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We show that any expression of the relational division operator in the relational algebra with union, difference, projection, selection, and equijoins, must produce intermediate results of quadratic size. To prove this result, we show a dichotomy theorem about intermediate sizes of relational algebra expressions (they are either all linear, or at least one is quadratic); we link linear relational algebra expressions to expressions using only semijoins instead of joins; and we link these semijoin algebra expressions to the guarded fragment of first-order logic.
The semijoin algebra is the variant of the relational algebra obtained by replacing the join operator by the semijoin operator. We discuss some interesting connections between the semijoin algebra and the guarded fragment of first-order logic. We also provide an Ehrenfeucht-Fraïssé game, characterizing the discerning power of the semijoin algebra. This game gives a method for showing that certain queries are not expressible in the semijoin algebra.
We introduce a new abstract model of database query processing, finite cursor machines, that incorporates certain data streaming aspects. The model describes quite faithfully what happens in so-called "one-pass" and "two-pass query processing". Technically, the model is described in the framework of abstract state machines. Our main results are upper and lower bounds for processing relational algebra queries in this model, specifically, queries of the semijoin fragment of the relational algebra.
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