Lovefield
Lovefield is a relational database for web apps
...It provides a typed schema system, indices, and a declarative API that compiles to efficient execution plans over IndexedDB (and other storage backends where available). Queries support joins, aggregations, ordering, pagination, and predicates, enabling complex client-side data operations with transactional semantics. The engine includes a cost-based optimizer and a planner that chooses index scans and join orders to reduce latency. Because it is embedded, apps can keep responsive, offline-capable experiences while synchronizing with a server on their own schedule. ...