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RE: System indexes are never unique indexes( was RE: [HACKERS] mdnblocksis

From: "Hiroshi Inoue" <Inoue(at)tpf(dot)co(dot)jp>
To: "Bruce Momjian" <maillist(at)candle(dot)pha(dot)pa(dot)us>, "Jan Wieck" <wieck(at)debis(dot)com>
Cc: <pgsql-hackers(at)postgresql(dot)org>
Subject: RE: System indexes are never unique indexes( was RE: [HACKERS] mdnblocksis
Date: 1999-10-27 00:52:55
Message-ID: 000201bf2015$9ae0f7a0$ (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
> > > Not sure.  I don't remember which ones.  I can take a look when I add
> > > more indexes for 7.0.
> >
> >     Don't  remember  if  really  or  what,  but wasn't there some
> >     problem with cached  system  relations,  unique  indices  and
> >     concurrency?
> >
> I don't remember anything about that.

I don't know old PostgreSQL at all.
Only one thing I could suppose is the following.

Before MVCC it was unnecessary to read dirty(uncommited) tuples
to check uniqueness because a table level exclusive lock was acquired
automatically. As for user tuples,the consistency was perserved because
the lock was held until transaction end.  As for system tuples,the
could be broken if the lock was a short term lock.

After MVCC,dirty(uncommitted) tuples are taken into account to check
uniqueness and  any lock is no longer needed.

AFAIK,there are no other means to check(lock ?)  (logically) non-existent
rows now(Referencial Integrity would provide the second one).
So probably PostgreSQL couldn't guarantee the uniquness of system
tuples in many cases.

Anyway,I want to change the implementation of mdcreate() to reuse
existent files but the uniqueness of table name is preserved by the
current implementation narrowly.

First of all,I would change pg_type,pg_class.
It's OK ?


Hiroshi Inoue

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