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The function calls GetLatestSnapshot() to acquire a fresh snapshot, makes it active, and was meant to pass it to table_tuple_lock(), but instead called GetLatestSnapshot() again to acquire yet another snapshot. It was harmless because the heap AM and all other known table AMs ignore the 'snapshot' argument anyway, but let's be tidy. In the long run, this perhaps should be redesigned so that snapshot was not needed in the first place. The table AM API uses TID + snapshot as the unique identifier for the row version, which is questionable when the row came from an index scan with a Dirty snapshot. You might lock a different row version when you use a different snapshot in the table_tuple_lock() call (a fresh MVCC snapshot) than in the index scan (DirtySnapshot). However, in the heap AM and other AMs where the TID alone identifies the row version, it doesn't matter. So for now, just fix the obvious albeit harmless bug. This has been wrong ever since the table AM API was introduced in commit 5db6df0c01, so backpatch to all supported versions. Discussion: https://www.postgresql.org/message-id/83d243d6-ad8d-4307-8b51-2ee5844f6230@iki.fi Backpatch-through: 13
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. PostgreSQL has many language interfaces, many of which are listed here: https://www.postgresql.org/download/ See the file INSTALL for instructions on how to build and install PostgreSQL. That file also lists supported operating systems and hardware platforms and contains information regarding any other software packages that are required to build or run the PostgreSQL system. Copyright and license information can be found in the file COPYRIGHT. A comprehensive documentation set is included in this distribution; it can be read as described in the installation instructions. The latest version of this software may be obtained at https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.