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Previously, we relied on HEAP2_NEW_CID records and XACT_INVALIDATION records to know if the transaction has modified the catalog, and that information is not serialized to snapshot. Therefore, after the restart, if the logical decoding decodes only the commit record of the transaction that has actually modified a catalog, we will miss adding its XID to the snapshot. Thus, we will end up looking at catalogs with the wrong snapshot. To fix this problem, this changes the snapshot builder so that it remembers the last-running-xacts list of the decoded RUNNING_XACTS record after restoring the previously serialized snapshot. Then, we mark the transaction as containing catalog changes if it's in the list of initial running transactions and its commit record has XACT_XINFO_HAS_INVALS. To avoid ABI breakage, we store the array of the initial running transactions in the static variables InitialRunningXacts and NInitialRunningXacts, instead of storing those in SnapBuild or ReorderBuffer. This approach has a false positive; we could end up adding the transaction that didn't change catalog to the snapshot since we cannot distinguish whether the transaction has catalog changes only by checking the COMMIT record. It doesn't have the information on which (sub) transaction has catalog changes, and XACT_XINFO_HAS_INVALS doesn't necessarily indicate that the transaction has catalog change. But that won't be a problem since we use snapshot built during decoding only to read system catalogs. On the master branch, we took a more future-proof approach by writing catalog modifying transactions to the serialized snapshot which avoids the above false positive. But we cannot backpatch it because of a change in the SnapBuild. Reported-by: Mike Oh Author: Masahiko Sawada Reviewed-by: Amit Kapila, Shi yu, Takamichi Osumi, Kyotaro Horiguchi, Bertrand Drouvot, Ahsan Hadi Backpatch-through: 10 Discussion: https://postgr.es/m/81D0D8B0-E7C4-4999-B616-1E5004DBDCD2%40amazon.com
The PostgreSQL contrib tree
---------------------------
This subtree contains porting tools, analysis utilities, and plug-in
features that are not part of the core PostgreSQL system, mainly
because they address a limited audience or are too experimental to be
part of the main source tree. This does not preclude their
usefulness.
User documentation for each module appears in the main SGML
documentation.
When building from the source distribution, these modules are not
built automatically, unless you build the "world" target. You can
also build and install them all by running "make all" and "make
install" in this directory; or to build and install just one selected
module, do the same in that module's subdirectory.
Some directories supply new user-defined functions, operators, or
types. To make use of one of these modules, after you have installed
the code you need to register the new SQL objects in the database
system by executing a CREATE EXTENSION command. In a fresh database,
you can simply do
CREATE EXTENSION module_name;
See the PostgreSQL documentation for more information about this
procedure.