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Alvaro Herrera b0e96f3119 Catalog not-null constraints ... We now create contype='n' pg_constraint rows for not-null constraints. We propagate these constraints to other tables during operations such as adding inheritance relationships, creating and attaching partitions and creating tables LIKE other tables. We also spawn not-null constraints for inheritance child tables when their parents have primary keys. These related constraints mostly follow the well-known rules of conislocal and coninhcount that we have for CHECK constraints, with some adaptations: for example, as opposed to CHECK constraints, we don't match not-null ones by name when descending a hierarchy to alter it, instead matching by column name that they apply to. This means we don't require the constraint names to be identical across a hierarchy. For now, we omit them for system catalogs. Maybe this is worth reconsidering. We don't support NOT VALID nor DEFERRABLE clauses either; these can be added as separate features later (this patch is already large and complicated enough.) psql shows these constraints in \d+. pg_dump requires some ad-hoc hacks, particularly when dumping a primary key. We now create one "throwaway" not-null constraint for each column in the PK together with the CREATE TABLE command, and once the PK is created, all those throwaway constraints are removed. This avoids having to check each tuple for nullness when the dump restores the primary key creation. pg_upgrading from an older release requires a somewhat brittle procedure to create a constraint state that matches what would be created if the database were being created fresh in Postgres 17. I have tested all the scenarios I could think of, and it works correctly as far as I can tell, but I could have neglected weird cases. This patch has been very long in the making. The first patch was written by Bernd Helmle in 2010 to add a new pg_constraint.contype value ('n'), which I (Álvaro) then hijacked in 2011 and 2012, until that one was killed by the realization that we ought to use contype='c' instead: manufactured CHECK constraints. However, later SQL standard development, as well as nonobvious emergent properties of that design (mostly, failure to distinguish them from "normal" CHECK constraints as well as the performance implication of having to test the CHECK expression) led us to reconsider this choice, so now the current implementation uses contype='n' again. During Postgres 16 this had already been introduced by commit e056c557ae, but there were some problems mainly with the pg_upgrade procedure that couldn't be fixed in reasonable time, so it was reverted. In 2016 Vitaly Burovoy also worked on this feature[1] but found no consensus for his proposed approach, which was claimed to be closer to the letter of the standard, requiring an additional pg_attribute column to track the OID of the not-null constraint for that column. [1] https://postgr.es/m/CAKOSWNkN6HSyatuys8xZxzRCR-KL1OkHS5-b9qd9bf1Rad3PLA@mail.gmail.com Author: Álvaro Herrera <alvherre@alvh.no-ip.org> Author: Bernd Helmle <mailings@oopsware.de> Reviewed-by: Justin Pryzby <pryzby@telsasoft.com> Reviewed-by: Peter Eisentraut <peter.eisentraut@enterprisedb.com> Reviewed-by: Dean Rasheed <dean.a.rasheed@gmail.com>		2 years ago
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src	Catalog not-null constraints	2 years ago
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