Hi,
postgres_fdw isn't expected to push down joins with placeholder vars.
But the check for that in foreign_join_ok() only considers
joinrel->relids. For a child-join relids contains the child relids but
PlaceHolderInfo refers to the top parent's relids. Hence postgres_fdw
tries to push down a child-join with PlaceHolderVars in it and fails
with error "unsupported expression type for deparse: 198". 198 being
T_PlaceHolderVar.
The fix is to use joinrel->top_parent_relids for a child-join.
Attached patch for the same.
--
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company