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On a primary, sets of XLOG_RUNNING_XACTS records are generated on a periodic basis to allow recovery to build the initial state of transactions for a hot standby. The set of transaction IDs is created by scanning all the entries in ProcArray. However it happens that its logic never counted on the fact that two-phase transactions finishing to prepare can put ProcArray in a state where there are two entries with the same transaction ID, one for the initial transaction which gets cleared when prepare finishes, and a second, dummy, entry to track that the transaction is still running after prepare finishes. This way ensures a continuous presence of the transaction so as callers of for example TransactionIdIsInProgress() are always able to see it as alive. So, if a XLOG_RUNNING_XACTS takes a standby snapshot while a two-phase transaction finishes to prepare, the record can finish with duplicated XIDs, which is a state expected by design. If this record gets applied on a standby to initial its recovery state, then it would simply fail, so the odds of facing this failure are very low in practice. It would be tempting to change the generation of XLOG_RUNNING_XACTS so as duplicates are removed on the source, but this requires to hold on ProcArrayLock for longer and this would impact all workloads, particularly those using heavily two-phase transactions. XLOG_RUNNING_XACTS is also actually used only to initialize the standby state at recovery, so instead the solution is taken to discard duplicates when applying the initial snapshot. Diagnosed-by: Konstantin Knizhnik Author: Michael Paquier Discussion: https://postgr.es/m/0c96b653-4696-d4b4-6b5d-78143175d113@postgrespro.ru Backpatch-through: 9.3
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/.