On Tue, Jun 7, 2016 at 4:07 PM, Amit Langote <langote_amit...@lab.ntt.co.jp>

> On 2016/06/07 19:13, Ashutosh Bapat wrote:
> > I thought, columns of inner relation will be set to null during
> projection
> > from ForeignScan for joins. But I was wrong. If we want to push-down
> joins
> > in this case, we have two solutions
> > 1. Build queries with subqueries at the time of deparsing. Thus a base
> > relation or join has to include placeholders while being deparsed as a
> > subquery. This means that the deparser should deparse expression
> > represented by the placeholder. This may not be possible always.
> > 2. At the time of projection from ForeignScan recognize the nullable
> > placeholders and nullify them if the corresponding relation is nullified.
> > That looks like a major surgery.
> > So, your patch looks to be the correct approach (even after we support
> > deparsing subqueries). Can you please include a test in regression?
> I added a slightly modified version of your test to the originally posted
> patch.
> Looks good to me. If we add a column from the outer relation, the
"NULL"ness of inner column would be more clear. May be we should tweak the
query to produce few more rows, some with non-NULL columns from both the
relations. Also add a note to the comment in the test mentioning that such
a join won't be pushed down for a reader to understand the EXPLAIN output.
Also, you might want to move that test, closer to other un-pushability

Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company

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