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Re: The mystery of concurrent access

From: Jason Earl <jdearl(at)yahoo(dot)com>
To: "P(dot)V(dot) Subramanian" <pvsmian(at)hotmail(dot)com>, pgsql-novice(at)postgresql(dot)org
Subject: Re: The mystery of concurrent access
Date: 2001-10-05 21:52:05
Message-ID: 20011005215205.92617.qmail@web10005.mail.yahoo.com (view raw or flat)
Thread:
Lists: pgsql-novice
What you really need to do is read the following
chapter from the PostgreSQL User's guide:

http://www.postgresql.org/idocs/index.php?mvcc.html

It explains all you need to know about how PostgreSQL
handles concurrent access.  Pay special attention to
section 9.3 as Read Committed Isolation Level is the
default for PostgreSQL.

--- "P.V. Subramanian" <pvsmian(at)hotmail(dot)com> wrote:
> Hi all
> 
> I thought enough experimentation would reveal this
> mystery, but its very 
> confusing, so I'll ask.

Yes, I imagine that experimenting would be very
confusing.  Fortunately the documentation is very
good.

> When 2 processes/people are simultaneously working
> on a table
> 
> * does Postgres automatically enforce some sort of
> locking?

Yes.  And it does so at the row level.  It's very
cool.

> * do the changes made by process A become visible to
> process B immediately 
> (of course, the result of a SELECT wouldn't change
> dynamically, but suppose 
> process B did another SELECT, would it access
> process A's records?)

It depends on the isolation level, and on what you
mean by "immediately."  In the default isolation level
if transaction A starts and then transaction B starts
transaction A cannot see any of the changes from
transaction B until transaction B is committed.

Bruce Momjian explains it much better than I ever
could, you should also take a look at the relevant
chapter of his book:

http://www.ca.postgresql.org/docs/aw_pgsql_book/node98.html

> Thanks
> 
> PVS


I hope this is helpful,
Jason


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