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List:       postgresql-general
Subject:    [GENERAL] Re: [SQL] Reading and writing at the same time
From:       Herouth Maoz <herouth () oumail ! openu ! ac ! il>
Date:       1998-10-25 9:49:03
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At 20:57 +0200 on 23/10/98, G. Anthony Reina wrote:

> I know that Postgres has table-level locking so that once a write is
> being performed on a table, no other reads or writes can be performed
> until that write finishes.
>
> I have a C program that performs several updates and inserts to a table.
> I found that I am able, despite any table locking, to use the front-end
> psql program at the same time to select data from the same table that is
> being written to. So in essence, I am able to have one program that is
> writing while at the same time another program reads (without running
> into aborted backends).

Are you sure you interpret what you see correctly? The backend in fact
locks out any readers while updates take place. I have a hunch that you did
not declare a transaction in your C program, and thus, each of the updates
ani inserts in it is a separate transaction.

When that happens, the reads (from your psql frontend) and the writes (from
your C) are interspersed, because the lock only holds while a transaction
is active. The next update in your C program may wait until the read in
PSQL has been done.

Herouth

--
Herouth Maoz, Internet developer.
Open University of Israel - Telem project
http://telem.openu.ac.il/~herutma

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