Implement scalar sub-query in SELECT clause.
Fix https://github.com/julianhyde/optiq/issues/175, "Modify Schema SPI to allow caching".

Added "cache" attribute of Schema JSON element; add method contentsHaveChangedSince to Schema SPI (breaking change, if you've written a schema that does not extend AbstractSchema); big rework of how OptiqSchema builds maps of tables, functions and sub-schemas and how it handles case-insensitive matches.
Tweak parallelism of optiq-plus test suite.
Fix https://github.com/julianhyde/optiq/issues/37, "Document JSON model file format"
  Get sub-schemas defined by a Schema SPI, and cache their OptiqSchema wrappers. (Tobi Vollebregt and Julian Hyde)
Move optimizer to linq4j, improve translation of strict functions
Mark SqlFunctions as @Deterministic (with some exceptions), support static final constants generated by linq4j
Upgrade to linq4j-0.3.
Upgrade maven-release-plugin.
Fix https://github.com/julianhyde/optiq/issues/252, "Scalar sub-query that returns 0 rows should become NULL value".
Provide an option to create root schema without the "metadata" schema.
Hush warnings from SplunkAdapterTest if Splunk is not available.
Avoid excessive box and unbox of primitives when using Object[] storage.
Add Apache incubator proposal (draft)
Update INCUBATOR_PROPOSAL.md
Update INCUBATOR_PROPOSAL.md
Update INCUBATOR_PROPOSAL.md
Represent ARRAY columns as Java lists; implement CARDINALITY(ARRAY) SQL operator.
very minor precisifications
Fix return type inference for aggregate calls: it should consider number of items in GROUP BY clause.

Alter min/max return type inference to ARG0_NULLABLE_IF_EMPTY (nullable when no keys in GROUP BY).

Fix PushAggregateThroughUnionRule.

Rework PullConstantsThroughAggregatesRule in terms of ProjectRel instead of CalcRel.

Log final plan in Prepare.

Make emp.mgr nullable (just to increase test coverage).