Enhance compatibility for SqlServer 2008

[MartinKoerner]

The code contains several statements like "GROUP BY 1". This is not supported in MS SQL, but can easily rewritten to "GROUP BY ascendent_id", which works for all SQL implementations.

[seuros]

We will need a benchmark with Postgresql and MYSQL. MSSql is not supported officially.

[MartinKoerner]

Do you have a preferred one?

I compared spec runs on my local laptop with this gist: https://gist.github.com/MartinKoerner/20f57509a2b3cebc5d69,
but it deviated around 4sec even without any change and me not touching the laptop, so it is not helpful for this small level problem.

[mceachen]

Fwiw, I've never seen a performance impact from ordinal versus named group_by clauses. I'd be more concerned with correctness here than speed, given the fairly intricate SQL generation from this gem. I think tests are fairly comprehensive, but I'm sure there are tons of consumers that have exercised the code in ways that don't have test coverage.

[mceachen]

This is innocuous to merge.