On Sun, 31 Mar 2019 at 05:50, Robert Haas <robertmh...@gmail.com> wrote: > > On Sat, Mar 30, 2019 at 12:16 PM Amit Langote <amitlangot...@gmail.com> wrote: > > Fwiw, I had complained when reviewing the run-time pruning patch that > > creating those maps in the planner and putting them in > > PartitionPruneInfo might not be a good idea, but David insisted that > > it'd be good for performance (in the context of using cached plans) to > > compute this information during planning. > > Well, he's not wrong about that, I expect.
I'm aware that there have been combinations of objects to either having these arrays and/or editing them during execution. I don't recall Amit's complaint, but I do recall Tom's. He suggested we not resequence the arrays in the executor and just maintain NULL elements in the Append/MergeAppend subplans. I did consider this when writing run-time pruning but found that performance suffers. I demonstrated this on a thread somewhere. IIRC, I wrote this code because there was no way to translate the result of the pruning code into Append/MergeAppend subplan indexes. Robert has since added a map of Oids to allow the executor to have those details, so it perhaps would be possible to take the result of the pruning code then lookup the Oids of the partitions that survived pruning, then map those to the subplans using the array Robert added. Using the array for that wouldn't be very efficient due to a lookup being O(n) per surviving partition. Maybe it could be thrown into a hashtable to make that faster. This solution would need to take into account mixed hierarchy Appends. e.g SELECT * FROM partitioned_table WHERE partkey = $1 UNION ALL SELECT * FROM something_else; so it would likely need to be a hashtable per partitioned table. If the pruning code returned a partition whose Oid we didn't know about, then it must be from a partition that was added concurrently since the plan was built... However, that shouldn't happen today since Robert went to great lengths for it not to. Further discussions are likely best put in their own thread. As far as I know, nothing is broken with the code today. -- David Rowley http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services
