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#423 Use MergeAppend for merging sorted runs from datanodes
open
5
2013-05-23
2013-05-23
No
While I hear Robert's presentation on query planning go wrong, I heard (first time) about MergeAppend plan. It looks like, we have something better to do with the way we are pushing the ORDER BY clauses and doing merge at the coordinator. Right now we are using Sort node and in ExecSort we start merging the rows. Instead, I think we should be using MergeAppend node. The only bad thing about this approach is MergeAppend node expects separate plans for each run of the sorted data. To do this, we need to replicate the RemoteQuery node as many times as there are nodes and put them in the list to the MergeAppend plan. I think that's going to solve the problem with materialisation (see mail thread "Using remote sorting for merge-join") and thus improve performance. It also takes away the need to have xc_node_id in the tupleslot of tuplestore. Obviously, simplifying the code.
