Re: index-only scans

On Tue, Oct 11, 2011 at 12:19 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> I wrote:
>> I have mostly-working code for approach #3, but I haven't tried to make
>> EXPLAIN work yet.  While looking at that I realized that there's a
>> pretty good argument for adding the above-mentioned explicit TargetEntry
>> list representing the index columns to index-only plan nodes.  Namely,
>> that if we don't do it, EXPLAIN will have to go to the catalogs to find
>> out what's in that index, and this will fall down for "hypothetical"
>> indexes injected into the planner by index advisors.  We could imagine
>> adding some more hooks to let the advisor inject bogus catalog data at
>> EXPLAIN time, but on the whole it seems easier and less fragile to just
>> have the planner include a data structure it has to build anyway into
>> the finished plan.
>
>> The need for this additional node list field also sways me in a
>> direction that I'd previously been on the fence about, namely that
>> I think index-only scans need to be their own independent plan node type
>> instead of sharing a node type with regular indexscans.  It's just too
>> weird that a simple boolean indexonly property would mean completely
>> different contents/interpretation of the tlist and quals.
>
> Attached is a draft patch for this.  It needs some more review before
> committing, but it does pass regression tests now.
>
> One point worth commenting on is that I chose to rename the OUTER and
> INNER symbols for special varnos to OUTER_VAR and INNER_VAR, along with
> adding a new special varno INDEX_VAR.  It's bothered me for some time
> that those macro names were way too generic/susceptible to collision;
> and since I had to look at all the uses anyway to see if the INDEX case
> needed to be added, this seemed like a good time to rename them.

+1 for that renaming, for sure.

Have you given any thought to what would be required to support
index-only scans on non-btree indexes - particularly, GIST? ISTM we
might have had a thought that some GIST opclasses but not others would
be able to regurgitate tuples, or maybe even that it might vary from
index tuple to index tuple. But that discussion was a long time ago,
and my memory is fuzzy.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company