Update link optiq-mat-plugin -> mat-calcite-plugin
Fix Linq4j.product; the cartesian product of 0 attributes is one row of 0 attributes.
Better handling of null values due to GROUPING SETS.

Expand GROUPING SETS at validate time, into AggregatingSelectScope, so that we know which columns become nullable, and use the same information at SQL-to-rel time.

Fix an infinite loop when validating a call in a GROUP BY clause.
Add GROUPING function
[CALCITE-470] Print warning when column type hint is not understood
Update EMPS.deptno column Integer->int
[CALCITE-464] Make parser accept configurable max length for SQL identifier

Add new SqlParser.Config interface and SqlParser.ConfigBuilder class; change parser .constructor and factory methods to use it.

Contribution from Jinfeng Ni, some fix-up by Julian Hyde.