On Tue, Jun 7, 2016 at 4:07 PM, Amit Langote <langote_amit...@lab.ntt.co.jp> wrote:
> 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 tests. -- Best Wishes, Ashutosh Bapat EnterpriseDB Corporation The Postgres Database Company
