row in set (0.02 sec)
60 Conclusion
61 CONCLUsiON Essential tool for SQL analysis Not the only information you need Other Presentations Understanding MySQL Indexes Improving Performance with Better Indexes
62 Ronald Bradford
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