MySQL: why 'explain' command yields different results on same SQL statement?

问题

I migrated a MySQL database from one environment to another, and discovered that after migrating, a particular query runs extremely slower than it was.

I was investigating the statement with 'explain' keyword (below) and found that the 'explain' command gives different output, on different servers.

explain select distinct j.job,f.path,p.path  
from fixes f
join  jobs j on  f.job=j.id 
join  paths p  on p.id =f.path  
where p.path like '//depot1/Dev\-trunk/%' ;

For the original one, it gives:

    1   SIMPLE  p   range   PRIMARY,path    path    752     **19**  Using where; Using index; Using temporary
    1   SIMPLE  f   ref path,job    path    8   p.p.id  **6448356** 
    1   SIMPLE  j   eq_ref  PRIMARY PRIMARY 8   p.f.job **1**   

And the same statement yields different output on the new environment:

    1   SIMPLE  j   index   PRIMARY job 62      **68486**   Using index; Using temporary
    1   SIMPLE  f   ref path,job    job 8   p.j.id  **812** 
    1   SIMPLE  p   eq_ref  PRIMARY,path    PRIMARY 8   p.f.path    **1**   Using where

回答1:

FYI, it turns out that when migrating the database, there are some stats used for the engine optimizer to decide the best strategy to e.g. join tables, that apparently was not copied.

The solution is to run ANALYZE on each of the tables to 'rebuild' such information. So after doing the ANALYZE to each of the table, and run the explain command again, the output agrees and the query speed increases dramatically.

来源：https://stackoverflow.com/questions/13390729/mysql-why-explain-command-yields-different-results-on-same-sql-statement