On 24/03/2019 18:50, Andrey Borodin wrote:
I was working on new version of gist check in amcheck and understand one more thing:/* Can this page be recycled yet? */ bool gistPageRecyclable(Page page) { return PageIsNew(page) || (GistPageIsDeleted(page) && TransactionIdPrecedes(GistPageGetDeleteXid(page), RecentGlobalXmin)); } Here RecentGlobalXmin can wraparound and page will become unrecyclable for half of xid cycle. Can we prevent it by resetting PageDeleteXid to InvalidTransactionId before doing RecordFreeIndexPage()? (Seems like same applies to GIN...)
True, and B-tree has the same issue. I thought I saw a comment somewhere in the B-tree code about that earlier, but now I can't find it. I must've imagined it.
We could reset it, but that would require dirtying the page. That would be just extra I/O overhead, if the page gets reused before XID wraparound. We could avoid that if we stored the full XID+epoch, not just XID. I think we should do that in GiST, at least, where this is new. In the B-tree, it would require some extra code to deal with backwards-compatibility, but maybe it would be worth it even there.
- Heikki
