Re: Why a bitmap scan in this case?

On 12/20/24 09:16, Frédéric Yhuel wrote:
> 
> 
> On 12/19/24 20:09, Jon Zeppieri wrote:
>> 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."
> 
> So you have 100% heap fetches. Are you sure that your table is freshly 
> vacuumed? Please note that VACUUM FULL doesn't create the visibility 
> map, so you still have to run a plain VACUUM for this.
> 
> 

And if you still have heap fetches, you could try VACUUM (INDEX_CLEANUP 
TRUE), so that it doesn't bypass index vacuuming.

(In case the pages that contain theses 27701 tuples amount to less than 
2% of the total number of pages of the table).

Nice explanation here: 

https://www.cybertec-postgresql.com/en/making-the-postgresql-visibility-map-visible/#vacuum-doesnt-update-the-visibility-map-as-it-should