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When generating paths for the ORDER BY clause, one thing we need to ensure is that the output paths project the correct final_target. To achieve this, in create_ordered_paths, we compare the pathtarget of each generated path with the given 'target', and add a post-sort projection step if the two targets do not match. Currently we perform a simple pointer comparison between the two targets. It turns out that this is not sufficient. Each sorted_path generated in create_ordered_paths initially projects the correct target required by the preceding steps of sort. If it is the same pointer as sort_input_target, pointer comparison suffices, because sort_input_target is always identical to final_target when no post-sort projection is needed. However, sorted_path's initial pathtarget may not be the same pointer as sort_input_target, because in apply_scanjoin_target_to_paths, if the target to be applied has the same expressions as the existing reltarget, we only inject the sortgroupref info into the existing pathtargets, rather than create new projection paths. As a result, pointer comparison in create_ordered_paths is not reliable. Instead, we can compare PathTarget.exprs to determine whether a projection step is needed. If the expressions match, we can be confident that a post-sort projection is not required. It could be argued that this change adds extra check cost each time we decide whether a post-sort projection is needed. However, as explained in apply_scanjoin_target_to_paths, by avoiding the creation of projection paths, we save effort both immediately and at plan creation time. This, I think, justifies the extra check cost. There are two ensuing plan changes in the regression tests, but they look reasonable and are exactly what we are fixing here. So no additional test cases are added. No backpatch as this could result in plan changes. Author: Richard Guo Reviewed-by: Peter Eisentraut, David Rowley, Tom Lane Discussion: https://postgr.es/m/CAMbWs48TosSvmnz88663_2yg3hfeOFss-J2PtnENDH6J_rLnRQ@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/.