1 files changed, 10 insertions, 4 deletions

diff --git a/src/backend/access/nbtree/README b/src/backend/access/nbtree/README
index 2d0f8f4b79a..216d419841c 100644
--- a/src/backend/access/nbtree/README
+++ b/src/backend/access/nbtree/README
@@ -223,9 +223,10 @@ right to make this happen --- a scan moving in the opposite direction
 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).  Page deletion always begins from an empty leaf page.  An
-internal page can only be deleted as part of a branch leading to a leaf
-page, where each internal page has only one child and that child is also to
-be deleted.
+internal page can only be deleted as part of deleting an entire subtree.
+This is always a "skinny" subtree consisting of a "chain" of internal pages
+plus a single leaf page.  There is one page on each level of the subtree,
+and each level/page covers the same key space.
 
 Deleting a leaf page is a two-stage process.  In the first stage, the page
 is unlinked from its parent, and marked as half-dead.  The parent page must
@@ -243,7 +244,12 @@ it, but it's still linked to its siblings.
 
 (Note: Lanin and Shasha prefer to make the key space move left, but their
 argument for doing so hinges on not having left-links, which we have
-anyway.  So we simplify the algorithm by moving the key space right.)
+anyway.  So we simplify the algorithm by moving the key space right.  This
+is only possible because we don't match on a separator key when ascending
+the tree during a page split, unlike Lehman and Yao/Lanin and Shasha -- it
+doesn't matter if the downlink is re-found in a pivot tuple whose separator
+key does not match the one encountered when inserter initially descended
+the tree.)
 
 To preserve consistency on the parent level, we cannot merge the key space
 of a page into its right sibling unless the right sibling is a child of