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PostgreSQL 10 Administration Cookbook - Fourth Edition
book

PostgreSQL 10 Administration Cookbook - Fourth Edition

by Simon Riggs, Gianni Ciolli
May 2018
Intermediate to advanced content levelIntermediate to advanced
576 pages
30h 25m
English
Packt Publishing
Content preview from PostgreSQL 10 Administration Cookbook - Fourth Edition

Adding a constraint without checking existing rows

A table constraint is a guarantee that must be satisfied by all the rows in the table. Therefore, adding a constraint to a table is a two-phase procedure—first, the constraint is created, and then all the existing rows are checked. Both happen in the same transaction, and the table cannot be accessed in the meantime. The constraint becomes visible after the check, yielding perfect consistency, which is usually the desired behavior—at the expense of availability, which is not that great.

This recipe demonstrates another case—how to enforce a constraint on future transactions only, without checking existing rows. This may be desirable in some specific cases, such as the following two:

  • Enabling ...
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Publisher Resources

ISBN: 9781788474924Supplemental Content