Cursor estimated row count

An occasionally asked question is "How can I find out how many rows
a cursor will return?" to which the answer is "Fetch them all." But
what about a way to get the planner's estimate? Would anybody find
that useful? Does the code below look close to being correct?

test=> EXPLAIN SELECT * FROM pg_class;
QUERY PLAN
------------------------------------------------------------
Seq Scan on pg_class (cost=0.00..6.88 rows=188 width=163)
(1 row)

test=> BEGIN;
BEGIN
test=> DECLARE curs CURSOR FOR SELECT * FROM pg_class;
DECLARE CURSOR
test=> SELECT cursor_plan_rows('curs');
cursor_plan_rows
------------------
188
(1 row)

#include "postgres.h"
#include "fmgr.h"

#include "nodes/pg_list.h"
#include "nodes/plannodes.h"
#include "utils/portal.h"

Datum cursor_plan_rows(PG_FUNCTION_ARGS);

PG_FUNCTION_INFO_V1(cursor_plan_rows);

Datum
cursor_plan_rows(PG_FUNCTION_ARGS)
{
char *portalname = PG_GETARG_CSTRING(0);
Portal portal;
Plan *plan;

portal = GetPortalByName(portalname);

if (!PortalIsValid(portal)) {
ereport(ERROR,
(errcode(ERRCODE_UNDEFINED_CURSOR),
errmsg("cursor \"%s\" does not exist", portalname)));
}

if (!portal->planTrees) {
ereport(ERROR,
(errcode(ERRCODE_INVALID_CURSOR_STATE),
errmsg("cursor \"%s\" has no plan trees", portalname)));
}

plan = linitial(portal->planTrees);

if (!plan) {
ereport(ERROR,
(errcode(ERRCODE_INVALID_CURSOR_STATE),
errmsg("cursor \"%s\" plan is NULL", portalname)));
}

PG_RETURN_FLOAT8(plan->plan_rows);
}

--
Michael Fuhr

Michael Fuhr <mike(at)fuhr(dot)org> writes:
> An occasionally asked question is "How can I find out how many rows
> a cursor will return?" to which the answer is "Fetch them all." But
> what about a way to get the planner's estimate? Would anybody find
> that useful?

Given how far off it frequently is, I can't believe that any of the
people who ask for the feature would find this a satisfactory answer :-(

regards, tom lane

On Sat, Nov 12, 2005 at 01:50:20PM -0500, Tom Lane wrote:
> Michael Fuhr <mike(at)fuhr(dot)org> writes:
> > An occasionally asked question is "How can I find out how many rows
> > a cursor will return?" to which the answer is "Fetch them all." But
> > what about a way to get the planner's estimate? Would anybody find
> > that useful?
>
> Given how far off it frequently is, I can't believe that any of the
> people who ask for the feature would find this a satisfactory answer :-(

That aside, do you see any problems with the code? Is it at least
returning the right wrong answer? ;-)

--
Michael Fuhr