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Summary

We will implement a new Agg stage, $union, that allows to merge results of n pipelines preserving duplicates. In order to enable merging data from multiple collections, we will also introduce an explicit stage to reference a collection, $collection.

Motivation

Union is a fundamental operation in relational algebra. We have several specific scenarios:

• BIC connector for completeness with SQL.
• TimeSeries scenario to combine data stored in per-period collections into one logical collection.
• Combining collections in Data Lake, e.g. archival and recent data, data from different regions.

For analytical scenarios, customers expect a complete set of fundamental operations. For example for Tableau, union and unpivot were top requested features after joins. In the future, we will be improving $lookup but delivering general and performant joins is a hard task. At the same time, union-like logic is already supported for operations that require merging results across shards in the backend.

Documentation

Scope Document
Design Document