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This commit only implements one prerequisite part for allowing logical decoding. The commit message contains an explanation of the overall design, which later commits will refer back to. Overall design: 1. We want to enable logical decoding on standbys, but replay of WAL from the primary might remove data that is needed by logical decoding, causing error(s) on the standby. To prevent those errors, a new replication conflict scenario needs to be addressed (as much as hot standby does). 2. Our chosen strategy for dealing with this type of replication slot is to invalidate logical slots for which needed data has been removed. 3. To do this we need the latestRemovedXid for each change, just as we do for physical replication conflicts, but we also need to know whether any particular change was to data that logical replication might access. That way, during WAL replay, we know when there is a risk of conflict and, if so, if there is a conflict. 4. We can't rely on the standby's relcache entries for this purpose in any way, because the startup process can't access catalog contents. 5. Therefore every WAL record that potentially removes data from the index or heap must carry a flag indicating whether or not it is one that might be accessed during logical decoding. Why do we need this for logical decoding on standby? First, let's forget about logical decoding on standby and recall that on a primary database, any catalog rows that may be needed by a logical decoding replication slot are not removed. This is done thanks to the catalog_xmin associated with the logical replication slot. But, with logical decoding on standby, in the following cases: - hot_standby_feedback is off - hot_standby_feedback is on but there is no a physical slot between the primary and the standby. Then, hot_standby_feedback will work, but only while the connection is alive (for example a node restart would break it) Then, the primary may delete system catalog rows that could be needed by the logical decoding on the standby (as it does not know about the catalog_xmin on the standby). So, it’s mandatory to identify those rows and invalidate the slots that may need them if any. Identifying those rows is the purpose of this commit. Implementation: When a WAL replay on standby indicates that a catalog table tuple is to be deleted by an xid that is greater than a logical slot's catalog_xmin, then that means the slot's catalog_xmin conflicts with the xid, and we need to handle the conflict. While subsequent commits will do the actual conflict handling, this commit adds a new field isCatalogRel in such WAL records (and a new bit set in the xl_heap_visible flags field), that is true for catalog tables, so as to arrange for conflict handling. The affected WAL records are the ones that already contain the snapshotConflictHorizon field, namely: - gistxlogDelete - gistxlogPageReuse - xl_hash_vacuum_one_page - xl_heap_prune - xl_heap_freeze_page - xl_heap_visible - xl_btree_reuse_page - xl_btree_delete - spgxlogVacuumRedirect Due to this new field being added, xl_hash_vacuum_one_page and gistxlogDelete do now contain the offsets to be deleted as a FLEXIBLE_ARRAY_MEMBER. This is needed to ensure correct alignment. It's not needed on the others struct where isCatalogRel has been added. This commit just introduces the WAL format changes mentioned above. Handling the actual conflicts will follow in future commits. Bumps XLOG_PAGE_MAGIC as the several WAL records are changed. Author: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com> Author: Andres Freund <andres@anarazel.de> (in an older version) Author: Amit Khandekar <amitdkhan.pg@gmail.com> (in an older version) Reviewed-by: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com> Reviewed-by: Andres Freund <andres@anarazel.de> Reviewed-by: Robert Haas <robertmhaas@gmail.com> Reviewed-by: Fabrízio de Royes Mello <fabriziomello@gmail.com> Reviewed-by: Melanie Plageman <melanieplageman@gmail.com> |
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