MySQL Forums
Forum List  »  Performance

Re: MySQL 5.6 update doubled IOs
Posted by: Øystein Grøvlen
Date: April 03, 2013 12:46AM

Hi Rick,

MRR is only beneficial if you need to follow several references from the index into the base table. This may be the case even if LIMIT 1 is specified. E.g., if there are extra conditons that needs to be evaluated so that the matching rows are few and far apart, or if you first need to access and sort many rows before picking the "top" one.

In the case discussed in this thread, the interesting row was always the first row that was accessed. Hence, MRR would not be beneficial in this case. The optimizer could definitely do a better job here, but currently LIMIT clauses are not taken into account when deciding whether to use MRR or not.

Øystein Grøvlen,
Senior Principal Software Engineer,
MySQL Group, Oracle,
Trondheim, Norway

Options: ReplyQuote


Subject
Views
Written By
Posted
1260
March 20, 2013 08:52AM
1168
March 27, 2013 11:44PM
1198
March 28, 2013 07:49PM
1126
April 02, 2013 12:20AM
1173
April 02, 2013 09:57PM
Re: MySQL 5.6 update doubled IOs
1234
April 03, 2013 12:46AM
1146
April 10, 2013 08:31AM


Sorry, you can't reply to this topic. It has been closed.

Content reproduced on this site is the property of the respective copyright holders. It is not reviewed in advance by Oracle and does not necessarily represent the opinion of Oracle or any other party.