Joins over UNION ALL Queries in Teradata®: Demonstration of Optimized Execution.

The UNION ALL set operator is useful for combining data from multiple sources. With the emergence and prevalence of big data ecosystems in which data is typically stored on multiple systems, UNION ALL has become even more important in many analytical queries. In this project, we demonstrate novel cost-based optimization techniques implemented in Teradata Database for join queries involving UNION ALL views and derived tables. Instead of the naive and traditional way of spooling each UNION ALL branch to a common spool prior to performing join operations, which can be prohibitively expensive, we demonstrate new techniques developed in Teradata Database including: 1) Cost-based pushing of joins into UNION ALL branches, 2) Branch grouping strategy prior to join pushing, 3) Geography adjustment of the pushed relations to avoid unnecessary redistribution or duplication, 4) Iterative join decomposition of a pushed join to multiple joins, and 5) Combining multiple join steps into a single multisource join step. In the demonstration, we use the Teradata Visual Explain tool, which offers a rich set of visual rendering capabilities of query plans, the display of various metadata information for each plan step, and several interactive UGI options for end-users.