On Wed, Nov 21, 2012 at 7:31 PM, Kohei KaiGai <kai...@kaigai.gr.jp> wrote:
> 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. Regards, -- Shigeru HANADA
