Currently, PhoenixInputFormat#getQueryPlan already generates a queryPlan and we use this plan to get the scans and splits for the MR job. In PhoenixInputFormat#createRecordReader which is called inside each mapper, we again create a queryPlan and pass this to the PhoenixRecordReader instance.
There are multiple problems with this approach:
- The mappers already have information about the scans from the driver code. We potentially just need to wrap these scans in an iterator and create a subsequent ResultSet.
- The mappers don't need most of the information embedded within a queryPlan, so they shouldn't need to regenerate the plan.
- There are weird corner cases that can occur if we replan the query in each mapper. For ex: If there is an index creation or metadata change in between when the MR job was created, and when the mappers actually launch. In this case, the mappers have the scans created for the first queryPlan, but the mappers will use iterators created for the second queryPlan. In such cases, the issued scans would not match the queryPlan embedded in the mappers' iterators/ResultSet. We could potentially miss some scans/be looking for more than we actually require since we check the original scans for this size. The resolved table would be as per the new queryPlan, and there could be a mismatch here as well (considering the index creation case). There are potentially other repercussions in case of intermediary metadata changes as well.
Serializing a subset of the information (like the projector, which iterator to use, etc.) of a QueryPlan and passing it from the driver to the mappers without having them regenerate the plans seems like the best way forward.