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Diffstat (limited to 'src/backend/access/nbtree/README')
| -rw-r--r-- | src/backend/access/nbtree/README | 29 |
1 files changed, 28 insertions, 1 deletions
diff --git a/src/backend/access/nbtree/README b/src/backend/access/nbtree/README index 9fe84e320e2..e53315a83fb 100644 --- a/src/backend/access/nbtree/README +++ b/src/backend/access/nbtree/README @@ -1,4 +1,4 @@ -$PostgreSQL: pgsql/src/backend/access/nbtree/README,v 1.20 2008/03/21 13:23:27 momjian Exp $ +$PostgreSQL: pgsql/src/backend/access/nbtree/README,v 1.21 2009/12/19 01:32:32 sriggs Exp $ Btree Indexing ============== @@ -401,6 +401,33 @@ of the WAL entry.) If the parent page becomes half-dead but is not immediately deleted due to a subsequent crash, there is no loss of consistency, and the empty page will be picked up by the next VACUUM. +Scans during Recovery +--------------------- + +The btree index type can be safely used during recovery. During recovery +we have at most one writer and potentially many readers. In that +situation the locking requirements can be relaxed and we do not need +double locking during block splits. Each WAL record makes changes to a +single level of the btree using the correct locking sequence and so +is safe for concurrent readers. Some readers may observe a block split +in progress as they descend the tree, but they will simply move right +onto the correct page. + +During recovery all index scans start with ignore_killed_tuples = false +and we never set kill_prior_tuple. We do this because the oldest xmin +on the standby server can be older than the oldest xmin on the master +server, which means tuples can be marked as killed even when they are +still visible on the standby. We don't WAL log tuple killed bits, but +they can still appear in the standby because of full page writes. So +we must always ignore them in standby, and that means it's not worth +setting them either. + +Note that we talk about scans that are started during recovery. We go to +a little trouble to allow a scan to start during recovery and end during +normal running after recovery has completed. This is a key capability +because it allows running applications to continue while the standby +changes state into a normally running server. + Other Things That Are Handy to Know ----------------------------------- |