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Before now, the cutoffs that VACUUM used to determine which XIDs/MXIDs to freeze were determined at the start of each VACUUM by taking related cutoffs that represent which XIDs/MXIDs VACUUM should treat as still running, and subtracting an XID/MXID age based value controlled by GUCs like vacuum_freeze_min_age. The FreezeLimit cutoff (XID freeze cutoff) was derived by subtracting an XID age value from OldestXmin, while the MultiXactCutoff cutoff (MXID freeze cutoff) was derived by subtracting an MXID age value from OldestMxact. This approach didn't match the approach used nearby to determine whether this VACUUM operation should be an aggressive VACUUM or not. VACUUM now uses the standard approach instead: it subtracts the same age-based values from next XID/next MXID (rather than subtracting from OldestXmin/OldestMxact). This approach is simpler and more uniform. Most of the time it will have only a negligible impact on how and when VACUUM freezes. It will occasionally make VACUUM more robust in the event of problems caused by long running transaction. These are cases where OldestXmin and OldestMxact are held back by so much that they attain an age that is a significant fraction of the value of age-based settings like vacuum_freeze_min_age. There is no principled reason why freezing should be affected in any way by the presence of a long-running transaction -- at least not before the point that the OldestXmin and OldestMxact limits used by each VACUUM operation attain an age that makes it unsafe to freeze some of the XIDs/MXIDs whose age exceeds the value of the relevant age-based settings. The new approach should at least make freezing degrade more gracefully than before, even in the most extreme cases. Author: Peter Geoghegan <pg@bowt.ie> Reviewed-By: Nathan Bossart <nathandbossart@gmail.com> Reviewed-By: Matthias van de Meent <boekewurm+postgres@gmail.com> Discussion: https://postgr.es/m/CAH2-WzkOv5CEeyOO=c91XnT5WBR_0gii0Wn5UbZhJ=4TTykDYg@mail.gmail.com |
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