attachjoin

Attachjoin is a term used in some data processing contexts to denote a join operation in which a second relation or data stream is appended to a primary one and joined on a specified key. The phrase underscores the idea of "attaching" one input to another to produce combined records, typically in streaming or federated querying environments. Definitions vary by domain, and attachjoin is not recognized as a formal standard by major database bodies; it tends to appear in vendor documentation, papers on streaming analytics, and ETL tools.

In practice, an attachjoin takes two inputs, A and B, and a join predicate. As records arrive, B is attached to A when the join key matches, producing output rows that merge fields from both inputs. Implementations may support inner joins, left/right outer joins, or semi-joins; late-arriving data or out-of-order arrivals are commonly handled through buffering, watermarks, or windowing strategies. State management is typical to maintain the join context across time.

Usage scenarios include real-time analytics and enrichment pipelines, where a live event stream is joined with a static reference dataset, or two streaming sources that must be correlated on a key. Attachjoin can be realized as a composition of existing join primitives in streaming frameworks, often augmented with buffering and windowing to meet latency and correctness requirements.

See also: join, stream processing, windowing, enrichment, data federation.