I happen to be reading about this last night except I was reading about it in the PostgreSQL manual.
In the case of PostgreSQL, the planner searches for the least expensive plan and uses different plans based upon the "selectivity" of the constraint. The PostgreSQL doc may give you a clue what MySQL is doing... maybe not. http://www.postgresql.org/docs/8.3/static/using-explain.html HTH, Perry -- Posted via http://www.ruby-forum.com/. --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Ruby on Rails: Talk" group. To post to this group, send email to [email protected] To unsubscribe from this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/rubyonrails-talk?hl=en -~----------~----~----~----~------~----~------~--~---
