In Section 16.3.2 we pointed out that in a multidatabase system, data could not be communicated directly from one database site to another and that even indirect communication was difficult since a query site cannot send data to a database site. However, the following “end run” might be considered for computing a semi-join. The query site, A, executes a SELECT statement at the first database site, B, which returns the projection, P, used in computing the semi-join. Site A then uses the projection to dynamically construct another SELECT statement whose result set is the semi-join. Using Example 16.3.1 on page 699 of Section 16.3.1 involving tables Student and Transcript, give the two queries. Under what circumstances could such an approach be considered?
What will be an ideal response?
The projection of Transcript is returned by query
SELECT T.StudId
FROM Transcript, T
If the result of this query is the set of values a, b, ... z, then the following query
submitted by A to C returns the semi-join
SELECT S.Id, S.Major
FROM Student, S
WHERE S.Id = ’a’ OR S.Id = ’b’ OR ... OR S.Id = ’z’
Such an approach would only make sense if the first query produced a very small result set.
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