Question

I am trying to develop a query that will delete all but the most recently added row in a database. This is based on a Timestamp field, that is stored as a string and a User ID field that is stored as a string..

table.Timestamp -> text field
table.Retrieving_User -> text field

This is the query I have developed. We have around 50K records in this database and it runs very slowly. I hope its not because of the string to date conversion that I’m doing, because this needs to be done.

DELETE 
FROM `table` main
WHERE (main.Retrieving_User, STR_To_DATE( main.Timestamp , '%a %b %d %H:%i:%s CST %Y' )) NOT IN 
    (SELECT  sub.Retrieving_User, MAX( STR_To_DATE( sub.Timestamp , '%a %b %d %H:%i:%s CST %Y' )) 
    FROM `table` sub
    WHERE sub.Retrieving_User = 'userID'
    GROUP BY sub.Retrieving_User )
AND main.Retrieving_User = 'userID'

Does anyone know of a more efficient way of doing what I’m trying to do?

Answer 1

Something like this might work faster because it does not use the IN statement that might be looping again and again over an in memory table. Backup and try

DELETE 
FROM `table` main
WHERE STR_To_DATE( main.Timestamp , '%a %b %d %H:%i:%s CST %Y' )<
  (SELECT  MAX( STR_To_DATE( sub.Timestamp , '%a %b %d %H:%i:%s CST %Y' ) 
   FROM `table` sub
   WHERE sub.Retrieving_User = main.Retrieving_User )
AND main.Retrieving_User = 'userID'