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The term B-tree may refer to a specific design or it may refer to a general class of designs. In the narrow sense, a B-tree stores keys in its internal nodes but need not store those keys in the records at the leaves. The general class includes variations such as the B+ tree, the B * tree and the B *+ tree.
A B+ tree can be viewed as a B-tree in which each node contains only keys (not key–value pairs), and to which an additional level is added at the bottom with linked leaves. The primary value of a B+ tree is in storing data for efficient retrieval in a block-oriented storage context — in particular, filesystems .
This implementation is a hybrid between the basic bitmap index (without compression) and the list of Row Identifiers (RID-list). Overall, the index is organized as a B+tree. When the column cardinality is low, each leaf node of the B-tree would contain long list of RIDs. In this case, it requires less space to represent the RID-lists as bitmaps.
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Certainly a programmer *could* pick U=2L. That type of B-tree is the simplest kind for a teacher to try to explain. But often programmers pick some maximum number of elements U that is is an odd number, such as 2-3 trees, L=2, U=3. Donald Knuth seems to like U = L * 3/2, in the form of a B*-tree.--DavidCary 20:52, 30 July 2005 (UTC)
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