> I thought about the Relations line a bit more and noticed that there are > cases where the table reference names for joining relations in the > Relations line are printed incorrectly. Here is an example: > > postgres=# explain verbose select * from (select t1.a, t2.a from ft1 t1, > ft2 t2 where t1.a = t2.a union select t1.a, t2.a from ft1 t1, ft2 t2 where > t1.a = t2.a) as t(t1a, t2a); > QUERY PLAN > > -------------------------------------------------------------------------------------------------------------------- > Unique (cost=204.12..204.13 rows=2 width=8) > Output: t1.a, t2.a > -> Sort (cost=204.12..204.12 rows=2 width=8) > Output: t1.a, t2.a > Sort Key: t1.a, t2.a > -> Append (cost=100.00..204.11 rows=2 width=8) > -> Foreign Scan (cost=100.00..102.04 rows=1 width=8) > Output: t1.a, t2.a > Relations: (public.ft1 t1) INNER JOIN (public.ft2 t2) > Remote SQL: SELECT r1.a, r2.a FROM (public.t1 r1 > INNER JOIN public.t2 r2 ON (((r1.a = r2.a)))) > -> Foreign Scan (cost=100.00..102.04 rows=1 width=8) > Output: t1_1.a, t2_1.a > Relations: (public.ft1 t1) INNER JOIN (public.ft2 t2) > Remote SQL: SELECT r1.a, r2.a FROM (public.t1 r1 > INNER JOIN public.t2 r2 ON (((r1.a = r2.a)))) > (14 rows) > > The table reference names for ft1 and ft2 in the Relations line for the > second Foreign Scan should be t1_1 and t2_1 respectively. >
Relations line prints the names of foreign tables that are being joined and the type of join. I find t1_1 and t2_1 more confusing since the query that user has provided does not mention t1_1 and t2_1. > > > Would we really need the Relations line? If joining relations are printed > by core like "Foreign Join on public.ft1 t1_1, public.ft2 t2_1" as proposed > upthread, we can see those relations from that, not the Relations line. The join type is missing in that description. > Also we can see the join tree structure from the deparsed query in the > Remote SQL line. The remote SQL has the names of the table on the foreign server. It does not help to identify the local names. -- Best Wishes, Ashutosh Bapat EnterpriseDB Corporation The Postgres Database Company
