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pg_stat_replication is documented to keep the last measured lag values for a short time after the standby catches up, and then set them to NULL when there is no WAL activity. However, previously lag values could become NULL prematurely even while WAL activity was ongoing, especially in logical replication. This happened because the code cleared lag when two consecutive reply messages indicated that the apply location had caught up with the send location. It did not verify that the reported positions were unchanged, so lag could be cleared even when positions had advanced between messages. In logical replication, where the apply location often quickly catches up, this issue was more likely to occur. This commit fixes the issue by clearing lag only when the standby reports that it has fully replayed WAL (i.e., both flush and apply locations have caught up with the send location) and the write/flush/apply positions remain unchanged across two consecutive reply messages. The second message with unchanged positions typically results from wal_receiver_status_interval, so lag values are cleared after that interval when there is no activity. This avoids showing stale lag data while preventing premature NULL values. Even with this fix, lag may rarely become NULL during activity if identical position reports are sent repeatedly. Eliminating such duplicate messages would address this fully, but that change is considered too invasive for stable branches and will be handled in master only later. Backpatch to all supported branches. Author: Shinya Kato <shinya11.kato@gmail.com> Reviewed-by: Chao Li <li.evan.chao@gmail.com> Reviewed-by: Fujii Masao <masao.fujii@gmail.com> Discussion: https://postgr.es/m/CAOzEurTzcUrEzrH97DD7+Yz=HGPU81kzWQonKZvqBwYhx2G9_A@mail.gmail.com Backpatch-through: 14
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/.