commit d4cace385fa8f314a947aae4d9b55d0ef71cb111
author Gian Merlino <gianmerlino@gmail.com> Sat Oct 23 17:18:43 2021 -0700
committer GitHub <noreply@github.com> Sat Oct 23 17:18:43 2021 -0700
tree 28ebcada3b6f16acdaf04ff0da9e64db341fbfce
parent 98ecbb21cd5715139da40ede8b24759f8ea4e47a

SQL: Allow Scans to be used as outer queries. (#11831)

* SQL: Allow Scans to be used as outer queries.

This has been possible in the native query system for a while, but the capability
hasn't yet propagated into the SQL layer. One example of where this is useful is
a query like:

  SELECT * FROM (... LIMIT X) WHERE <filter>

Because this expands the kinds of subquery structures the SQL layer will consider,
it was also necessary to improve the cost calculations. These changes appear in
PartialDruidQuery and DruidOuterQueryRel. The ideas are:

- Attach per-column penalties to the output signature of each query, instead of to
  the initial projection that starts a query. This encourages moving projections
  into subqueries instead of leaving them on outer queries.
- Only attach penalties to projections if there are actually expressions happening.
  So, now, projections that simply reorder or remove fields are free.
- Attach a constant penalty to every outer query. This discourages creating them
  when they are not needed.

The changes are generally beneficial to the test cases we have in CalciteQueryTest.
Most plans are unchanged, or are changed in purely cosmetic ways. Two have changed
for the better:

- testUsingSubqueryWithLimit now returns a constant from the subquery, instead of
  returning every column.
- testJoinOuterGroupByAndSubqueryHasLimit returns a minimal set of columns from
  the innermost subquery; two unnecessary columns are no longer there.

* Fix various DS operator conversions.

These were all implemented as direct conversions, which isn't appropriate
because they do not actually map onto native functions. These are only
usable as post-aggregations.

* Test case adjustment.