Re: Index selection bug

Andriy I Pilipenko <bamby(at)marka(dot)net(dot)ua> writes:
> create function func() returns int as 'select 1' language 'sql';

> set enable_seqscan to 'off';

> explain select * from t where f = 1;

> Index Scan using i on t (cost=0.00..2.01 rows=1 width=4)

> explain select * from t where f = func();

> Seq Scan on t (cost=100000000.00..100000001.34 rows=1 width=4)

Not a bug, because you didn't declare the function 'iscachable'.
For all the system knows, func() is like random() and will return a
different result at every row. An indexscan can't be used unless it's
safe to fold the function call down to a constant. See
http://www.postgresql.org/users-lounge/docs/7.0/postgres/sql-createfunction.htm

regards, tom lane