828e94c9d2
For a mergejoin, if the given outer path or inner path is not already well enough ordered, we need to do an explicit sort. Currently, we only consider explicit full sort and do not account for incremental sort. In this patch, for the outer path of a mergejoin, we choose to use explicit incremental sort if it is enabled and there are presorted keys. For the inner path, though, we cannot use incremental sort because it does not support mark/restore at present. The rationale is based on the assumption that incremental sort is always faster than full sort when there are presorted keys, a premise that has been applied in various parts of the code. In addition, the current cost model tends to favor incremental sort as being cheaper than full sort in the presence of presorted keys, making it reasonable not to consider full sort in such cases. It could be argued that what if a mergejoin with an incremental sort as the outer path is selected as the inner path of another mergejoin. However, this should not be a problem, because mergejoin itself does not support mark/restore either, and we will add a Material node on top of it anyway in this case (see final_cost_mergejoin). There is one ensuing plan change in the regression tests, and we have to modify that test case to ensure that it continues to test what it is intended to. No backpatch as this could result in plan changes. Author: Richard Guo Reviewed-by: David Rowley, Tomas Vondra Discussion: https://postgr.es/m/CAMbWs49x425QrX7h=Ux05WEnt8GS757H-jOP3_xsX5t1FoUsZw@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.
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