On Fri, Feb 16, 2024 at 8:42 AM Andrei Lepikhov <a.lepik...@postgrespro.ru> wrote: > Live example: right now, I am working on the code like MSSQL has - a > combination of NestLoop and HashJoin paths and switching between them in > real-time. It requires both paths in the path list at the moment when > extensions are coming. Even if one of them isn't referenced from the > upper pathlist, it may still be helpful for the extension.
There is no guarantee that every path presented to add_path will be preserved. Suboptimal paths are freed as and when add_path discovers that they are suboptimal. So I don't think an extension can rely on existence of a path. But having a refcount makes it easy to preserve the required paths by referencing them. > > >> About partitioning. As I discovered planning issues connected to > >> partitions, the painful problem is a rule, according to which we are > >> trying to use all nomenclature of possible paths for each partition. > >> With indexes, it quickly increases optimization work. IMO, this can help > >> a 'symmetrical' approach, which could restrict the scope of possible > >> pathways for upcoming partitions if we filter some paths in a set of > >> previously planned partitions. > > > > filter or free? > Filter. > I meant that Postres tries to apply IndexScan, BitmapScan, > IndexOnlyScan, and other strategies, passing throughout the partition > indexes. The optimizer spends a lot of time doing that. So, why not > introduce a symmetrical strategy and give away from the search some > indexes of types of scan based on the pathifying experience of previous > partitions of the same table: if you have dozens of partitions, Is it > beneficial for the system to find a bit more optimal IndexScan on one > partition having SeqScans on 999 other? > IIUC, you are suggesting that instead of planning each partition/partitionwise join, we only create paths with the strategies which were found to be optimal with previous partitions. That's a good heuristic but it won't work if partition properties - statistics, indexes etc. differ between groups of partitions. -- Best Wishes, Ashutosh Bapat
