Обсуждение: "internal error" triggered by EXISTS()
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
> > 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@illustra.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. -
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@candle.pha.pa.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)
David Gould wrote: > > > > > 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 > > 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). We do. Vadim
Bruce Momjian wrote: > > We only support subqueries in the target list. May have that expaned > for 6.4. Not sure that EXISTS is allowed in target list... Vadim
> Bruce Momjian wrote: > > > > We only support subqueries in the target list. May have that expaned > > for 6.4. > > Not sure that EXISTS is allowed in target list... > > Vadim > I meant to say we only support subqueries in the "WHERE" clause. We do NOT support subqueries in the target list. That may change in 6.4. -- Bruce Momjian | 830 Blythe Avenue maillist@candle.pha.pa.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)