Re: EXPLAIN IndexOnlyScan shows disabled when enable_indexonlyscan=on

On Wed, 23 Oct 2024 at 02:08, Melanie Plageman
<melanieplageman(at)gmail(dot)com> wrote:
> However, it seems like there should be a way to force an index-only
> scan even if it is not the cheapest path. Perhaps I only think this as
> a developer needing to test something. But if enable_indexscan
> disables index-only scan then I don't see how I can force an
> index-only scan when it is not cheapest.

The way to do that is to turn off enable_seqscan and enable_bitmapscan
and ensure your query can support IOS.

The important part to remember here is that when creating the index
paths, the Index Only Scan is always assumed to be better than Index
Scan whenever it's possible to use IOS. There's no opportunity that if
an IOS is possible that you'll get an Index Scan instead. See
check_index_only() and build_index_paths() around where
check_index_only() is used.

David