Dear hackers, My PoC does not read and copy logical mappings files to new node, but I did not analyzed in detail whether it is correct. Now I have done this and considered that they do not have to be copied because transactions which executed at the same time as rewriting are no longer decoded. How do you think? Followings my analysis.
## What is logical mappings files? Logical mappings file is used to track the system catalogs while logical decoding even if its heap file is written. Sometimes catalog heaps files are modified, or completely replaced to new files via VACUUM FULL or CLUSTER, but reorder buffer cannot not track new one as-is. Mappings files allow to do them. The file contains key-value relations for old-to-new tuples. Also, the name of files contains the LSN where the triggered event is happen. Mappings files are needed when transactions which modify catalogs are decoded. If the LSN of files are older than restart_lsn, they are no longer needed then removed. Please see CheckPointLogicalRewriteHeap(). ## Is it needed? I think this is not needed. Currently pg_upgrade dumps important information from old publisher and then execute pg_create_logical_replication_slot() on new one. Apart from pg_copy_logical_replication_slot(), retart_lsn and confirmed_flush_lsn for old slot is not taken over to the new slot. They are recalculated on new node while creating. This means that transactions which have modified catalog heaps on the old publisher are no longer decoded on new publisher. Therefore, the mappings files on old publisher are not needed for new one. Best Regards, Hayato Kuroda FUJITSU LIMITED
