SQL common optimization

sql Optimize

1. Small tables drive large tables

First, query the table with less data , Then execute the query of the table with more data .

2. Index building , A table does not exceed 5 An index

Avoid large memory consumption .

3. Go to the index , Try to meet the leftmost match , Avoid index invalidation

Index failure ：
（1）select *
（2）>、<、!=、between
（3） In front of % Of like Inquire about .
（4）where The field value type is inconsistent with the database , There is an automatic conversion .
（5）or Connect different fields .
（6）or Connect the same field , But there is >、<、!= Non indexed query .（ Only when both left and right queries are indexes .）
（7） Function or operation causes index invalidation .
（8）IS NULL Don't walk index ,IS NOT NULL Go to the index （ Table design ： When not necessary , The field should not be NULL, Set the default empty string or 0）
（9） When querying the range of composite cables , The following index is invalid .
（10）IN Can walk the index , But when IN When the value range of is large, the index will be invalid .

• ep_range_index_dive_limit This parameter affects in Use index or not ,MySQL 5.6 Default 10, MySQL.5.7 Default 200. But our code tends to be controlled in 50 Inside . A Table data is greater than B Table data , choice in Than exists High execution efficiency . contrary ,A Table data is less than B Table data , choice exists More efficient . in Execute subquery first ,exists First perform the appearance .

（11）not in Will invalidate the index , In either case not exists All ratio not in Efficient .

• Use left join or not exists To optimize not in operation .

4. Try to separate hot and cold data

Reduce the number of query table columns , Avoid cold data filtering .

5. Adjust the order of index columns

The uniqueness is better 、 Field is short 、 The frequently used column is placed on the far left of the union index

6. Override indexes are a priority for frequent queries

Query the data together when searching .

7. Avoid implicit conversions of data types

Avoid index invalidation .

8. No use SELECT * You have to use SELECT < Field list > Inquire about

There will be redundant fields , Consume more CPU And network bandwidth Will not be able to use overlay index .

9. Avoid subqueries , The subquery can be optimized to join operation

Subqueries will generate a large number of temporary tables without indexes .

10. Avoid using JOIN Too many tables associated

The more associated tables, the larger the associated cache .

11. Used when there is obviously no duplicate value UNION ALL instead of UNION

UNION ALL No de duplication operation , Speed up query .

12. Big SQL Split , More than one species SQL Batch processing

One SQL You can only use one cpu Calculate , After splitting, multiple cpu Parallel computing .

13. WHERE Functional transformations and calculations for columns are forbidden in clauses

Avoid index invalidation .

14. Use count(*) instead of count( Name )？

Comprehensive performance ：count( Non primary key columns ) < count( Primary key ) < count(1) ≈ count()
count() yes SQL92 The syntax for defining the number of standard statistics lines , It's not about the database ,count(*) The statistical value is NULL The line of , and
count( Name ) This column is not counted NULL Row of values .