On 09/11/2014 12:22 PM, Etsuro Fujita wrote:
(2014/09/11 4:32), Heikki Linnakangas wrote:
I had a cursory look at this patch and the discussions around this.

Thank you!

ISTM there are actually two new features in this: 1) allow CHECK
constraints on foreign tables, and 2) support inheritance for foreign
tables. How about splitting it into two?

That's right.  There are the two in this patch.

I'm not sure if I should split the patch into the two, because 1) won't
make sense without 2).  As described in the following note added to the
docs on CREATE FOREIGN TABLE, CHECK constraints on foreign tables are
intended to support constraint exclusion for partitioned foreign tables:

+     Constraints on foreign tables are not enforced on insert or update.
+     Those definitions simply declare the constraints hold for all rows
+     in the foreign tables.  It is the user's responsibility to ensure
+     that those definitions match the remote side.  Such constraints are
+     used for some kind of query optimization such as constraint exclusion
+     for partitioned tables

The planner can do constraint exclusion based on CHECK constraints even without inheritance. It's not enabled by default because it can increase planning time, but if you set constraint_exclusion=on, it will work.

For example:

postgres=# create table foo (i int4 CHECK (i > 0));
postgres=# explain select * from foo WHERE i < 0;
                      QUERY PLAN
 Seq Scan on foo  (cost=0.00..40.00 rows=800 width=4)
   Filter: (i < 0)
 Planning time: 0.335 ms
(3 rows)

postgres=# show constraint_exclusion ;
(1 row)

postgres=# set constraint_exclusion ='on';
postgres=# explain select * from foo WHERE i < 0;
                QUERY PLAN
 Result  (cost=0.00..0.01 rows=1 width=0)
   One-Time Filter: false
 Planning time: 0.254 ms
(3 rows)


- Heikki

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