Java MySQL slow query optimization

0 Preface

　　This article introduces MySQL, the related slow query optimization.

　　Mysql slow query log is a log, mysql statement can be used to record the execution time exceeds the specified long_query_time SQL statement, the default value is 10s long_query_time
　　slow query log under the circumstances is not turned on by default, because the data is saved to a log have some impact on performance, the test environment can be opened manually, but then pay attention to manually open only works on this start, restart restore the default state after mysql closed, in order to take effect lasting change my.ini configuration file (under Window system), other system variables also do so. By '% slow_query_log%' show varaibles like to view the log open case

1 Query Optimization

　　1.1 explain + SQL statement : execute this statement allows developers to see the details of the implementation of the select statement, developers can step on slowly query SQL query statements captured in the analysis, the low efficiency of inquiry to determine the possible causes

　　　　id: Number select query

id are the same, perform a top-down order   

id is not the same, the greater the value of id higher priority, the first to be executed   

id have the same of different, the same id as a group, according to different sets of values id rules two priority execution, the same group id is sequentially executed in accordance with the rules of a  

　　　　select_type: select the type of query

simple: it indicates that the query is not connected subqueries and UNION queries   
outermost query has a subquery: Primary   
subquery: inner layer when subqueries nested query   
derived: select included in the called derived from (derived) , mysql will recursively these sub-queries, the results in a temporary table   
union: union of the second or the last   
union result: the result of the union  

　　　　table: the table with the implementation of the current SQL statement

　　　　partitions: the representative of the current partition table used

　　　　type: show how to use the query, according to several common query order of best to worst system> const> eq_ref> ref> range> index> all

system, const: mysql can be part of the query optimization enables these convert it to a constant (system returns only one, const multiple rows), as the primary key of a row manner into the WHERE clause to select this line primary key, MySQL will be able to convert this query into a constant. Then the table can be efficiently removed from the coupling execution   
eq_ref: using the lookup index, mysql return up to a known data can be used when using the master key or a unique index lookup   
ref: non-unique index lookup index   
range: scanning range, for example between or with>, <, in other   
index: index scanning all lines   
all: scanning all rows of data

　　　　The actual use of the index and the index may be used on behalf of: possible_keys / kesy

　　　　key_len: number of bytes in the index

　　　　ref: it shows a lookup table prior to the value recorded in the index key column used in columns or constants

　　　　rows: mysq to find the estimated number of rows meet the conditions required for scanning of line

　　Show Profile 1.2 : mysql Show Profile is used to analyze resource usage of SQL query tool

　　We perform some tests run SQL statements after the statements show profiles

　　We can choose to specify items specified SQL statement analysis

　　Our view is that the general properties and cpu block io two modules

　　note:

If any of the following situations arise, we have said it was a bad SQL statement that needs to be optimized

1. convering heap to MyIsam query result is too large, not enough memory, you need to record to disk
2. creating tmp table to create a temporary table to store data, delete exhausted after
3. copying to tmp table on disk to store data in the temporary table to disk
4. locked

2 Index Tuning

　　What circumstances need to build the index?

1. Primary key, a unique index
2. Often used as the query conditions index
3. Often we need sorting, grouping, indexing statistics
4. Query associated with other tables, foreign key relationships indexes

　　What case do not build the index?

1. Table records too, you do not need to build the index about one million
2. Frequent additions and deletions to the list
3. Data is repeatedly and uniformly distributed data: such as true, false
4. Frequent Updates field
5. where conditions are less than in the field