On Wed, 23 Oct 2024 at 13:51, David G. Johnston
<david.g.johnston@gmail.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.
