Stamatis,
Thank you for your explanation, this makes sense now.
On Fri, Sep 13, 2019 at 2:58 PM Stamatis Zampetakis <zabe...@gmail.com>
wrote:
> Hi Thomas,
>
> It is not safe to push the filter in the null generating side of a join
> since
> it changes the semantics of the query. Consider the example below.
>
> SELECT b.title
> FROM Book b
> LEFT JOIN Author a ON b.author = a.id
> WHERE a.name <> 'Victor'
>
> Author
> --------------
> id | name |
> 1 | Victor |
>
> Book
> -----------------------------------
> id | title |author
> 1 | Les Miserables | 1
>
> If you apply the a.name <> 'Victor' condition before the join
> (over the author table) then you will get the following result.
>
> 'Les Miserables'
>
> If you apply the condition after the join you will get the empty result.
>
> Best,
> Stamatis
>
> On Thu, Sep 12, 2019 at 11:55 PM Thomas D'Silva <tdsi...@twilio.com.invalid
> >
> wrote:
>
> > We are using calcite to query data in Kudu. We have implemented our own
> > sort rule that merges rows returned from kudu when a queries orders by
> > columns in the same order as the primary key of the kudu table.
> > For a query that does a left join, in order to get the planner to use my
> > sort rule I had to disable the FILTER_ON_JOIN rule and use
> > the DUMB_FILTER_ON_JOIN (which does not try to simplify outer joins). I
> > also had to modify the following code in FilterJoinRule.perform()
> >
> > // Try to push down above filters. These are typically where clause
> > // filters. They can be pushed down if they are not on the NULL
> > // generating side.
> > boolean filterPushed = false;
> > if (RelOptUtil.classifyFilters(
> > join,
> > aboveFilters,
> > joinType,
> > needsPushInto(join),
> > !joinType.generatesNullsOnLeft(),
> > !joinType.generatesNullsOnRight(),
> > joinFilters,
> > leftFilters,
> > rightFilters)) {
> > filterPushed = true;
> > }
> >
> > The above code sets the pushLeft and pushRight boolean flags only if the
> > join type does not generate nulls on the left or right side. I was
> > wondering if any one knows why this is required?
> > If a filter only references column present in the left or right table
> > shouldn't we always be able to push it down?
> >
>
