MySQL Query Tuning 101. Sveta Smirnova, Alexander Rubin April, 16, 2015

Transcription

1 MySQL Query Tuning 101 Sveta Smirnova, Alexander Rubin April, 16, 2015

2 Agenda 2 Introduction: where to find slow queries Indexes: why and how do they work All about EXPLAIN More tools Where to find more information

3 When you see slow query first 3 You develop an application and find out that some queries are running slow After a while you find some slow queries in the slow query log All such queries are always slow We would not talk about cases when concurrency affects performance

4 Why query can run slow 4 MySQL has to do some job to execute a select query In the worst case scenario it will do a full table scan CREATE INDEX Incorrect index can be used by MySQL

5 Full table scan 5 select * from table select * from table where id=

6 MySQL Indexes 6 When you add index (except for MEMORY) MySQL will use B-Tree Support equality and range operations

7 B-Tree: Equality search 7 select * from table where id = 12 Scan thru the tree and go directly to 1 leaf Stop

8 B-Tree: Range 8 select * from table where id in (6, 12, 18) Scan thru the tree and visit many leafs/nodes

9 How to find out how MySQL uses indexes 9 EXPLAIN Estimates what happens during query execution EXTENDED FORMAT=JSON PARTITIONS INFORMATION SCHEMA.OPTIMIZER TRACE Real data, collected after query was executed Advanced topic

10 Effect of indexes: before 10 mysql> explain select * from t1\g *************************** 1. row ***************************... rows: 12 Extra: NULL mysql> explain select * from t1 where f2=12\g *************************** 1. row ***************************... key: NULL... rows: 12 Same number of examined rows for both queries Extra: Using where

11 Effect of indexes: after 11 mysql> alter table t1 add index(f2); Query OK, 12 rows affected (0.07 sec) Records: 12 Duplicates: 0 Warnings: 0 mysql> explain select * from t1 where f2=12\g *************************** 1. row ***************************... key: f2 key_len: 5 ref: const rows: 1 Extra: NULL 1 row in set (0.00 sec) Much more effective! Only 1 row examined

12 EXPLAIN: overview 12 Number of select Tables, for which information is printed Possible keys Length of the key Number of examined rows Additional information mysql> explain extended select * from t1 join t2 where t1.int_key=1; id select_type table type possible_keys key key_len ref rows filtered Extra SIMPLE t1 ref int_key,ik int_key 5 const NULL 1 SIMPLE t2 index NULL pk 9 NULL Using index; Using join buffer Product of rows here: how many rows in all tables will be accessed Table, for which information is printed (Block Nested Loop) For this example estimated value is 4*6 = 24 2 rows in set, 1 warning (0.00 sec) Select type How data is accessed Key, which was actually used % of filtered rows rows x filtered / 100 number of rows, which will be joined with another table Which columns were compared with the index Note (Code 1003): /* select#1 */ select test. t1. pk AS pk, test. t1. int_key AS int_key, test. t2. pk AS pk, test. t2. int_key AS int_key from test. t1 join test. t2 where ( test. t1. int_key = 1) Actual (optimized) query as executed by MySQL Server

13 EXPLAIN in details 13 mysql> explain extended select * from t1 join t2 where *** id select_type table type *** *** 1 SIMPLE t1 ref *** 1 SIMPLE t2 index *** *** 2 rows in set, 1 warning (0.00 sec) SIMPLE;PRIMARY;UNION;DEPENDENT UNION;UNION RESULT; SUBQUERY;DEPENDENT SUBQUERY;DERIVED;MATERIALIZED system const eq ref ref fulltext ref or null index merge unique subquery index subquery range index ALL

14 EXPLAIN in details: keys 14 Keys, which can be used for resolving the query Actual length of the key (Important for multiple-column keys) mysql> explain extended select * from t1 join t2 where t1.int_key=1; *** *** *** possible_keys key key_len ref *** *** *** *** int_key,ik int_key 5 const *** *** NULL pk 9 NULL *** *** *** 2 rows in set, 1 warning (0.00 sec) Constant Numeric in our case Index used to resolve rows Only one key was actually used Which columns were compared with the index

15 EXPLAIN in details: rows 15 Number of rows accessed % of rows filtered Additional information: how query is resolved Using filesort Using temporary etc. mysql> explain extended select * from t1 join t2 where t1.int_key=1; *** *** rows filtered Extra *** X6 *** NULL = *** Using index; Using join buffer (Block Nested Loop) *** rows in set, 1 warning (0.00 sec) All rows used

16 EXPLAIN type by example: setup 16 MySQL or Percona Server 5.6 Employees test database XML: Fusheng Wang and Carlo Zaniolo SQL: Giuseppe Maxia and Patrick Crews More information Download: Install: cd employees db mysql <employees.sql

17 EXPLAIN type by example: ALL 17 mysql> explain select count(*) from employees where hire_date > \ ********************** 1. row ********************** id: 1 select_type: SIMPLE table: employees type: ALL possible_keys: NULL key: NULL key_len: NULL ref: NULL rows: Extra: Using where 1 row in set (0.00 sec) All rows in the table examined Worst plan ever!

18 EXPLAIN type by example: index 18 mysql> explain select count(*) from titles where title= Senior Engineer \G ********************** 1. row ********************** id: 1 select_type: SIMPLE table: titles type: index possible_keys: NULL key: emp_no key_len: 4 ref: NULL rows: Extra: Using where; Using index 1 row in set (0.11 sec) No row in the table was accessed to resolve the query! Only index used Still all records in the index were scanned

19 EXPLAIN type by example: range 19 We need to add index to table employees first mysql> alter table employees \ -> add index(hire_date); Query OK, 0 rows affected (3.48 sec) Records: 0 Duplicates: 0 Warnings: 0

20 EXPLAIN type by example: range 20 mysql> explain select count(*) from employees where hire_date > \ ********************** 1. row ********************** id: 1 Only rows from given range used select_type: SIMPLE table: employees type: range possible_keys: hire_date key: hire_date key_len: 3 Compare with ALL: ref: NULL /68654 = rows: times less rows examined! Extra: Using where; Using index 1 row in set (0.00 sec)

21 Combined indexes 21 Consists of two or more columns Only leftmost part used mysql> alter table City add key comb(countrycode, District, Population), drop key CountryCode;

22 Combined indexes: example 1 22 mysql> explain select * from City where CountryCode = USA \G ********************** 1. row ****************** table: City type: ref possible_keys: comb key: comb key_len: 3 Uses first field from the comb key ref: const rows: 273

23 Combined indexes: example 2 23 mysql> explain select * from City where \ -> District = California and population > 10000\G ********************** 1. row ****************** table: City type: ALL possible_keys: NULL key: NULL key_len: NULL ref: NULL rows: 3868 Can t use combined index: not a leftmost part Does not have the CountryCode in the where clause = can t use comb index

24 Combined indexes: key len 24 Key len = total size (in bytes) of index parts used Index: comb(countrycode, District, Population) Explain: Fields: key: comb CountryCode char(3) key len: 3 District char(20) Population int(11) 3 ->Char(3) ->First field is used

25 Covered indexes 25 Covered index = cover all fields in query select name from City where CountryCode = USA and District = Alaska and population > mysql> alter table City add key cov1(countrycode, District, population, name); 1. Where part 2. Group By/Order (not used now) 3. Select part Uses all fields in the query in particular order

26 EXPLAIN by example: covered indexes 26 mysql> explain select name from City where CountryCode = USA \ -> and District = Alaska and population > 10000\G *************************** 1. row *********** table: City type: range possible_keys: cov1 key: cov1 key_len: 27 ref: NULL rows: 1 Extra: Using where; Using index Covered index is used MySQL will only use index Will not go to the data file

27 EXPLAIN type by example: ref 27 mysql> explain select * from dept_emp where dept_no = d005 \G ************************ 1. row ************************ id: 1 select_type: SIMPLE table: dept_emp type: ref possible_keys: dept_no key: dept_no key_len: 4 ref: const rows: Extra: Using where 1 row in set (0.00 sec)

28 EXPLAIN type by example: eq ref 28 mysql> explain select * from dept_manager join employees using(emp_no) limit 10\G ************************ 1. row ************************ id: 1 select_type: SIMPLE table: dept_manager type: ALL possible_keys: PRIMARY,emp_no key: NULL key_len: NULL ref: NULL rows: 24 Extra:

29 EXPLAIN type by example: eq ref 29 ************************ 2. row ************************ id: 1 select_type: SIMPLE table: employees type: eq_ref possible_keys: PRIMARY key: PRIMARY key_len: 4 ref: employees.dept_manager.emp_no rows: 1 Extra: 2 rows in set (0.00 sec)

30 EXPLAIN type by example: const 30 mysql> explain select * from departments where dept_no= d005 \G ************************ 1. row ************************ id: 1 select_type: SIMPLE table: departments type: const possible_keys: PRIMARY key: PRIMARY key_len: 4 ref: const rows: 1 Extra: 1 row in set (0.00 sec)

31 EXPLAIN limitations and extensions 31 Limitations It shows estimates only Extensions EXTENDED PARTITIONS FORMAT=JSON

32 EXPLAIN PARTITIONS 32 mysql> explain partitions select count(*) \ -> from employees_part where hire_date > \G ************************ 1. row ************************ id: 1 select_type: SIMPLE table: employees_part partitions: p1,p2 type: index possible_keys: NULL key: PRIMARY key_len: 7 ref: NULL rows: Extra: Using where; Using index

33 EXPLAIN FORMAT=JSON 33 Gives more information Real execution path of the query Pushed conditions Temporary table and index creation are more precise Reflects execution order of group by and order by operations Displays table materializations

34 EXPLAIN FORMAT=JSON 34 mysql> explain format=json insert into salaries(emp_no, from_date) select emp_no, min(from_date) from titles group by emp_no\g *********************** 1. row *********************** EXPLAIN: { "query_block": { "select_id": 1, "grouping_operation": { "using_filesort": false, "table": { "table_name": "titles",

35 EXPLAIN for DML: not only SELECT 35 mysql> explain insert into salaries(emp_no, from_date) \ -> select emp_no, min(from_date) from titles group by emp_no\g ********************** 1. row ********************** id: 1 select_type: SIMPLE table: titles type: index possible_keys: NULL key: emp_no

36 EXPLAIN for DML: not only SELECT 36 mysql> explain insert into salaries(emp_no, from_date) select emp_no, min(from_date) from titles group by emp_no\g... key_len: 4 ref: NULL rows: Extra: Using index 1 row in set, 2 warnings (0.00 sec) Warning (Code 1364): Field salary doesn t have a default value Warning (Code 1364): Field to_date doesn t have a default value

37 Other tools 37 Status variables Handler % Performance schema events stages % events statements % INFORMATION SCHEMA.OPTIMIZER TRACE

38 More to learn 38 MySQL User Reference Manual MySQL Troubleshooting book High Performance MySQL book MySQL Performance Blog Planet MySQL Troubleshooting Performance Companion slides

39 Thank you! 39

40 Questions? 40?