Re: Why isn't an index scan being used?

On Tue, Feb 19, 2019, 8:00 PM Andrew Gierth <andrew(at)tao11(dot)riddles(dot)org(dot)uk
wrote:

> >>>>> "Abi" == Abi Noda <a(at)abinoda(dot)com> writes:
>
> Abi> However, when I index the closed column, a bitmap scan is used
> Abi> instead of an index scan, with slightly slower performance. Why
> Abi> isn't an index scan being used, given that the exact same number
> Abi> of rows are at play as in my query on the state column?
>
> Most likely difference is the correlation estimate for the conditions.
> The cost of an index scan includes a factor based on how well correlated
> the physical position of rows is with the index order, because this
> affects the number of random seeks in the scan. But for nulls this
> estimate cannot be performed, and bitmapscan is cheaper than plain
> indexscan on poorly correlated data.
>

Does this imply that the optimizer would always prefer the bitmapscan
rather than index scan even if random page cost = 1, aka sequential cost,
when the correlation is unknown like a null? Or only when it thinks random
access is more expensive by some significant factor?

> --
> Andrew (irc:RhodiumToad)
>
>