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Relational algebra and functional programming

Data flows up a relational algebra expression


project(
    join(Artist,
        join(
            select(Album, year <= 1977),
            select(Track, length < 180))),
    name)

Data access starts at the leaf expressions.
Results flow to parents.
Parent holds on to data for as short a time as possible.
- Ideally one row (from child) at a time.
- But possibly all rows from child, saving them in memory, or on disk if necessary. E.g., sort has to accumulate all input before generating any output.
Query result emerges from the root.

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