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014f9a831a
apply_scanjoin_target_to_paths wants to avoid useless work and platform-specific dependencies by throwing away the path list created prior to applying the final scan/join target and constructing a whole new one using the final scan/join target. However, this is only valid when we'll consider all the same strategies after the pathlist reset as before. After resetting the path list, we reconsider Append and MergeAppend paths with the modified target list; therefore, it's only valid for a partitioned relation. However, what the previous coding missed is that it cannot be a partitioned join relation, because that also has paths that are not Append or MergeAppend paths and will not be reconsidered. Thus, before this patch, we'd sometimes choose a partitionwise strategy with a higher total cost than cheapest non-partitionwise strategy, which is not good. We had a surprising number of tests cases that were relying on this bug to work as they did. A big part of the reason for this is that row counts in regression test cases tend to be low, which brings the cost of partitionwise and non-partitionwise strategies very close together, especially for merge joins, where the real and perceived advantages of a partitionwise approach are minimal. In addition, one test case included a row-count-inflating join. In such cases, a partitionwise join can easily be a loser on cost, because the total number of tuples passing through an Append node is much higher than it is with a non-partitionwise strategy. That test case is adjusted by adding additional join clauses to avoid the row count inflation. Although the failure of the planner to choose the lowest-cost path is a bug, we generally do not back-patch fixes of this type, because planning is not an exact science and there is always a possibility that some user will end up with a plan that has a lower estimated cost but actually runs more slowly. Hence, no backpatch here, either. The code change here is exactly what was originally proposed by Ashutosh, but the changes to the comments and test cases have been very heavily rewritten by me, helped along by some very useful advice from Richard Guo. Reported-by: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com> Author: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com> Author: Robert Haas <rhaas@postgresql.org> Reviewed-by: Jakub Wartak <jakub.wartak@enterprisedb.com> Reviewed-by: Arne Roland <arne.roland@malkut.net> Reviewed-by: Richard Guo <guofenglinux@gmail.com> Discussion: http://postgr.es/m/CAExHW5toze58+jL-454J3ty11sqJyU13Sz5rJPQZDmASwZgWiA@mail.gmail.com |
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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/.