Skip site navigation (1) Skip section navigation (2)

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

From: dg(at)illustra(dot)com (David Gould)
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: 1998-07-14 22:50:19
Message-ID: 9807142250.AA14382@hawk.illustra.com (view raw or flat)
Thread:
Lists: pgsql-hackers
> 
> 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
> 

Should have given a syntax error probably. But you might try:

select 1 where exists (select...);

Should be faster if and only if we are doing the existential query
optimization trick (stop on the first qualifying row).

-dg


David Gould            dg(at)illustra(dot)com           510.628.3783 or 510.305.9468 
Informix Software  (No, really)         300 Lakeside Drive  Oakland, CA 94612
 - If simplicity worked, the world would be overrun with insects. -

In response to

Responses

pgsql-hackers by date

Next:From: Vadim MikheevDate: 1998-07-15 02:29:31
Subject: Re: [HACKERS] SEQUENCES and COPY FROM
Previous:From: Tom LaneDate: 1998-07-14 21:31:36
Subject: "internal error" triggered by EXISTS()

Privacy Policy | About PostgreSQL
Copyright © 1996-2014 The PostgreSQL Global Development Group