At execute_query(), it stores the retrieved rows onto tuplestore of festate->tuples at once. Doesn't it make problems when remote- table has very big number of rows?
No. postgres_fdw uses single-row processing mode of libpq when
retrieving query results in execute_query, so memory usage will
be stable at a certain level.
IIRC, the previous code used cursor feature to fetch a set of rows to avoid over-consumption of local memory. Do we have some restriction if we fetch a certain number of rows with FETCH? It seems to me, we can fetch 1000 rows for example, and tentatively store them onto the tuplestore within one PG_TRY() block (so, no need to worry about PQclear() timing), then we can fetch remote rows again when IterateForeignScan reached end of tuplestore.
As you say, postgres_fdw had used cursor to avoid possible memory
exhaust on large result set. I switched to single-row processing mode
(it could be said "protocol-level cursor"), which was added in 9.2,
because it accomplish same task with less SQL calls than cursor.
