Avoid pin scan for replay of XLOG_BTREE_VACUUM in all cases

Replay of XLOG_BTREE_VACUUM during Hot Standby was previously thought to
require complex interlocking that matched the requirements on the
master. This required an O(N) operation that became a significant
problem with large indexes, causing replication delays of seconds or in
some cases minutes while the XLOG_BTREE_VACUUM was replayed.

This commit skips the “pin scan” that was previously required, by
observing in detail when and how it is safe to do so, with full
documentation. The pin scan is skipped only in replay; the VACUUM code
path on master is not touched here.

No tests included. Manual tests using an additional patch to view WAL records
and their timing have shown the change in WAL records and their handling has
successfully reduced replication delay.

This is a back-patch of commits 687f2cd7a015, 3e4b7d87988f, b60284261375
by Simon Riggs, to branches 9.4 and 9.5.  No further backpatch is
possible because this depends on catalog scans being MVCC.  I (Álvaro)
additionally updated a slight problem in the README, which explains why
this touches the 9.6 and master branches.

diff --git a/src/backend/access/nbtree/README b/src/backend/access/nbtree/README
index 067d15c803916e3524e992dbbf4dbf2e40559fac..a3f11da8d5aa7242060086cc747eb0cf5d5cbbcb 100644 (file)
--- a/src/backend/access/nbtree/README
+++ b/src/backend/access/nbtree/README
@@ -521,11 +521,12 @@ because it allows running applications to continue while the standby
 changes state into a normally running server.
 
 The interlocking required to avoid returning incorrect results from
-MVCC scans is not required on standby nodes. That is because
+non-MVCC scans is not required on standby nodes. That is because
 HeapTupleSatisfiesUpdate(), HeapTupleSatisfiesSelf(),
 HeapTupleSatisfiesDirty() and HeapTupleSatisfiesVacuum() are only
 ever used during write transactions, which cannot exist on the standby.
-This leaves HeapTupleSatisfiesMVCC() and HeapTupleSatisfiesToast().
+MVCC scans are already protected by definition, so HeapTupleSatisfiesMVCC()
+is not a problem.  That leaves concern only for HeapTupleSatisfiesToast().
 HeapTupleSatisfiesToast() doesn't use MVCC semantics, though that's
 because it doesn't need to - if the main heap row is visible then the
 toast rows will also be visible. So as long as we follow a toast