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For binary upgrades from v16 or newer, pg_upgrade transfers the files for pg_largeobject_metadata from the old cluster, as opposed to using COPY or ordinary SQL commands to reconstruct its contents. While this approach adds complexity, it can greatly reduce pg_upgrade's runtime when there are many large objects. Large objects with comments or security labels are one source of complexity for this approach. During pg_upgrade, schema restoration happens before files are transferred. Comments and security labels are transferred in the former step, but the COMMENT and SECURITY LABEL commands will fail if their corresponding large objects do not exist. To deal with this, pg_upgrade first copies only the rows of pg_largeobject_metadata that are needed to avoid failures. Later, pg_upgrade overwrites those rows by replacing pg_largeobject_metadata's files with its files in the old cluster. Unfortunately, there's a subtle problem here. Simply put, there's no guarantee that pg_upgrade will overwrite all of pg_largeobject_metadata's files on the new cluster. For example, the new cluster's version might more aggressively extend relations or create visibility maps, and pg_upgrade's file transfer code is not sophisticated enough to remove files that lack counterparts in the old cluster. These extra files could cause problems post-upgrade. More fortunately, we can simultaneously fix the aforementioned problem and further optimize binary upgrades for clusters with many large objects. If we teach the COMMENT and SECURITY LABEL commands to allow nonexistent large objects during binary upgrades, pg_upgrade no longer needs to transfer pg_largeobject_metadata's contents beforehand. This approach also allows us to remove the associated dependency tracking from pg_dump, even for upgrades from v12-v15 that use COPY to transfer pg_largeobject_metadata's contents. In addition to what is described in the previous paragraph, this commit modifies the query in getLOs() to only retrieve LOs with comments or security labels for upgrades from v12 or newer. We have long assumed that such usage is rare, so this should reduce pg_upgrade's memory usage and runtime in many cases. We might also be able to remove the "upgrades from v12 or newer" restriction on the recent batch of optimizations by adding special handling for pg_largeobject_metadata's hidden OID column on older versions (since this catalog previously used the now-removed WITH OIDS feature), but that is left as a future exercise. Reported-by: Andres Freund <andres@anarazel.de> Reviewed-by: Andres Freund <andres@anarazel.de> Discussion: https://postgr.es/m/3yd2ss6n7xywo6pmhd7jjh3bqwgvx35bflzgv3ag4cnzfkik7m%40hiyadppqxx6w |
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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/.