Query: allow providers to easily override null compensation and provide simpler translations instead

[divega]

We have pretty heavy logic in query that allows translating two-value logic expressions into SQL, which does three-value logic natively. Many popular relational databases besides SQL Server implement alternative patterns to perform equality comparisons with in-memory semantics, e.g. x == y in a LINQ expression could be translated to

Translation	Supported by
x is y	SQLite
x <=> y	MySQL and MariaDB
decode(x, y, 0, 1) = 0	DB2 and Oracle
EXISTS (SELECT x INTERSECT SELECT y)	PostgreSQL, SQLite, SQL Server (as search condition)
EXISTS (VALUES(x) INTERSECT VALUES(y))	DB2, PostgreSQL, SQL Server
x IS NOT DISTINCT FROM y	ANSI SQL:2003, PostgreSQL (apparently not sargable)

This article presents an excellent survey of all the alternatives: http://modern-sql.com/feature/is-distinct-from.

For cases in which we know if either x or y are non-nullable, we can probably produce a more concise translation, but in more complex cases, the options above could lead to better SQL, even for SQL Server.

In any case, it should be possible for a provider to pick the best translation.