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Incorrect behaviour when using a GiST index on points

From: Alexander Korotkov <aekorotkov(at)gmail(dot)com>
To: pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Incorrect behaviour when using a GiST index on points
Date: 2012-02-16 07:43:11
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers

Oleg and me found an incorrect behaviour when using a GiST index on points.
It's quite easy to reproduce.

test=# create table test as values (0,0.0000001);
test=# insert into test values (0,0.0000001);
test=# select * from test where point(x,y) <@ box(point(0,0),point(0,0));
 x | y
(0 rows)

test=# create index gist_test on test using gist (point(x,y));
test=# set enable_seqscan = off;
test=# select * from test where point(x,y) <@ box(point(0,0),point(0,0));
 x |   y
 0 | 1e-07
(1 row)

So, "point <@ box" operator don't thinks (0, 1e-07) to be equal to (0, 0),
while GiST index does. "point <@ box" operator uses on_ob function which
uses comparison operators of float numbers.

Point   *pt = PG_GETARG_POINT_P(0);
 BOX   *box = PG_GETARG_BOX_P(1);

PG_RETURN_BOOL(pt->x <= box->high.x && pt->x >= box->low.x &&
   pt->y <= box->high.y && pt->y >= box->low.y);

GiST consistent method uses box_contained function, which uses FP* macros
for float comparison.

/* box_contained - is box1 contained by box2?
BOX   *box1 = PG_GETARG_BOX_P(0);
 BOX   *box2 = PG_GETARG_BOX_P(1);

PG_RETURN_BOOL(FPle(box1->high.x, box2->high.x) &&
   FPge(box1->low.x, box2->low.x) &&
   FPle(box1->high.y, box2->high.y) &&
   FPge(box1->low.y, box2->low.y));

Correspondingly, FP* compares numbers with some error.

#define EPSILON 1.0E-06

#ifdef EPSILON
#define FPzero(A) (fabs(A) <= EPSILON)
#define FPeq(A,B) (fabs((A) - (B)) <= EPSILON)
#define FPne(A,B) (fabs((A) - (B)) > EPSILON)
#define FPlt(A,B) ((B) - (A) > EPSILON)
#define FPle(A,B) ((A) - (B) <= EPSILON)
#define FPgt(A,B) ((A) - (B) > EPSILON)
#define FPge(A,B) ((B) - (A) <= EPSILON)
#define FPzero(A) ((A) == 0)
#define FPeq(A,B) ((A) == (B))
#define FPne(A,B) ((A) != (B))
#define FPlt(A,B) ((A) < (B))
#define FPle(A,B) ((A) <= (B))
#define FPgt(A,B) ((A) > (B))
#define FPge(A,B) ((A) >= (B))

Described differences leads to incorrect behaviour of GiST index.
The question is: what is correct way to fix it? Should on_pb also use FP*
or consistent method should behave like on_pb?

With best regards,
Alexander Korotkov.


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