On Mon, Jun 26, 2023 at 11:27 PM Andres Freund <and...@anarazel.de> wrote: > On 2023-06-26 21:53:12 -0700, Peter Geoghegan wrote: > > It should be safe to allow searchers to see a version of the root page > > that is out of date. The Lehman & Yao design is very permissive about > > these things. There aren't any special cases where the general rules > > are weakened in some way that might complicate this approach. > > Searchers need to check the high key to determine if they need to move > > right -- same as always. > > Wouldn't we at least need a pin on the root page, or hold a snapshot, to > defend against page deletions?
You need to hold a snapshot to prevent concurrent page recycling -- though not page deletion itself (I did say "anything that you'd usually think of as an interlock"). I'm pretty sure that a concurrent page deletion is possible, even when you hold a pin on the page. (Perhaps not, but if not then it's just an accident -- a side-effect of the interlock that protects against concurrent heap TID recycling.) You can't delete a rightmost page (on any level). Every root page is a rightmost page. So the root would have to be split, and then once again emptied before it could be deleted -- only then would there be a danger of some backend with a locally cached root page having an irredeemably bad picture of what's going on with the index. That's another angle that you could approach the problem from, I suppose. -- Peter Geoghegan
