On 29.01.2020 20:37, Pavel Stehule wrote:
st 29. 1. 2020 v 18:21 odesílatel Konstantin Knizhnik
<k.knizh...@postgrespro.ru <mailto:k.knizh...@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
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company
