diff options
Diffstat (limited to 'src/backend/access/nbtree/README')
| -rw-r--r-- | src/backend/access/nbtree/README | 8 |
1 files changed, 5 insertions, 3 deletions
diff --git a/src/backend/access/nbtree/README b/src/backend/access/nbtree/README index edf33d90b1e..6d438a7662e 100644 --- a/src/backend/access/nbtree/README +++ b/src/backend/access/nbtree/README @@ -261,12 +261,14 @@ we need to be sure we don't miss or re-scan any items. A deleted page can only be reclaimed once there is no scan or search that has a reference to it; until then, it must stay in place with its -right-link undisturbed. We implement this by waiting until all -transactions that were running at the time of deletion are dead; which is +right-link undisturbed. We implement this by waiting until all active +snapshots and registered snapshots as of the deletion are gone; which is overly strong, but is simple to implement within Postgres. When marked dead, a deleted page is labeled with the next-transaction counter value. VACUUM can reclaim the page for re-use when this transaction number is -older than the oldest open transaction. +older than RecentGlobalXmin. As collateral damage, this implementation +also waits for running XIDs with no snapshots and for snapshots taken +until the next transaction to allocate an XID commits. Reclaiming a page doesn't actually change its state on disk --- we simply record it in the shared-memory free space map, from which it will be |