On Wed, 20 Feb 2019 at 13:11, Abi Noda <a...@abinoda.com> wrote: > However, when I index the closed column, a bitmap scan is used instead of an > index scan, with slightly slower performance. Why isn't an index scan being > used, given that the exact same number of rows are at play as in my query on > the state column?
That's down to the planner's cost estimates. Likely it thinks that either doing a bitmap scan is cheaper, or close enough that it does not matter. > How do I index closed in a way where an index scan is used? The costing does account for the size of the index. If the "closed_index" index is large than the "state_index", then doing an Index scan on "closed_index" is going to be costed higher. Most of this likely boils down to random_page_cost being a guess. You may want to check your effective_cache_size is set to something like 75% of the machine's memory, and/or tweak random page cost down, if it's set to the standard 4 setting. modern SSDs are pretty fast at random reads. HDDs, not so much. -- David Rowley http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services
