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From Julian Hyde <jh...@apache.org>
Subject Re: Question on Subquery plan
Date Thu, 02 Mar 2017 17:22:26 GMT
The LogicalAggregate will ALWAYS return one row (just as "SELECT
COUNT(*) FROM t" always returns one row, even if t is empty, or in
this case "SELECT MIN(TRUE) FROM t" always returns one row). If the
result is non-empty the column value will be TRUE. If the result is
empty the column value will be NULL (aka UNKNOWN).

So, "IS NOT NULL($9)" is a proxy for the "EXISTS ...". It always
returns the same value.

And thankfully EXISTS (unlike IN) never returns UNKNOWN so we don't
have to worry about 3-valued logic.

Julian




On Wed, Mar 1, 2017 at 10:05 PM, baofeng.zbf
<baofeng.zbf@alibaba-inc.com> wrote:
>
> Hi,
>
> When I do non-correlated subquery using NOT EXISTS, the behavior of
> TableScan on empty result confused me.
>
> For example, `select * from foo where not exists (select col1, col2 from bar
> where id<-1) order by 1 limit 10;` will generate:
>
> xxxx
>     LogicalFilter(condition=[NOT(IS NOT NULL($9))])
>       LogicalJoin(condition=[true], joinType=[left])
>         TableScan(table=[foo], projects=[...])
>         LogicalAggregate(group=[{}], agg#0=[MIN($0)])
>           LogicalProject($f0=[true])
>             LogicalProject(id=[$0])
>               LogicalFilter(condition=[<($0, -1)])
>                 TableScan(table=[bar], projects=[[0, 1]])
>
> When `select col1, col2 from bar where id<-1` returns empty result,
> TableScan givens no rows.
>
> In this cases, the left join will not perform, unless empty result will
> always return Row(null, null) if it is empty.
>
> Is Calcite`s subquery conversion based on this assumption?
>
> Thx.
> Baofeng Zhang.

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