PipelineDB, an open-source relational streaming-SQL database, publicly released version (0.7.7) today and made the product available as open-source via their website and GitHub. PipelineDB is based on, and is wire compatible with, PostgreSQL 9.4 and has added functionality including continuous SQL queries, probabilistic data structures, sliding windowing, and stream-table joins. For a full description of PipelineDB and its capabilities see their technical documentation.
PipelineDB’s fundamental abstraction is what is called a continuous view. These are much like regular SQL views, except that their defining SELECT queries can include streams as a source to read from. The most important property of continuous views is that they only store their output in the database. That output is then continuously updated incrementally as new data flows through streams, and raw stream data is discarded once all continuous views have read it. Let's look at a canonical example:
CREATE CONTINUOUS VIEW v AS SELECT COUNT(*) FROM stream
Only one row would ever physically exist in PipelineDB for this continuous view, and its value would simply be incremented for each new event ingested.
For more information on PipelineDB as a company, product and for examples and benefits, please check out their first blog post on their new website.