postgres=# create table t1 (a int, b int);

CREATE TABLE

postgres=# insert into t1 values (generate_series(1,1000),1);

INSERT 0 1000

and so on...

postgres=# insert into t1 values (generate_series(4001,5000),5);

INSERT 0 1000

postgres=# analyze t1;

ANALYZE

postgres=# explain select * from t1 where b=5;

QUERY PLAN

------------------------------------------------------

Seq Scan on t1 (cost=0.00..85.50 rows=1000 width=8)

Filter: (b = 5)

(2 rows)

postgres=# explain select * from t1 where b=6;

QUERY PLAN

---------------------------------------------------

Seq Scan on t1 (cost=0.00..85.50 rows=1 width=8)

Filter: (b = 6)

(2 rows)

do you know if there is a formula for such a cardinality based on the size of the table or is it always one row?

can you point to the source code location?

Thanks,

Igor