When we decide we need to make a derived clause equating a.x and
b.y, we already will re-use a previously-made clause "a.x = b.y".
But we might instead have "b.y = a.x", which is perfectly usable
because equivclass.c has never promised anything about the
operand order in clauses it builds. Saving construction of a
new RestrictInfo doesn't matter all that much in itself --- but
because we cache selectivity estimates and so on per-RestrictInfo,
there's a possibility of saving a fair amount of duplicative
effort downstream.
Hence, check for commutative matches as well as direct ones when
seeing if we have a pre-existing clause. This changes the visible
clause order in several regression test cases, but they're all
clearly-insignificant changes.
Checking for the reverse operand order is simple enough, but
if we wanted to check for operator OID match we'd need to call
get_commutator here, which is not so cheap. I concluded that
we don't really need the operator check anyway, so I just
removed it. It's unlikely that an opfamily contains more than
one applicable operator for a given pair of operand datatypes;
and if it does they had better give the same answers, so there
seems little need to insist that we use exactly the one
select_equality_operator chose.
Using the current core regression suite as a test case, I see
this change reducing the number of new join clauses built by
create_join_clause from 9673 to 5142 (out of 26652 calls).
So not quite 50% savings, but pretty close to it.
Discussion: https://postgr.es/m/78062.1666735746@sss.pgh.pa.us
Commit 147e3722f7 changed Tid scan so that it calls table_beginscan()
and uses the scan option for seq scan. This change caused two issues.
(1) The change caused Tid scan to take a predicate lock on the entire
relation in serializable transaction even when relation-level
lock is not necessary. This could lead to an unexpected
serialization error.
(2) The change caused Tid scan to increment the number of seq_scan
in pg_stat_*_tables views even though it's not seq scan. This
could confuse the users.
This commit adds the scan option for Tid scan and makes Tid scan
use it, to avoid those issues.
Back-patch to v12, where the bug was introduced.
Author: Tatsuhito Kasahara
Reviewed-by: Kyotaro Horiguchi, Masahiko Sawada, Fujii Masao
Discussion: https://postgr.es/m/CAP0=ZVKy+gTbFmB6X_UW0pP3WaeJ-fkUWHoD-pExS=at3CY76g@mail.gmail.com
Up to now we've not worried much about joins where the join key is a
relation's CTID column, reasoning that storing a table's CTIDs in some
other table would be pretty useless. However, there are use-cases for
this sort of query involving self-joins, so that argument doesn't really
hold water.
With larger relations, a merge or hash join is desirable. We had a btree
opclass for type "tid", allowing merge joins on CTID, but no hash opclass
so that hash joins weren't possible. Add the missing infrastructure.
This also potentially enables hash aggregation on "tid", though the
use-cases for that aren't too clear.
Discussion: https://postgr.es/m/1853.1545453106@sss.pgh.pa.us
Up to now we've not worried much about joins where the join key is a
relation's CTID column, reasoning that storing a table's CTIDs in some
other table would be pretty useless. However, there are use-cases for
this sort of query involving self-joins, so that argument doesn't really
hold water.
This patch allows generating plans for joins on CTID that use a nestloop
with inner TidScan, similar to what we might do with an index on the join
column. This is the most efficient way to join when the outer side of
the nestloop is expected to yield relatively few rows.
This change requires upgrading tidpath.c and the generated TidPaths
to work with RestrictInfos instead of bare qual clauses, but that's
long-postponed technical debt anyway.
Discussion: https://postgr.es/m/17443.1545435266@sss.pgh.pa.us
TidScan plan nodes were not systematically tested before. These additions
raise the LOC coverage number for the basic regression tests from 52% to
92% in nodeTidscan.c, and from 60% to 93% in tidpath.c.
Andres Freund, tweaked a bit by me
Discussion: https://postgr.es/m/20170320062511.hp5qeurtxrwsvfxr@alap3.anarazel.de
Tom Lane
Fujii Masao