Re: Why a bitmap scan in this case?

On Thu, Dec 19, 2024 at 1:39 PM Greg Sabino Mullane <htamfids(at)gmail(dot)com> wrote:
>>
>> Why wouldn't it do an index (or, really, an index only) scan in this case
>
>
> Well, it did do an index scan (and a bitmap scan is a pretty good solution here), but as to why no indexonly scan, there is probably not enough assurance that it won't have to hit the heap heavily anyway. Try doing a SET enable_bitmapscan=0; and re-run with EXPLAIN ANALYZE. If you see a large number of "Heap Fetches", that could be why. Vacuum the table and try again after doing SET enable_bitmapscan=1;
>

The table is freshly vacuumed. If I disable bitmap scans, it will do
an index only scan, which performs better. For the bitmap heap scan,
it says "Heap Blocks: exact=27393," whereas for the index only scan,
it's "Heap Fetches: 27701."

The row estimate is not good. The query estimates 317919 rows but
there are only 27701. There is some correlation here; if end_on is
null, start_on is a lot more likely to be recent, so maybe extended
statistics would be useful here.

- Jon