diff options
Diffstat (limited to 'src/backend/access/nbtree/README')
| -rw-r--r-- | src/backend/access/nbtree/README | 11 |
1 files changed, 5 insertions, 6 deletions
diff --git a/src/backend/access/nbtree/README b/src/backend/access/nbtree/README index e53315a83f..57d6308ada 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 1.21 2009/12/19 01:32:32 sriggs Exp $ +$PostgreSQL: pgsql/src/backend/access/nbtree/README,v 1.22 2010/02/08 04:33:53 tgl Exp $ Btree Indexing ============== @@ -171,9 +171,9 @@ We consider deleting an entire page from the btree only when it's become completely empty of items. (Merging partly-full pages would allow better space reuse, but it seems impractical to move existing data items left or right to make this happen --- a scan moving in the opposite direction -might miss the items if so. We could do it during VACUUM FULL, though.) -Also, we *never* delete the rightmost page on a tree level (this -restriction simplifies the traversal algorithms, as explained below). +might miss the items if so.) Also, we *never* delete the rightmost page +on a tree level (this restriction simplifies the traversal algorithms, as +explained below). To delete an empty page, we acquire write lock on its left sibling (if any), the target page itself, the right sibling (there must be one), and @@ -266,8 +266,7 @@ transactions that were running at the time of deletion are dead; 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. (NOTE: VACUUM FULL can reclaim -such pages immediately.) +older than the oldest open transaction. 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 |