Re: BUG #15334: Partition elimination not working as expected whenusing enum as partition key

>>>>> "Damir" == Damir Ciganović-Janković <damir.ciganovic.jankovic@gmail.com> writes:

 >> It's the Postgres EXPLAIN's code for expression deparsing that puts
 >> the '::cstring' there. I have to wonder why it couldn't just skip
 >> adding that and print it as simply 'A'::ab. However, I'm not sure if
 >> answer to that question is related to why partition pruning doesn't
 >> occur.

I think what's happening there is this: the parameter value needs a cast
from text to "ab"; this cast is an IO cast, so it becomes
enum_in(textout('A'),oid), and simplify_function can process
textout('A') as a constant (since textout is immutable) but not
enum_in('A',oid) (because enum_in is only stable, since it depends on
the enum label definitions). So the simplified expression becomes
('A'::cstring)::ab even though that's not actually a valid cast in
normal circumstances.

I haven't checked the partition code, but my guess is that since that's
not a constant (due to the stable cast), it can't be used for pruning in
pg10 (in pg11 it seems to get pruned at execution time).