Re: [Proposal] Global temporary tables

From: Konstantin Knizhnik <k(dot)knizhnik(at)postgrespro(dot)ru>
To: Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, 曾文旌(义从) <wenjing(dot)zwj(at)alibaba-inc(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>, 蔡松露(子嘉) <zijia(at)taobao(dot)com>, "Cai, Le" <le(dot)cai(at)alibaba-inc(dot)com>, 萧少聪(铁庵) <shaocong(dot)xsc(at)alibaba-inc(dot)com>
Subject: Re: [Proposal] Global temporary tables
Date: 2020-01-24 09:43:10
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On 24.01.2020 12:09, Pavel Stehule wrote:
> pá 24. 1. 2020 v 9:39 odesílatel Konstantin Knizhnik
> <k(dot)knizhnik(at)postgrespro(dot)ru <mailto:k(dot)knizhnik(at)postgrespro(dot)ru>> napsal:
> On 23.01.2020 23:47, Robert Haas wrote:
> > On Sat, Jan 11, 2020 at 8:51 PM Tomas Vondra
> > <tomas(dot)vondra(at)2ndquadrant(dot)com
> <mailto:tomas(dot)vondra(at)2ndquadrant(dot)com>> wrote:
> >> I proposed just ignoring those new indexes because it seems
> much simpler
> >> than alternative solutions that I can think of, and it's not
> like those
> >> other solutions don't have other issues.
> > +1.
> >
> >> For example, I've looked at the "on demand" building as
> implemented in
> >> global_private_temp-8.patch, I kinda doubt adding a bunch of
> index build
> >> calls into various places in index code seems somewht suspicious.
> > +1. I can't imagine that's a safe or sane thing to do.
> >
> As far as you know there are two versions of GTT implementations now.
> And we are going to merge them into single patch.
> But there are some principle question concerning provided
> functionality
> which has to be be discussed:
> should we prohibit DDL on GTT if there are more than one sessions
> using
> it. It includes creation/dropping indexes, dropping table,
> altering table...
> If the answer is "yes", then the question whether to populate new
> indexes with data is no relevant at all, because such situation
> will not
> be possible.
> But in this case we will get incompatible behavior with normal
> (permanent) tables and it seems to be very inconvenient from DBA
> point
> of view:
> it will be necessary to enforce all clients to close their
> sessions to
> perform some DDL manipulations with GTT.
> Some DDLs will be very difficult to implement if GTT is used by more
> than one backend, for example altering table schema.
> My current solution is to allow creation/droping index on GTT and
> dropping table itself, while prohibit alter schema at all for GTT.
> Wenjing's approach is to prohibit any DDL if GTT is used by more than
> one backend.
> When I create index on GTT in one session, then I don't expect
> creating same index in all other sessions that uses same GTT.
> But I can imagine to creating index on GTT enforces index in current
> session, and for other sessions this index will be invalid to end of
> session.

So there are three possible alternatives:

1. Prohibit index creation of GTT when it used by more than once session.
2. Create index and populate them with data in all sessions using this GTT.
3. Create index only in current session and do not allow to use it in
all other sessions already using this GTT (but definitely allow to use
it in new sessions).

1 is Wenjing's approach, 2 - is my approach, 3 - is your suggestion :)

I can construct the following table with pro/cons of each approach:

Compatibility with normal table
User (DBA) friendly
Complexity of implementation
1: requires restart of all sessions to perform operation
2: requires global cache of GTT
3/: /no man, no problem
3: if index is created then it is actually needed, isn't it? 1: use
existed functionality to create index
2: if alter schema is prohibited
2: requires restart of all sessions to use created index
3: requires some mechanism for prohibiting index created after first
session access to GTT
1: can perform DDL but do no see effect of it

Konstantin Knizhnik
Postgres Professional:
The Russian Postgres Company

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