Re: Push down more full joins in postgres_fdw

On Wed, Dec 7, 2016 at 1:57 AM, Robert Haas <robertmhaas@gmail.com> wrote:
> On Mon, Dec 5, 2016 at 6:20 AM, Ashutosh Bapat
> <ashutosh.bapat@enterprisedb.com> wrote:
>> 4. I am still not happy with this change
>> +        /*
>> +         * Since (1) the expressions in foreignrel's reltarget doesn't contain
>> +         * any PHVs and (2) foreignrel's local_conds is empty, the tlist
>> +         * created by build_tlist_to_deparse must be one-to-one with the
>> +         * expressions.
>> +         */
>> +        Assert(list_length(tlist) ==
>> list_length(foreignrel->reltarget->exprs));
>> the assertion only checks that the number of elements in both the lists are
>> same but does not check whether those lists are same i.e. they contain the same
>> elements in the same order. This equality is crucial to deparsing logic. If
>> somehow build_tlist_to_deparse() breaks that assumption in future, we have no
>> way to detect it, unless a regression test fails.
>
> If there's an easy way to do a more exact comparison, great.  But if
> we can't get an awesome Assert(), a helpful Assert() is still better
> than a kick in the head.

The assert is not a problem in itself, but the reason we have to add
the assert. The problem is explained in [1], point #9.

[1] https://www.postgresql.org/message-id/CAFjFpRfwoSsJr9418b2jA7g0nwagjZSWhPQPUmK9M6z5XSOAqQ%40mail.gmail.com


-- 
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company