On 2015/10/14 12:07, Kouhei Kaigai wrote: >> On 2015/10/07 15:39, Etsuro Fujita wrote: >> I noticed that the approach using a column to populate the foreign >> scan's slot directly wouldn't work well in some cases. For example, >> consider: >> >> SELECT * FROM verysmall v LEFT JOIN (bigft1 JOIN bigft2 ON bigft1.x = >> bigft2.x) ON v.q = bigft1.q AND v.r = bigft2.r FOR UPDATE OF v; >> >> The best plan is presumably something like this as you said before: >> >> LockRows >> -> Nested Loop >> -> Seq Scan on verysmall v >> -> Foreign Scan on bigft1 and bigft2 >> Remote SQL: SELECT * FROM bigft1 JOIN bigft2 ON bigft1.x = >> bigft2.x AND bigft1.q = $1 AND bigft2.r = $2 >> >> Consider the EvalPlanQual testing to see if the updated version of a >> tuple in v satisfies the query. If we use the column in the testing, we >> would get the wrong results in some cases.
> In this case, does ForeignScan have to be reset prior to ExecProcNode()? > Once ExecReScanForeignScan() gets called by ExecNestLoop(), it marks EPQ > slot is invalid. So, more or less, ForeignScan needs to kick the remote > join again based on the new parameter come from the latest verysmall tuple. > Please correct me, if I don't understand correctly. > In case of unparametalized ForeignScan case, the cached join-tuple work > well because it is independent from verysmall. > > Once again, if FDW driver is responsible to construct join-tuple from > the base relation's tuple cached in EPQ slot, this case don't need to > kick remote query again, because all the materials to construct join- > tuple are already held locally. Right? Sorry, maybe I misunderstand your words, but we are talking here about an approach using a whole-row var that would populate a join tuple that is returned by an FDW and stored in the scan slot in the corresponding ForeingScanState node in the parent state tree. Best regards, Etsuro Fujita -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
