mirror of https://github.com/citusdata/citus.git
76ab7040cb
Custom Scan is a node in the planned statement which helps external providers to abstract data scan not just for foreign data wrappers but also for regular relations so you can benefit your version of caching or hardware optimizations. This sounds like only an abstraction on the data scan layer, but we can use it as an abstraction for our distributed queries. The only thing we need to do is to find distributable parts of the query, plan for them and replace them with a Citus Custom Scan. Then, whenever PostgreSQL hits this custom scan node in its Vulcano style execution, it will call our callback functions which run distributed plan and provides tuples to the upper node as it scans a regular relation. This means fewer code changes, fewer bugs and more supported features for us! First, in the distributed query planner phase, we create a Custom Scan which wraps the distributed plan. For real-time and task-tracker executors, we add this custom plan under the master query plan. For router executor, we directly pass the custom plan because there is not any master query. Then, we simply let the PostgreSQL executor run this plan. When it hits the custom scan node, we call the related executor parts for distributed plan, fill the tuple store in the custom scan and return results to PostgreSQL executor in Vulcano style, a tuple per XXX_ExecScan() call. * Modify planner to utilize Custom Scan node. * Create different scan methods for different executors. * Use native PostgreSQL Explain for master part of queries. |
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