go/zodb/btree: Draft overview

Very brief and incomplete.

go/zodb/btree/btree.go

@@ -8,6 +8,25 @@
 //
 //	http://btrees.readthedocs.io
 //	https://github.com/zopefoundation/BTrees
+//
+// A B⁺ tree consists of nodes. Only leaf tree nodes point to data.
+// Intermediate tree nodes contains keys and pointer to next-level tree nodes.
+//
+// A well-balanced B⁺ tree always have uniform depth - that is the path to any
+// leaf node from the tree root is the same. However historically ZODB/py does
+// not always balance B⁺ trees well(*), so this property is not preserved.
+// Nevertheless an intermediate B⁺ tree node always has children of the same
+// kind: they are either all leafs or all intermediate nodes(+).
+//
+// BTree and Bucket represent an intermediate and a leaf tree node correspondingly.
+//
+// node.Get(key) performs point-query.
+//
+// --------
+//
+// (*) https://github.com/zopefoundation/ZODB/blob/3.10.7-4-gb8d7a8567/src/BTrees/Development.txt#L211
+//
+// (+) https://github.com/zopefoundation/ZODB/blob/3.10.7-4-gb8d7a8567/src/BTrees/Development.txt#L231
 package btree
 
 //go:generate ./gen-btree IO int32 ziobtree.go