diff options
| -rw-r--r-- | src/backend/access/nbtree/README | 14 | ||||
| -rw-r--r-- | src/backend/access/nbtree/nbtpage.c | 23 |
2 files changed, 20 insertions, 17 deletions
diff --git a/src/backend/access/nbtree/README b/src/backend/access/nbtree/README index 266c1cd4cd9..c5b0a30e4eb 100644 --- a/src/backend/access/nbtree/README +++ b/src/backend/access/nbtree/README @@ -262,11 +262,15 @@ long as we're keeping the leaf locked. The internal pages in the chain cannot acquire new children afterwards either, because the leaf page is marked as half-dead and won't be split. -Removing the downlink to the top of the to-be-deleted chain effectively -transfers the key space to the right sibling for all the intermediate levels -too, in one atomic operation. A concurrent search might still visit the -intermediate pages, but it will move right when it reaches the half-dead page -at the leaf level. +Removing the downlink to the top of the to-be-deleted subtree/chain +effectively transfers the key space to the right sibling for all the +intermediate levels too, in one atomic operation. A concurrent search might +still visit the intermediate pages, but it will move right when it reaches +the half-dead page at the leaf level. In particular, the search will move to +the subtree to the right of the half-dead leaf page/to-be-deleted subtree, +since the half-dead leaf page's right sibling must be a "cousin" page, not a +"true" sibling page (or a second cousin page when the to-be-deleted chain +starts at leaf page's grandparent page, and so on). In the second stage, the topmost page in the chain is unlinked from its siblings, and the half-dead leaf page is updated to point to the next page diff --git a/src/backend/access/nbtree/nbtpage.c b/src/backend/access/nbtree/nbtpage.c index 8ade165f7a4..e7c40cba973 100644 --- a/src/backend/access/nbtree/nbtpage.c +++ b/src/backend/access/nbtree/nbtpage.c @@ -1301,17 +1301,6 @@ _bt_lock_branch_parent(Relation rel, BlockNumber child, BTStack stack, _bt_relbuf(rel, lbuf); } - /* - * Perform the same check on this internal level that - * _bt_mark_page_halfdead performed on the leaf level. - */ - if (_bt_is_page_halfdead(rel, *rightsib)) - { - elog(DEBUG1, "could not delete page %u because its right sibling %u is half-dead", - parent, *rightsib); - return false; - } - return _bt_lock_branch_parent(rel, parent, stack->bts_parent, topparent, topoff, target, rightsib); } @@ -1621,7 +1610,17 @@ _bt_mark_page_halfdead(Relation rel, Buffer leafbuf, BTStack stack) * parent, unless it is the only child --- in which case the parent has to * be deleted too, and the same condition applies recursively to it. We * have to check this condition all the way up before trying to delete, - * and lock the final parent of the to-be-deleted branch. + * and lock the final parent of the to-be-deleted subtree. + * + * However, we won't need to repeat the above _bt_is_page_halfdead() check + * for parent/ancestor pages because of the rightmost restriction. The + * leaf check will apply to a right "cousin" leaf page rather than a + * simple right sibling leaf page in cases where we actually go on to + * perform internal page deletion. The right cousin leaf page is + * representative of the left edge of the subtree to the right of the + * to-be-deleted subtree as a whole. (Besides, internal pages are never + * marked half-dead, so it isn't even possible to directly assess if an + * internal page is part of some other to-be-deleted subtree.) */ rightsib = leafrightsib; target = leafblkno; |