On Tue, 29 Oct 2024 at 13:30, David G. Johnston <david.g.johnston@gmail.com> wrote: > > On Mon, Oct 28, 2024 at 3:54 PM David Rowley <dgrowleyml@gmail.com> wrote: >> I'm concerned about the wording "all index-scan related". It's not >> that clear if that would include Bitmap Index Scans or not. > > > That was partially the point of writing "all" there - absent other information, and seeing how index-only scans were treated, I presumed it was indeed actually or effectively a switch for all. If it is not it should be made clear which node types with the word index in them are not affected.
I'm very much against mentioning which things are *not* affected by settings. It doesn't seem like a very sustainable way to write documentation.
The documentation presently uses the term "index-scan related" and it is unclear what exactly that is supposed to cover. My addition of the word "all" doesn't materially change this other than for certain covering the "index-only-scan related" nodes that gets clarified and is cross-referenced. If you are uncertain whether adding "all" is meant to cover Bitmap Index Scans then your uncertainty still exists in the current wording. I just added "all" to be explicit about that fact, or at least that is what I thought I did.
For me, the answer to "are bitmap index scans disabled" by setting enable_indexscans to off is "yes" and does not require explanation. If the real answer is "no" then please propose a change that can disabuse me of my belief.
> Is there a listing of all node types produced by PostgreSQL (with the explain output naming) along with which ones are affected by which enable_* knobs (possibly multiple for something like Bitmap Index Scan)?
No. We purposefully do our best not to document executor nodes. The enable_* GUCs is one place where it's hard to avoid.
For education, mainly mine, not to add to the documentation; though our lack of detail here for what are user-facing things is IMO unfortunate.
>> >> Could we just add "The <xref linkend="guc-enable-indexscan"/> setting >> must also be enabled to have the query planner consider >> index-only-scans"? > > > I'd like to stick with a conjunction there but agree the "must be enabled" wording is preferrable, avoiding the double-negative. > > "The default is on, but the <xref> setting must also be enabled." > > The 'to have the...' part seems to just be redundant.
I think it's confusing to include this as part of the mention of what the default value is. The default value and enable_indexscans being the master switch aren't at all related.
Fair point. I'm good with your proposed change here.
