Re: [Proposal] Global temporary tables - Mailing list pgsql-hackers

On 29.01.2020 20:37, Pavel Stehule wrote:

st 29. 1. 2020 v 18:21 odesílatel Konstantin Knizhnik <k.knizhnik@postgrespro.ru> napsal:

On 29.01.2020 20:08, Pavel Stehule wrote:

2. Actually I do not propose some completely new approach. I try to
provide behavior with is compatible with regular tables.
If you create index for regular table, then it can be used in all
sessions, right?

I don't understand to this point. Regular tables shares data, shares files. You cannot to separate it. More - you have to uses relatively aggressive locks to be this operation safe.

Nothing from these points are valid for GTT.

GTT shares metadata.
As far as them are not sharing data, then GTT are safer than regular table, aren't them?
"Safer" means that we need less "aggressive" locks for them: we need to protect only metadata, not data itself.

My point is that if we allow other sessions to access created indexes for regular tables, then it will be not more complex to support it for GTT.
Actually "not more complex" in this case means "no extra efforts are needed".

It is hard to say. I see a significant difference. When I do index on regular table, then I don't change a context of other processes. I have to wait for lock, and after I got a lock then other processes waiting.

With GTT, I don't want to wait for others - and other processes should build indexes inside - without expected sequence of operations. Maybe it can have positive effect, but it can have negative effect too. In this case I prefer (in this moment) zero effect on other sessions. So I would to build index in my session and I don't would to wait for other sessions, and if it is possible other sessions doesn't need to interact or react on my action too. It should be independent what is possible. The most simple solution is request on unique usage. I understand so it can be not too practical. Better is allow to usage GTT by other tables, but the changes are invisible in other sessions to session reset. It is minimalistic strategy. It has not benefits for other sessions, but it has not negative impacts too.

Building regular index requires two kinds of lock:
1. You have to lock pg_class to make changes in system catalog.
2. You need to lock heap relation  to pervent concurrent updates while building index.

GTT requires 1)  but not 2).
Once backend inserts information about new index in system catalog, all other sessions may use it. pg_class lock prevents any race condition here.
And building index itself doesn't affect any other backends.

-- 
Konstantin Knizhnik
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