Select Expressions Cache Entities

Closing issues resolved in 4.1.0

PR https://github.com/nhibernate/nhibernate-core/pull/522

The issue isn't with the "case"-ing of booleans (which has more to do with the way that NULL comparisons are handled in DBs vs C#).

The issue is that the expression e.Owner == bob (where bob is a captured in-memory entity) does different things when in the WHERE clause and the SELECT clause.

In the WHERE clause this is turned into HQL and eventually translated to SQL as _entity.OwnerId = 0, and the value of p0 is cached as part of the query plan.

In the SELECT clause e.Owner is nominated as HQL, but bob isn't because it's a constant expression. Therefore e.Owner == bob is not nominated either. The SELECT clause ends up containing statements to select the entire joined Owner object and a projection expression is extracted: input => input[0] == bob where input[0] is the Owner object and bob is still the original captured constant instance. The projection expression is then cached along with the query plan.

This has 2 issues:

1. We query a bunch more data than is needed and (in some cases) create a completely unnecessary join

2. When a query matching this one occurs in a different session, even if the new instance of bob is from the new session, the cached projection will be used, which will fail the object comparison because the objects are no longer the same instance.

However, if we are able to nominate the entire e.Owner == bob expression and generate HQL, then the SELECT clause looks identical to the WHERE clause and only the primary key is cached in the query plan.

Can you please consider my last pull request? It improves the "case"-ing of Booleans a lot

I'm working up a solution. Basically it involves modifying the SelectClauseHqlNominator to project constants into HQL for equality (==) expressions.

The hitch is that when I do this, some conditional expressions start getting nominated, and nominating conditional expressions is... tricky.

The HQL tree builder automatically wraps all non-boolean conditionals in a CAST of the conditional type. But only certain conditional types are cast-able. This is determined by HqlIdent constructor that takes a Type as the second argument.

The options are to make all cast-able conditional expressions nominated for HQL or to allow no conditionals to be nominated for HQL unless directly nominated (like by the GroupKeyNominator).

I'd like to nominate all conditionals, but I'll probably nominate none. It should be enough that equality is nominated so that it is checked by the DB, and by identifier for DB entities.