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RIGHT_SEMI joins rely on the HEAP_TUPLE_HAS_MATCH flag to guarantee that only the first match for each inner tuple is considered. However, in a parallel hash join, the inner relation is stored in a shared global hash table that can be probed by multiple workers concurrently. This allows different workers to inspect and set the match flags of the same inner tuples at the same time. If two workers probe the same inner tuple concurrently, both may see the match flag as unset and emit the same tuple, leading to duplicate output rows and violating RIGHT_SEMI join semantics. For now, we disable parallel plans for RIGHT_SEMI joins. In the long term, it may be possible to support parallel execution by performing atomic operations on the match flag, for example using a CAS or similar mechanism. Backpatch to v18, where RIGHT_SEMI join was introduced. Bug: #19094 Reported-by: Lori Corbani <Lori.Corbani@jax.org> Diagnosed-by: Tom Lane <tgl@sss.pgh.pa.us> Author: Richard Guo <guofenglinux@gmail.com> Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us> Discussion: https://postgr.es/m/19094-6ed410eb5b256abd@postgresql.org Backpatch-through: 18
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/.