On Fri, Jan 07, 2022 at 03:53:51PM +0000, Finnerty, Jim wrote: > I'd still like a plan to retire the "double xmax" representation eventually. > Previously I suggested that this could be done as a post-process, before > upgrade is complete, but that could potentially make upgrade very slow. > > Another way to retire the "double xmax" representation eventually could be to > disallow "double xmax" pages in subsequent major version upgrades (e.g. to > PG16, if "double xmax" pages are introduced in PG15). This gives the luxury > of time after a fast upgrade to convert all pages to contain the epochs, > while still providing a path to more maintainable code in the future.
Yes, but how are you planning to rewrite it? Is vacuum enough? I suppose it'd need FREEZE + DISABLE_PAGE_SKIPPING ? This would preclude upgrading "across" v15. Maybe that'd be okay, but it'd be a new and atypical restriction. How would you enforce that it'd been run on v15 before upgrading to pg16 ? You'd need to track whether vacuum had completed the necessary steps in pg15. I don't think it'd be okay to make pg_upgrade --check to read every tuple. The "keeping track" part is what reminds me of the online checksum patch. It'd be ideal if there were a generic solution to this kind of task, or at least a "model" process to follow. -- Justin
