Yep, my bad. Cognitive distortion - AND read as WHERE

There is one more option:

SELECT hs.*
    , isnull(fs1.ValidFrom, fs2.ValidFrom) as ValidFrom
FROM dbo.historicsummary hs
  ...

1. Your LEFT JOIN will be interpreted as INNER due to right table condition present in WHERE (fs.ValidFrom).
2. I don't think the index would be used at all under that...

I've expanded the description of the engines https://github.com/uratol/t-chess/blob/master/README.md .

The chess project is more as a tutorial demo and for fun, it plays terribly actually, SQL is very slow for such...