>> 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.
Ah, I see.
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).
