Subject: Re: printing results of query to file in different times
To: Mariel Cherkassky <mariel.cherkassky@gmail.com>
References: <634f2eb6-daf6-6314-e6f5-1b531b50a9ec@comcast.net>
 <CA+t6e1kS59uQ7HYSrx-KH2ieFLhJUsnprJ1rGBNPvO1n0YjvbA@mail.gmail.com>
Cc: pgsql-performance@postgresql.org
From: George Neuner <gneuner2@comcast.net>
Message-ID: <2d6aac29-0b20-1cd7-859e-4fc3a47f5477@comcast.net>
Date: Tue, 5 Sep 2017 13:12:24 -0400
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Geez ... I just saw how my last message got mangled.
Trying again.


On 9/5/2017 7:28 AM, Mariel Cherkassky wrote:
> I didn't understand what do you mean by REPEATABLE_READ.

I was referring to transaction isolation levels.  When multiple 
transactions are running concurrently, the DBMS can (or not) prevent 
them from seeing changes made by one another.  Consider 2 transactions A 
and B running concurrently:

   T1:   A reads table X
   T2:   B writes to table X
   T3:   B commits
   T4:   A reads table X again.

Depending on the isolation levels [and the specific query, obviously], A 
may or may not be able to see what changes B made to X.

The default isolation level in Postgresql is READ COMMITTED, which does 
allow transactions to see committed writes made by concurrently running 
transactions.  REPEATABLE READ is a higher level of isolation which 
effectively takes a snapshot of the table(s) when they are 1st read, and 
guarantees that any further reads (e.g., by cursors) of the tables made 
by the transaction continue to see the same results.


My thought was that your loop may be running slowly because the table is 
being changed underneath your cursor.  It may be better to pull the 
results into a temporary table and run your cursor loop over that.

For more information, see:
https://www.postgresql.org/docs/current/static/transaction-iso.html
https://www.postgresql.org/docs/9.6/static/sql-begin.html
https://stackoverflow.com/questions/6274457/set-isolation-level-for-postgresql-stored-procedures 


George


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