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Re: Adding missing FROM clause - thanks or no thanks?

From: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>
To: siracusa(at)mindspring(dot)com
Cc: Postgres General <pgsql-general(at)postgresql(dot)org>
Subject: Re: Adding missing FROM clause - thanks or no thanks?
Date: 2004-01-27 18:34:03
Message-ID: 200401271834.i0RIY3Q09540@candle.pha.pa.us (view raw or flat)
Thread:
Lists: pgsql-general
John Siracusa wrote:
> I recently had a problem where a bunch of postgres backends were taking up
> huge amounts of CPU time.  I found a bunch of log messages like this:
> 
> NOTICE:  adding missing FROM-clause entry for table "b"
> 
> Which I eventually tracked to an improperly constructed query that reduced
> to something like this:
> 
> select a1, a2 FROM a WHERE a1 = '...' AND b.b1 = '...';
> 
> Table b is pretty big and the conditions were such that it was (apparently)
> doing a join of every row in a with every possible row of b.  That's
> certainly now what I wanted.
> 
> Anyway, I fixed the query construction bug, but I have a question: is this
> "adding missing FROM-clause entry" behavior part of the SQL standard(s) or a
> "feature" of Postgres?  Prior to seeing this, I'd have expected a query like
> the above to cause a fatal error in the parsing stage.  Put another way, can
> someone give me an example of when this behavior is useful or desirable?

It is a PostgreSQL extension and can be turned off with a
postgresql.conf variable or via SET:

	add_missing_from = false

-- 
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman(at)candle(dot)pha(dot)pa(dot)us               |  (610) 359-1001
  +  If your life is a hard drive,     |  13 Roberts Road
  +  Christ can be your backup.        |  Newtown Square, Pennsylvania 19073

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Subject: Re: Adding missing FROM clause - thanks or no thanks?
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