tables: Only populate table cache with star-like selects

[theopolis]

This is a prototype fix for #6510. I wrote the test code without running it.

This has one limitation in that if you were to populate the table cache with:

select * from some_table;

You would expect a cache-hit with:

select some_col from some_table;

But cache-hits are only allowed with star (or similar) selects.

[muffins]

Out of curiosity, and as per your statement, do we have documentation about the table caching? Would it be worth adding a note about your caveat to the docs if they exist?

... cache-hits are only allowed with star (or similar) selects.

[amalone-scwx]

We could support cache-hits with less than '*' selects, as long as no indexed/additional/required columns are used, right? Change isCached() to true in those cases, but setCache() should not persist the results unless it's a star select.

[theopolis]

We could support cache-hits with less than '*' selects

Correct. I was debating adding that feature in this PR but I decided to keep it simple and fix the data inconsistency first.

Would it be worth adding a note about your caveat to the docs if they exist?

I think so, it would be nice to have an "advanced topics" wiki page that covered this and other behind-the-scenes features. Most of that documentation would help us reason about the expectation of code vs. implementation (and point out bugs where expectations are not met!). And some of these features are tunable with flags.