How to optimize MySQL

 

Now there is 3000W Left and right data , Perform the following tests .

1. According to the leftmost matching principle , Do not use in the first index column >

, It must be a full meter scan  

2. Coverage index optimization ： How to optimize the above situation ？ 

3.in and or When the amount of table data is large, the index will be used , When there are few table records, full table scanning will be selected

3000W Your watch didn't go through the full scan , go Range

 

  Only 5 Bar record ： The efficiency of walking full table scanning is higher than that of walking index

Index push down （Index Condition Pushdown,ICP）
effect ： Index push down will reduce the number of table returns

SELECT * FROM employees WHERE name like 'LiLei%'AND age = 22 AND position ='manager'
 

Pushing forward without index ： First query like And then return to the table , Finally, according to the results of the back table age and position Compare , Efficiency and efficiency

There is the practice of pushing down the index ： First like and age,position Compare together as a condition , Then go back to the table , It can effectively reduce the times of returning to the table

As follows ：

Why? >a Walk the whole watch ,>zzz Search by range ？ 

trace Tools to analyze execution costs . It is found that the time of full table scanning is less than that of indexing , Gu Zou full table scanning
 

order by The optimization of the ：

 

 

Look at the first situation ： key_len=74, Only name The index is gone ,position No index ,extra=Using index condition, It means that it is still sorted in memory , Not disk sort

The situation is the same as above ,key_len=78

  Look at this situation ： Do not follow the leftmost match , therefore using filesort, File sorting

  Do not follow the leftmost match, but also follow the file sorting

 

Look at this situation ： The ascending and descending order is inconsistent , Also go through file sorting

 

  One way two-way sorting ：

Using filesort Detailed explanation of document sorting principle
filesort Sort files
One way sorting ： Is to retrieve all fields of the qualified row at one time , And then in sort buffer In order
Two way sorting （ Also called table sorting mode ）： First, the corresponding sorting fields and rows can be directly located according to the corresponding conditions
Lines of data ID, And then in sort buffer In order , After sorting, you need to retrieve other required fields again ;
Determine which sort mode to use .

If The total length of the field is less than max_length_for_sort_data , So use One way sort mode ;
If The total length of the field is greater than max_length_for_sort_data , So use Two way sorting module ∙ type .