abcjoin

abcjoin is a three-way join operation used in relational databases and data-processing systems. It combines three input relations, typically labeled A, B, and C, by applying join predicates on their shared attributes to produce a result containing tuples that satisfy all specified conditions. In practice, abcjoin can be implemented as a single multiway operator or as a composition of binary joins (for example, (A join B) then join with C, or A join (B join C)).

Semantics of abcjoin are usually described for inner joins, but the concept can be extended to outer variants. The core form applies a conjunction of equi-join predicates, such as A.x = B.y and B.z = C.w, potentially along with non-equi predicates. Three-way joins can also be extended to implement left, right, or full outer semantics, though those variants require additional handling of unmatched tuples from one or more relations.

Implementation and optimization considerations include choosing a plan that minimizes intermediate results. Common strategies include multiway hash join, where hash partitions are computed over all participating relations, or staged binary joins with careful ordering to reduce data flow. Predicate pushdown, early filtering, and partitioning (hash or sort-based) are important for performance. Memory usage and disk I/O become critical for large datasets, since naive three-way joins can generate large intermediates.

Applications and limitations: abcjoin is used in complex analytics, data integration, and scenarios requiring direct three-way relational reasoning. Its performance hinges on the size of A, B, and C and the selectivity of the predicates; poor selectivity can lead to substantial intermediate results. See also: join, multiway join, hash join, sort-merge join.