SNIP
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>It seems to me that it would be relatively easy to use indices which include NOT. Probably it would also be possible to use filtered indices - by checking whether the filter is relevant for the particular query (for instance, if I have SET DELETED ON, and therefore the implicit condition on "... and not deleted()", an index filtered on "not deleted()" could be used without harm). But that would make the optimization engine more complicated.

I think it would also be relatively 'easy' to make Rushmore work with filtered indexes. And I have seen (and rated) a Wish to this effect.