Re: join removal

On Sun, Jul 19, 2009 at 10:56 PM, Tom Lane<tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> I think we want something along the lines of relation_is_distinct_for
> with a list of columns and a list of comparison operators, where the
> first-cut implementation will be to look for matching indexes.
> This will be different from query_is_distinct_for, but it's dealing
> with the same sorts of considerations about whether the operator
> semantics are the right things.

I took at a first crack at coding up an implementation of
relation_is_distinct_for() tonight. Pseudocode:

for each indexoptinfo
{
if (not unique or not predOK or contains expressions)
skip it;
for c = 0 .. ind->ncolumns
{
opid = distinct_col_search(ind->indexkeys[c], colnos, opids);
if (!OidIsValid(opid) || !equality_ops_are_compatible(opid, XXXXXXXX))
break;
}
if (found them all)
return true;
}
return false;

distinct_col_search() is going to return the relevant equality
operator from the argument list, which is ultimately going to come
from the RestrictInfo for the join clause. So I need to see whether
that's compatible with the index, but equality_ops_are_compatible()
wants two equality operators, and what I have is one equality operator
and one operator class.

Maybe it's sufficient to just check whether op_in_opfamily(opid,
ind->opfamily[c]), and skip equality_ops_are_compatible()?

I am having a hard time wrapping my brain around what it means to have
multiple, incompatible notions of equality... any help appreciated!

...Robert