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Re: [HACKERS] "internal error" triggered by EXISTS()

From: Bruce Momjian <maillist(at)candle(dot)pha(dot)pa(dot)us>
To: tgl(at)sss(dot)pgh(dot)pa(dot)us (Tom Lane)
Cc: pgsql-hackers(at)hub(dot)org
Subject: Re: [HACKERS] "internal error" triggered by EXISTS()
Date: 1998-07-15 02:45:30
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
We only support subqueries in the target list.  May have that expaned
for 6.4.

> I tried the following to find out whether a table has any records
> with field1 < X (for a constant X):
> tgl=> SELECT EXISTS(SELECT * FROM table WHERE field1 < X);
> ERROR:  internal error: do not know how to transform targetlist
> Is this a bug?  (I'm using development sources from yesterday.)
> Am I using EXISTS() incorrectly?  The examples I've been able to find
> only show it as a part of a WHERE clause.
> If it did work, would it be any faster than a table scan?  The code
> I was hoping to replace is like this:
> 	SELECT COUNT(field1) WHERE field1 < X;
> 	// test whether result > 0
> Since aggregates aren't optimized very well, this ends up reading
> much or all of the table, even if there is an index for field1.
> I was hoping EXISTS() might be smarter...
> 			regards, tom lane

Bruce Momjian                          |  830 Blythe Avenue
maillist(at)candle(dot)pha(dot)pa(dot)us              |  Drexel Hill, Pennsylvania 19026
  +  If your life is a hard drive,     |  (610) 353-9879(w)
  +  Christ can be your backup.        |  (610) 853-3000(h)

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