On Mon, Dec 2, 2024 at 12:11 PM Tom Lane <t...@sss.pgh.pa.us> wrote: > > Freezing a page, and setting a page all-visible are orthogonal. > > Sorry, sloppy wording on my part.
Freezing doesn't affect the contents of the visibility map in any way that seems relevant. The executor only cares about the all-visible bit (and never the all-frozen bit), and the rules around when and how VACUUM sets the all-visible bit (and how everybody else unsets the all-visible bit) haven't changed in forever. So I just can't see it. I guess it's natural to suspect more recent work -- commit 7c70996e is about 6 years old. But I the race condition that I suspect is at play here is very narrow. It's pretty unlikely that there'll be a dead-to-all TID returned to a scan (not just dead to our MVCC snapshot, dead to everybody's) that is subsequently concurrently removed from the index, and then set LP_UNUSED in the heap. It's probably impossible if you don't have a small table -- VACUUM just isn't going to be fast enough to get to the leaf page after the bitmap index scan, but still be able to get to the heap before its corresponding bitmap heap scan (that uses the VM as an optimization) can do the relevant visibility checks (while it could happen with a large table and a slow bitmap scan, the chances of the VACUUM being precisely aligned with the bitmap scan, in just the wrong way, seem remote in the extreme). Finally, none of this will happen if some other factor hinders VACUUM from setting the relevant heap page all-visible. AFAICT this is only a problem because of the involvement of the VM, specifically -- an MVCC snapshot *is* generally sufficient to make bitmap index scans safe from the dangers of concurrent TID recycling, as explained in "62.4. Index Locking Considerations". That only ceases to be true when the visibility map becomes involved (the VM lacks the granular visibility information required to make all this safe). This is essentially the same VM race issue that nbtree's _bt_drop_lock_and_maybe_pin protects against during conventional index-only scans. -- Peter Geoghegan
