Thread: Cursor rowcount

Short Version:

I've read the idocs and Notes and Googled a fair amount, honest. :-)

What's the most efficient way of determining the number of rows in a
cursor's result set if you really *DO* need that?  (Or, rather, if your
client specifically asked for features that require that.)


Long Version:
I'm not finding any way in the docs of asking a cursor how many rows total
are in the result set, even if I do "move 1000000 in foo", knowing a
priori that 1000000 is far more than could be returned.

Oracle docs seem to have a SQL.%ROWCOUNT which gives the answer, provided
one has moved beyond the last row...  If I'm reading the Oracle docs
right...

Anyway.  I could find nothing similar in PostgreSQL, even though it seems
reasonable, even for a Portal, provided one is willing to do the "move X"
for X sufficiently high -- And, in fact, psql outputs the precise number
of rows when I do that in the psql monitor, so at some level PostgreSQL
"knows" the answer I want, but I can't get that "MOVE XX" output into PHP,
as far as I can tell.  (Can I?)

I suppose I could, in theory, use PHP to fire up psql, but that's not
exactly going to be efficient, much less pleasant. :-)

Using PHP, if it matters.  I guess it does since maybe other APIs have
some way to access that number I want -- psql sure seems to print it out
when one goes over the edge.

Given that the count(*) queries take just as long as the actual
data-retrieval queries, and that some of my queries take too long as it is
(like, a minute for a 4-term full text search)...

I've written and am about to benchmark a binary search using a bunch of
"move X" "fetch 1" "move backward 1" "move backward X" and then using Ye
Olde Low/High guessing game algorithm to find the number of rows, but I'm
hoping for something better from the optimization experts.

Sorry this got a bit long, but I wanted to be clear about where I've been
and gone, rather than leave you guessing. :-)

Hope I didn't miss some obvious solution/documentation "out there"...

<typea@l-i-e.com> writes:
> I'm not finding any way in the docs of asking a cursor how many rows total
> are in the result set, even if I do "move 1000000 in foo", knowing a
> priori that 1000000 is far more than could be returned.

regression=# begin;
BEGIN
regression=# declare c cursor for select * from int8_tbl;
DECLARE CURSOR
regression=# move all in c;
MOVE 5                              <-----------------------
regression=# end;
COMMIT

            regards, tom lane

> <typea@l-i-e.com> writes:
>> I'm not finding any way in the docs of asking a cursor how many rows
>> total are in the result set, even if I do "move 1000000 in foo",
>> knowing a priori that 1000000 is far more than could be returned.
>
> regression=# begin;
> BEGIN
> regression=# declare c cursor for select * from int8_tbl;
> DECLARE CURSOR
> regression=# move all in c;
> MOVE 5                              <-----------------------
> regression=# end;
> COMMIT
>
>             regards, tom lane

Yes, but as noted in my longer version, that number does not seem to "come
through" the PHP API.

I've tried calling just about every function I can in the PHP API in a
test script, and none of them give me that number.

At least, none that I can find...

<typea@l-i-e.com> writes:
> Yes, but as noted in my longer version, that number does not seem to "come
> through" the PHP API.

Perhaps not, but you'd have to ask the PHP folk about it.  This question
surely doesn't belong on pgsql-performance ...

            regards, tom lane

On Mon, 2003-01-13 at 22:19, Tom Lane wrote:
> <typea@l-i-e.com> writes:
> > Yes, but as noted in my longer version, that number does not seem to "come
> > through" the PHP API.
>
> Perhaps not, but you'd have to ask the PHP folk about it.  This question
> surely doesn't belong on pgsql-performance ...

Wellllll, maybe it does, since the /performance/ of a SELECT COUNT(*)
followed by a cursor certainly is lower than getting the count from
a system variable.

But still I agree, the PHP list seems more appropriate...

--
+------------------------------------------------------------+
| Ron Johnson, Jr.     mailto:ron.l.johnson@cox.net          |
| Jefferson, LA  USA   http://members.cox.net/ron.l.johnson  |
|                                                            |
| "Basically, I got on the plane with a bomb. Basically, I   |
|  tried to ignite it. Basically, yeah, I intended to damage |
|  the plane."                                               |
|    RICHARD REID, who tried to blow up American Airlines    |
|                  Flight 63                                 |
+------------------------------------------------------------+