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Previously, subqueries were given names only after they were planned, which makes it difficult to use information from a previous execution of the query to guide future planning. If, for example, you knew something about how you want "InitPlan 2" to be planned, you won't know whether the subquery you're currently planning will end up being "InitPlan 2" until after you've finished planning it, by which point it's too late to use the information that you had. To fix this, assign each subplan a unique name before we begin planning it. To improve consistency, use textual names for all subplans, rather than, as we did previously, a mix of numbers (such as "InitPlan 1") and names (such as "CTE foo"), and make sure that the same name is never assigned more than once. We adopt the somewhat arbitrary convention of using the type of sublink to set the plan name; for example, a query that previously had two expression sublinks shown as InitPlan 2 and InitPlan 1 will now end up named expr_1 and expr_2. Because names are assigned before rather than after planning, some of the regression test outputs show the numerical part of the name switching positions: what was previously SubPlan 2 was actually the first one encountered, but we finished planning it later. We assign names even to subqueries that aren't shown as such within the EXPLAIN output. These include subqueries that are a FROM clause item or a branch of a set operation, rather than something that will be turned into an InitPlan or SubPlan. The purpose of this is to make sure that, below the topmost query level, there's always a name for each subquery that is stable from one planning cycle to the next (assuming no changes to the query or the database schema). Author: Robert Haas <rhaas@postgresql.org> Co-authored-by: Tom Lane <tgl@sss.pgh.pa.us> Reviewed-by: Alexandra Wang <alexandra.wang.oss@gmail.com> Reviewed-by: Richard Guo <guofenglinux@gmail.com> Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us> Reviewed-by: Junwang Zhao <zhjwpku@gmail.com> Discussion: http://postgr.es/m/3641043.1758751399@sss.pgh.pa.us |
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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/.