RE: [HACKERS] optimizer pruning problem

> 
> > When the join is between attnos < 0 (such as oids), the selectivity is
> > estimated as 0.5 (leading to very bad size estimates), since this code
> > in function compute_clause_selec (clausesel.c):
> 
> >   if (relid1 > 0 && relid2 > 0 && attno1 > 0 && attno2 > 0)
> >     ...
> >   else
> >     s1 = (Cost) (0.5);
> 
> > So what is the aim of the last two and conditions?
> 
> That's a bug, I guess.  -1 is used to signal "couldn't find the
> attribute", but there's no real need to check *both* relid and attno
> to determine that.  It should consider positive relid and negative
> attno to be valid.
>
> Since vacuum doesn't record statistics for the system attributes,
> there probably also needs to be a hack in the code that looks in
> pg_statistic so that it will produce reasonable estimates.   We
> should assume that OID has perfect disbursion, for sure.  I don't
> know if we can assume anything much about the other sys attributes...
>

CTID has perfect disbursion too. The selectivity is necessary
in order to compute rows of scan using TIDs,though we couldn't
use WHERE restriction on ctid now.

Regards.

Hiroshi Inoue
Inoue@tpf.co.jp