Definition
A join index is a collection of pairs {(r, s)} such that the record in table R with record ID (RID) r joins with the record in table S with RID s, according to the join predicate which defines the index.
Key Points
The purpose of a join index is to accelerate common joins, even equijoins. One of the advantages of join indices is that they can be represented in a very compact way, allowing for highly efficient access. For example, suppose that the DBMS is to evaluate a query, “Select R.a from R,S where R.a = S.b.” A conventional join would use a nested loop algorithm, with an indexed scan in the inner loop. With a join index, the join can be computed by scanning the join index, thus minimizing random I/O.
There are a variety of ways of implementing a join index. One can list pairs (clustered on R or clustered on S), or in the case of an equijoin, associate R and S RIDs with attribute values. The example below shows the join index for R.a = S.b, organized as a list of pairs.
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Recommended Reading
Li Z, Ross KA. Fast joins using join indices. VLDB J. 1999;8(1):1–24.
Valduriez P. Join indices. ACM Trans Database Syst. 1987;12(2):218–46.
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Johnson, T. (2018). Join Index. In: Liu, L., Özsu, M.T. (eds) Encyclopedia of Database Systems. Springer, New York, NY. https://doi.org/10.1007/978-1-4614-8265-9_892
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