Add RelationalCommandCaching based on parameter value nullability

[smitpatel]

Resolves #15892

I attempted to add to cosmos also but ran into #18033
Currently this takes naïve approach that if SelectExpression was modified during parameter based EV then we cannot cache it. If we work on #16375 then that would be quite accurate logic without us having to pass back the information. I filed #18034 to track.

[smitpatel]

Implementation follows what we had in previous versions.

[AndriySvyryd]

Tests?

[smitpatel]

Do you have any ideas in mind, how to test this? A way to catch regression would be our perf tests.

[AndriySvyryd]

You can add unit tests with a mock QuerySqlGenerator and assert how many times a command was created.

[AndriySvyryd]

We should also consider extracting some nested classes out of the partial classes. The fact that they are partial is a smell and it's ok to have a separate type even if it's only used in one place.

[smitpatel]

They are implementation detail of this visitor hence stayed inside. Is partial the issue or nesting?

[AndriySvyryd]

@smitpatel The partial means that the class is too big and should be split. And in practice you don't gain much by keeping the class nested.

[smitpatel]

@AndriySvyryd - Since this is also cache, do we need to do anything specific for cache eviction?

[AndriySvyryd]

@smitpatel Yes, use MemoryCache and set size for the entries

[smitpatel]

Filed #18493 for tests

[AndriySvyryd]

Use MemoryCache

[smitpatel]

I thought I was missing something. 😄