Re: is DB2 recursive query semantics inflationary?

#1

mikharakiri (AT) yahoo (DOT) com (Mikito Harakiri) writes:

Quote:

That's right: recursive view means that given temporary table state
T(N) at the step N we calculate T(N+1)

which means that temporary table is totally rewritten at each step
rather than incrementally inflated as Graeme suggests.

Am I missing some kind of optimization that reduces my execution to
Graeme's? (My guess would be that not every kind of recursive union
view would allow such transformation.)

First, you are confusing the language semantics with the evaluation
procedure. The semantics of the recursive view is the view instance
corresponding to T(n) where n is the least positive integer for which
T(n) = T(n+1). This is called the least fixed point of the query.

At step N you calculate T(N+1). If you compute T(N+1) from scratch,
it is called naive evaluation. For some queries, including all linear
recursive queries, you can do semi-naive evaluation which incorporates
the optimization of just computing the delta between T(N) and T(N+1) and
adding that into the temporary table for T(N) to get T(N+1).

Hope that helps.

Joseph

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Re: is DB2 recursive query semantics inflationary?

comp.databases.theory comp.databases.theory

Re: is DB2 recursive query semantics inflationary? - 10-08-2003 , 05:33 PM