Hi, Changeset extraction only works in the context of a single database but has to scan through xlog records from multiple databases. Most records are easy to skip because they contain the database in the relfilenode or are just not interesting for logical replication. The only exception are compact commits. So we have some alternatives: 1) don't do anything, in that case empty transactions will get replayed since the changes themselves will get skipped. 2) Don't use compact commits if wal_level=logical 3) unify compact and non-compact commits, trying to get the normal one smaller.
For 3) I am thinking of using 'xinfo' to store whether we have the other information or not. E.g. if there are subxacts in a compact commit we signal that by the flag 'XACT_COMMIT_CONTAINS_SUBXACTS' and store the number of subxacts after the xlog record. Similarly with relations, invalidation messages and the database id. That should leave compact commits without any subxacts at the former size, and those with at the former size + 4. Normal commits would get smaller in many cases since we don't store the empty fields. I personally think 3) is the best solution, any other opinions? Greetings, Andres Freund -- Andres Freund http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
