pgsql: Make entirely-dummy appendrels get marked as such in set_append_ - Mailing list pgsql-committers

From Tom Lane
Subject pgsql: Make entirely-dummy appendrels get marked as such in set_append_
Date
Msg-id E1ZJSOO-0000j3-L1@gemulon.postgresql.org
Whole thread Raw
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Make entirely-dummy appendrels get marked as such in set_append_rel_size.

The planner generally expects that the estimated rowcount of any relation
is at least one row, *unless* it has been proven empty by constraint
exclusion or similar mechanisms, which is marked by installing a dummy path
as the rel's cheapest path (cf. IS_DUMMY_REL).  When I split up
allpaths.c's processing of base rels into separate set_base_rel_sizes and
set_base_rel_pathlists steps, the intention was that dummy rels would get
marked as such during the "set size" step; this is what justifies an Assert
in indxpath.c's get_loop_count that other relations should either be dummy
or have positive rowcount.  Unfortunately I didn't get that quite right
for append relations: if all the child rels have been proven empty then
set_append_rel_size would come up with a rowcount of zero, which is
correct, but it didn't then do set_dummy_rel_pathlist.  (We would have
ended up with the right state after set_append_rel_pathlist, but that's
too late, if we generate indexpaths for some other rel first.)

In addition to fixing the actual bug, I installed an Assert enforcing this
convention in set_rel_size; that then allows simplification of a couple
of now-redundant tests for zero rowcount in set_append_rel_size.

Also, to cover the possibility that third-party FDWs have been careless
about not returning a zero rowcount estimate, apply clamp_row_est to
whatever an FDW comes up with as the rows estimate.

Per report from Andreas Seltenreich.  Back-patch to 9.2.  Earlier branches
did not have the separation between set_base_rel_sizes and
set_base_rel_pathlists steps, so there was no intermediate state where an
appendrel would have had inconsistent rowcount and pathlist.  It's possible
that adding the Assert to set_rel_size would be a good idea in older
branches too; but since they're not under development any more, it's likely
not worth the trouble.

Branch
------
REL9_5_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/7481c6c2aa379f8d3427819fcaa0eac5c93b1dcf

Modified Files
--------------
src/backend/optimizer/path/allpaths.c |  104 ++++++++++++++++++++-------------
src/test/regress/expected/join.out    |   20 +++++++
src/test/regress/sql/join.sql         |   12 ++++
3 files changed, 94 insertions(+), 42 deletions(-)


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