On Wed, Jun 6, 2012 at 3:07 PM, Andres Freund <and...@2ndquadrant.com> wrote: > Hm. For a short while I thought there would be an issue with heap_delete and > IOS because the deleting transaction can commit without any barriers happening > on the IOS side. But that only seems to be possible with non MVCC snapshots > which are currently not allowed with index only scans.
Well, one, commits are irrelevant; the page ceases to be all-visible as soon as the delete happens. And two, you can't do a delete or a commit without a memory barrier - every LWLockAcquire() or LWLockRelease() is a full barrier, so any operation that requires a buffer content lock is both preceded and followed by a full barrier. Proposed patch attached. This adds some more comments in various places, and implements your suggestion of retesting the visibility-map bit when we detect a possible mismatch with the page-level bit. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company
vm-test-cleanup.patch
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