1ea5bdb00b
For several types of plan nodes that use TupleHashTables, the planner estimated the expected size of the table as basically numEntries * (MAXALIGN(dataWidth) + MAXALIGN(SizeofHeapTupleHeader)). This is pretty far off, especially for small data widths, because it doesn't account for the overhead of the simplehash.h hash table nor for any per-tuple "additional space" the plan node may request. Jeff Janes noted a case where the estimate was off by about a factor of three, even though the obvious hazards such as inaccurate estimates of numEntries or dataWidth didn't apply. To improve matters, create functions provided by the relevant executor modules that can estimate the required sizes with reasonable accuracy. (We're still not accounting for effects like allocator padding, but this at least gets the first-order effects correct.) I added functions that can estimate the tuple table sizes for nodeSetOp and nodeSubplan; these rely on an estimator for TupleHashTables in general, and that in turn relies on one for simplehash.h hash tables. That feels like kind of a lot of mechanism, but if we take any short-cuts we're violating modularity boundaries. The other places that use TupleHashTables are nodeAgg, which took pains to get its numbers right already, and nodeRecursiveunion. I did not try to improve the situation for nodeRecursiveunion because there's nothing to improve: we are not making an estimate of the hash table size, and it wouldn't help us to do so because we have no non-hashed alternative implementation. On top of that, our estimate of the number of entries to be hashed in that module is so suspect that we'd likely often choose the wrong implementation if we did have two ways to do it. Reported-by: Jeff Janes <jeff.janes@gmail.com> Author: Tom Lane <tgl@sss.pgh.pa.us> Reviewed-by: David Rowley <dgrowleyml@gmail.com> Discussion: https://postgr.es/m/CAMkU=1zia0JfW_QR8L5xA2vpa0oqVuiapm78h=WpNsHH13_9uw@mail.gmail.com
PostgreSQL Database Management System
This directory contains the source code distribution of the PostgreSQL database management system.
PostgreSQL is an advanced object-relational database management system that supports an extended subset of the SQL standard, including transactions, foreign keys, subqueries, triggers, user-defined types and functions. This distribution also contains C language bindings.
Copyright and license information can be found in the file COPYRIGHT.
General documentation about this version of PostgreSQL can be found at https://www.postgresql.org/docs/devel/. In particular, information about building PostgreSQL from the source code can be found at https://www.postgresql.org/docs/devel/installation.html.
The latest version of this software, and related software, may be obtained at https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.