Estimation of SQL execution cost by MySQL query optimizer

summary

mysql in , There are query optimizers , Is specifically used to estimate the cost of query statements , Then choose one mysql The lowest cost scheme is considered to be implemented . Today, let's look at some common queries sql How is the cost of the statement estimated ,mysql What is the basis ?

Estimate the preparation knowledge points

1mysql What data is the estimate based on

a.mysql Since you want to estimate the query cost , Then there must be some calculations , It's like we're going to a place , Want to know how long it will take .
   The formula is distance divided by speed . Different vehicles have different speeds , The total distance of different routes is also different .
  
b.mysql There are data similar to the above .mysql Every table has statistics , Use command SHOW TABLE STATUS You can see that ,
   The command is followed by a parameter called the table name . For example, to perform show table status like '%user%'\G; command , We can see 
   Two fields Rows and Data_length,Rows Number of table records ,Data_length Indicates the number of bytes occupied by data (