If there can certainly be no more than one matching inner row for a given
outer row, then the executor can move on to the next outer row as soon as
it's found one match; there's no need to continue scanning the inner
relation for this outer row. This saves useless scanning in nestloop
and hash joins. In merge joins, it offers the opportunity to skip
mark/restore processing, because we know we have not advanced past the
first possible match for the next outer row.
Of course, the devil is in the details: the proof of uniqueness must
depend only on joinquals (not otherquals), and if we want to skip
mergejoin mark/restore then it must depend only on merge clauses.
To avoid adding more planning overhead than absolutely necessary,
the present patch errs in the conservative direction: there are cases
where inner_unique or skip_mark_restore processing could be used, but
it will not do so because it's not sure that the uniqueness proof
depended only on "safe" clauses. This could be improved later.
David Rowley, reviewed and rather heavily editorialized on by me
Discussion: https://postgr.es/m/CAApHDvqF6Sw-TK98bW48TdtFJ+3a7D2mFyZ7++=D-RyPsL76gw@mail.gmail.com
@ -1854,12 +1859,10 @@ SELECT t1.c1 FROM ft1 t1 WHERE EXISTS (SELECT 1 FROM ft2 t2 WHERE t1.c1 = t2.c1)
-> Foreign Scan on public.ft1 t1
Output: t1.c1
Remote SQL: SELECT "C 1" FROM "S 1"."T 1" ORDER BY "C 1" ASC NULLS LAST
-> Materialize
-> Foreign Scan on public.ft2 t2
Output: t2.c1
-> Foreign Scan on public.ft2 t2
Output: t2.c1
Remote SQL: SELECT "C 1" FROM "S 1"."T 1" ORDER BY "C 1" ASC NULLS LAST
(13 rows)
Remote SQL: SELECT "C 1" FROM "S 1"."T 1" ORDER BY "C 1" ASC NULLS LAST
(11 rows)
SELECT t1.c1 FROM ft1 t1 WHERE EXISTS (SELECT 1 FROM ft2 t2 WHERE t1.c1 = t2.c1) ORDER BY t1.c1 OFFSET 100 LIMIT 10;
c1
@ -1889,12 +1892,10 @@ SELECT t1.c1 FROM ft1 t1 WHERE NOT EXISTS (SELECT 1 FROM ft2 t2 WHERE t1.c1 = t2
-> Foreign Scan on public.ft1 t1
Output: t1.c1
Remote SQL: SELECT "C 1" FROM "S 1"."T 1" ORDER BY "C 1" ASC NULLS LAST
-> Materialize
-> Foreign Scan on public.ft2 t2
Output: t2.c2
-> Foreign Scan on public.ft2 t2
Output: t2.c2
Remote SQL: SELECT c2 FROM "S 1"."T 1" ORDER BY c2 ASC NULLS LAST
(13 rows)
Remote SQL: SELECT c2 FROM "S 1"."T 1" ORDER BY c2 ASC NULLS LAST
(11 rows)
SELECT t1.c1 FROM ft1 t1 WHERE NOT EXISTS (SELECT 1 FROM ft2 t2 WHERE t1.c1 = t2.c2) ORDER BY t1.c1 OFFSET 100 LIMIT 10;
c1
@ -3121,6 +3122,7 @@ select count(*), x.b from ft1, (select c2 a, sum(c1) b from ft1 group by c2) x w
Group Key: x.b
-> Hash Join
Output: x.b
Inner Unique: true
Hash Cond: (ft1.c2 = x.a)
-> Foreign Scan on public.ft1
Output: ft1.c2
@ -3133,7 +3135,7 @@ select count(*), x.b from ft1, (select c2 a, sum(c1) b from ft1 group by c2) x w
Output: ft1_1.c2, (sum(ft1_1.c1))
Relations: Aggregate on (public.ft1)
Remote SQL: SELECT c2, sum("C 1") FROM "S 1"."T 1" GROUP BY c2
(20 rows)
(21 rows)
select count(*), x.b from ft1, (select c2 a, sum(c1) b from ft1 group by c2) x where ft1.c2 = x.a group by x.b order by 1, 2;
count | b
@ -3252,6 +3254,7 @@ select sum(q.a), count(q.b) from ft4 left join (select 13, avg(ft1.c1), sum(ft2.
Output: sum(q.a), count(q.b)
-> Nested Loop Left Join
Output: q.a, q.b
Inner Unique: true
Join Filter: ((ft4.c1)::numeric <= q.b)
-> Foreign Scan on public.ft4
Output: ft4.c1, ft4.c2, ft4.c3
@ -3264,7 +3267,7 @@ select sum(q.a), count(q.b) from ft4 left join (select 13, avg(ft1.c1), sum(ft2.
Output: 13, (avg(ft1.c1)), NULL::bigint
Relations: Aggregate on ((public.ft2) LEFT JOIN (public.ft1))
Remote SQL: SELECT 13, avg(r1."C 1"), NULL::bigint FROM ("S 1"."T 1" r2 LEFT JOIN "S 1"."T 1" r1 ON (((r1."C 1" = r2."C 1"))))
(16 rows)
(17 rows)
select sum(q.a), count(q.b) from ft4 left join (select 13, avg(ft1.c1), sum(ft2.c1) from ft1 right join ft2 on (ft1.c1 = ft2.c1)) q(a, b, c) on (ft4.c1 <= q.b);
sum | count
@ -4048,20 +4051,18 @@ explain (verbose, costs off) select * from ft3 where f2 = 'foo' COLLATE "C";
explain (verbose, costs off) select * from ft3 f, loct3 l
where f.f3 = l.f3 COLLATE "POSIX" and l.f1 = 'foo';
Tom Lane
9 years ago