Differential D10895
Reverse meaning of task priority column Authored by epriestley on Nov 23 2014, 7:34 PM. Tags None Referenced Files
Subscribers
Details
Ref T6615. Mixing ASC and DESC ordering on a multipart key makes it dramatically less effective (or perhaps totally ineffective). Reverse the meaning of the priority column so it goes in the same direction as the id column (both ascending, lower values execute sooner).
Diff Detail
Event TimelineComment Actions Yeah -- my mental model of indexes is that MySQL just builds some kind of fancy magical tree inside itself, so it followed that after it walked down the "leaseOwner = ?" part of the tree to get to the "leaseOwner = null" node it would be looking at a "priority = ?" subtree, and it could arbitrarily start at the left or right side to do the order by bit, and once it had walked down that subtree to the left-most or right-most node it would have the same opportunity to start at the left or right side of the "id = ?" sub-sub-tree at each priority node. Clearly that's not what it actually does given that priority DESC, id ASC is >1500x slower than priority ASC, id ASC with 1.2M rows. Comment Actions My mental model is "you think you are in control, but who is in control of chaos? Better run EXPLAIN again." |