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Message-ID: <20060504181056.14509.qmail@web52510.mail.yahoo.com>
Date: Thu, 4 May 2006 11:10:56 -0700 (PDT)
From: Ash Grove <ash_grv7@yahoo.com>
Subject: is an explicit lock necessary?
To: pgsql-sql@postgresql.org
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
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Hi,

Does beginning a transaction put locks on the tables
queried within the transaction?

In the example below, is #2 necessary? My thought was
that I would need to use an explicit lock to make sure
that the sequence value I'm selecting in #4 is the
same one that is generated from #3. I'm worried about
another instance of the application doing an insert on
table1 between #3 and #4.

1) From my app, I turn off autocommit.
2) I lock table1 in access exclusive mode
3) I do an insert into table1 which generates a
primary key via nextval on sequence1
4) I grab grab the primary key value via currval on
sequence1
5) I do an insert on table2 which includes table1's
primary key so I can join the records later.
6) I manually commit

Thanks!
Ash

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