On Mon, Oct 28, 2024 at 6:03 PM David Rowley <dgrowle...@gmail.com> wrote:
> We don't seem to be agreeing on much here... :-( > > On Tue, 29 Oct 2024 at 13:30, David G. Johnston > <david.g.johns...@gmail.com> wrote: > > > > On Mon, Oct 28, 2024 at 3:54 PM David Rowley <dgrowle...@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. David J.
