Re: EXPLAIN IndexOnlyScan shows disabled when enable_indexonlyscan=on

On Mon, Oct 28, 2024 at 3:54 PM David Rowley <dgrowleyml(at)gmail(dot)com> wrote:

> On Wed, 23 Oct 2024 at 13:51, David G. Johnston
> <david(dot)g(dot)johnston(at)gmail(dot)com> wrote:
> > Went with a slightly different wording that seems to flow better with
> the xrefs I added between the two options.
>
> - Enables or disables the query planner's use of index-scan plan
> - types. The default is <literal>on</literal>.
> + Enables or disables the query planner's use of all index-scan
> related plan
>
> 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 think
> it's better to explicitly mention index-only-scans to make it clear
> which nodes are affected.
>

I hadn't considered Bitmap Index Scans but I would expect if you do not use
index scans then the ability to produce bitmaps from them would be
precluded.

I could see pointing out, in enable_bitmapscan, that enable_bitmapscan is
effectively disabled (for index inputs) when enable_indexscan is set to
off. Then, in enable_indexscan, add a "see also" to enable_bitmapscan with
a brief reason as well.

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)?

> + types. The default is <literal>on</literal>. The
> index-only-scan plan types
> + can be independently disabled by setting <xref
> linkend="guc-enable-indexonlyscan"/>
> + to <literal>off</literal>.
>
> I wondered if it's better to reference the enable_indexonlyscan GUC
> here rather than document what enable_indexonlyscan does from the
> enable_indexscan docs. Maybe just a "Also see enable_indexonlyscans."
> could be added?
>

I prefer to briefly explain why we advise the reader to go "see also" here.

> - The default is <literal>on</literal>.
> + The default is <literal>on</literal>. However, this setting
> has no effect if
> + <xref linkend="guc-enable-indexscan"/> is set to
> <literal>off</literal>.
>
> 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.

David J.