If I’m being picky, the subquery in the example is not a nested join (unless you think of the subquery as expressing an infinite relation as I do in imp, but that’s definitely not the standard semantics given to sql).

the semantics of relational algebra that are amenable to optimization a query that can be analyzed as to its dependencies on other queries

But if we ask specifically what in the semantics of relational algebra makes it amenable to optimization? Or what it makes it tractable to analyze the dependencies of a subquery, when analyzing the dependencies of a loop body in python is so intractable? It’s going to look something like that list of requirements.

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