MySQL 45 lecture learning notes (x) force index

One . Optimizer logic

      Choose the index yes The job of the optimizer

The purpose of the optimizer's index selection ：

• Find an optimal implementation plan , Execute the statement with the least cost
• The number of execution lines is one of the factors that affect the execution cost .
• The fewer lines to scan , It means that the fewer times you access disk data  , Consume CPU The less resources .   

The optimizer also combines Whether to use temporary table , Is it sorted? Other factors .

Two . How to judge the number of scanning lines    

show index Method View index cardinality

MySQL Get the cardinality of the index

Sampling statistics ：

• Take out the whole table and make statistics row by row , Although accurate results can be obtained, the cost is very high , Can only choose “ Sampling statistics ”
• InnoDB Default choice N Data pages , Count the different values on these pages , Get an average of , Then multiply by the number of pages in this index , Get index base .
• When the changed data exceeds 1/M It will automatically trigger an index statistics .

innodb_stats_persistent

Count disk or memory

• Set up on When , Statistics persistent storage
• Set up off When , Statistics of information in memory

analyze table t    It can be used to count index information

Index selection exceptions and handling

Most of the time, the optimizer can find the correct index , But occasionally you will encounter the above situation ：

• use force index Force an index

shortcoming ：

• Programmers don't like to use force index, Index name changed , This statement needs to be changed .             

advantage ：

•   Use force index The main problem is the timeliness of changes . Generally, when there is a problem online , Will modify SQL sentence , add force index. But after modification, it has to be tested and released . For production systems , This process is not agile enough .

2. Modify the statement , guide MySQL Use the index we expect

3. You can create a more suitable index , To provide the optimizer with choices , Or delete the misused index