Re: [Proposal] Global temporary tables

On Fri, Oct 25, 2019 at 12:22 PM Konstantin Knizhnik
<k.knizhnik@postgrespro.ru> wrote:
> On 25.10.2019 18:01, Robert Haas wrote:
> > On Fri, Oct 11, 2019 at 9:50 AM Konstantin Knizhnik
> > <k.knizhnik@postgrespro.ru> wrote:
> >> Just to clarify.
> >> I have now proposed several different solutions for GTT:
> >>
> >> Shared vs. private buffers for GTT:
> >> 1. Private buffers. This is least invasive patch, requiring no changes in relfilenodes.
> >> 2. Shared buffers. Requires changing relfilenode but supports parallel query execution for GTT.
> > I vote for #1. I think parallel query for temp objects may be a
> > desirable feature, but I don't think it should be the job of a patch
> > implementing GTTs to make it happen. In fact, I think it would be an
> > actively bad idea, because I suspect that if we do eventually support
> > temp relations for parallel query, we're going to want a solution that
> > is shared between regular temp tables and global temp tables, not
> > separate solutions for each.
>
> Sorry, may be I do not not understand you.
> It seems to me that there is only one thing preventing usage of
> temporary tables in parallel plans: private buffers.
> If global temporary tables are accessed as normal tables though shared
> buffers then them can be used in parallel queries
> and no extra support is required for it.
> At least I have checked that parallel queries are correctly worked for
> my implementation of GTT with shared buffers.
> So I do not understand about which "separate solutions" you are talking
> about.
>
> I can agree that private buffers may be  good starting point for GTT
> implementation, because it is less invasive and GTT access speed is
> exactly the same as of normal temp tables.
> But I do not understand your argument why it is "actively bad idea".

Well, it sounds like you're talking about ending up in a situation
where local temporary tables are still in private buffers, but global
temporary table data is in shared buffers. I think that would be
inconsistent. And it would mean that when somebody wanted to make
local temporary tables accessible in parallel query, they'd have to
write a patch for that.  In other words, I don't support dividing the
patches like this:

Patch #1: Support global temporary tables + allow global temporary
tables to used by parallel query
Patch #2: Allow local temporary tables to be used by parallel query

I support dividing them like this:

Patch #1: Support global temporary tables
Patch #2: Allow (all kinds of) temporary tables to be used by parallel query

The second division looks a lot cleaner to me, although as always I
might be missing something.

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