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exec_for_query() normally tries to prefetch a few rows at a time from the query being iterated over, so as to reduce executor entry/exit overhead. Unfortunately this is unsafe if we have COMMIT or ROLLBACK within the loop, because there might be TOAST references in the data that we prefetched but haven't yet examined. Immediately after the COMMIT/ROLLBACK, we have no snapshots in the session, meaning that VACUUM is at liberty to remove recently-deleted TOAST rows. This was originally reported as a case triggering the "no known snapshots" error in init_toast_snapshot(), but even if you miss hitting that, you can get "missing toast chunk", as illustrated by the added isolation test case. To fix, just disable prefetching in non-atomic contexts. Maybe there will be performance complaints prompting us to work harder later, but it's not clear at the moment that this really costs much, and I doubt we'd want to back-patch any complicated fix. In passing, adjust that error message in init_toast_snapshot() to be a little clearer about the likely cause of the problem. Patch by me, based on earlier investigation by Konstantin Knizhnik. Per bug #15990 from Andreas Wicht. Back-patch to v11 where intra-procedure COMMIT was added. Discussion: https://postgr.es/m/15990-eee2ac466b11293d@postgresql.org |
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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. 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/.