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Diffstat (limited to 'src/backend/access/nbtree/README')
| -rw-r--r-- | src/backend/access/nbtree/README | 24 |
1 files changed, 24 insertions, 0 deletions
diff --git a/src/backend/access/nbtree/README b/src/backend/access/nbtree/README index 7055c242d20..3c703948448 100644 --- a/src/backend/access/nbtree/README +++ b/src/backend/access/nbtree/README @@ -520,6 +520,30 @@ 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. +The interlocking required to avoid returning incorrect results from +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(), so +HeapTupleSatisfiesToast() is the only non-MVCC scan type used on standbys. +There is one minor exception, which is that the optimizer sometimes +looks at the boundaries of value ranges using SnapshotDirty, which +could result in returning a newer value for query statistics; this +would affect the query plan in rare cases, but not the correctness. +The risk window is small since the stats look at the min and max values +in the index, so the scan retrieves a tid then immediately uses it +to look in the heap. It is unlikely that the tid could have been +deleted, vacuumed and re-inserted in the time taken to look in the heap +via direct tid access. So we ignore that scan type as a problem. +This means if we re-check the results of any scan of a toast index we +will be able to completely avoid performing the "pin scan" operation +during replay of VACUUM WAL records. + +XXX FIXME: Toast re-checks are not yet added, so we still perform the +pin scan when replaying vacuum records of toast indexes. + + Other Things That Are Handy to Know ----------------------------------- |