A B-tree is a data structure that maintains an ordered set of data and allows efficient operations to find, delete, insert, and browse the data. In this discussion, each piece of data stored in a B-tree will be called a "key", because each key is unique and can occur in the B-tree in only one location.
A B-tree consists of "node" records containing the keys, and pointers that link the nodes of the B-tree together.
Every B-tree is of some "order n", meaning nodes contain from n to 2n keys, and nodes are thereby always at least half full of keys. Keys are kept in sorted order within each node. A corresponding list of pointers are effectively interspersed between keys to indicate where to search for a key if it isn't in the current node. A node containing k keys always also contains k+1 pointers.
For example, here is a portion of a B-tree with order 2 (nodes have at least 2 keys and 3 pointers). Nodes are delimited with [square brackets]. The keys are city names, and are kept sorted in each node. On either side of every key are pointers linking the key to subsequent nodes:
Start here | v [ Chicago Hoboken ] | | | +-----------+ | +------------+ | | | v v v [ Aptos Boston ] [ Denver Detroit ] [ San-Jose Seattle ] | | | | | | | | | v v v v v v v v v XTo find the key "Dallas", we begin searching at the top "root" node. "Dallas" is not in the node but sorts between "Chicago" and "Hoboken", so we follow the middle pointer to the next node. Again, "Dallas" is not in the node but sorts before "Denver", so we follow that node's first pointer down to the next node (marked with an "X"). Eventually, we will either locate the key, or encounter a "leaf" node at the bottom level of the B-tree with no pointers to any lower nodes and without the key we want, indicating the key is nowhere in the B-tree.
Below is another fragment of an order 1 B-tree (nodes have at least 1 key and 2 pointers). Searching for the key "Chicago" begins at "Marin", follows the first pointer to "Aptos" (since Chicago sorts before Marin), then follows that node's second pointer down to the next level (since Chicago sorts after Aptos), as marked with an "X".
| v [ Marin ] | | +--+ +---+ | | v v [ Aptos ] [ Seattle ] | | | | v v v v XSearching a B-tree for a key always begins at the root node and follows pointers from node to node until either the key is located or the search fails because a leaf node is reached and there are no more pointers to follow.
B-trees grow when new keys are inserted. Since the root node initially begins with just one key, the root node is a special exception and the only node allowed to have less than n keys in an order n B-tree.
Here is an order 2 B-tree with integer keys. Except for the special root node, order 2 requires every node to have from 2 to 4 keys and 3 to 5 pointers. Empty slots are marked with ".", showing where future keys have not yet been stored in the nodes:
[ 57 . . .] | | +---------------+ +---------------------+ | | v v [ 14 40 . .] [ 72 84 . .] | | | | | | +--------+ | +----------+ +----------+ | +-----------+ | | | | | | v v v v v v [01 12 . .] [15 16 17 .] [47 56 . .] [58 60 61 .] [74 75 76 78] [85 86 99 .]To insert the key "59", we first simply search for that key. If 59 is found, the key is already in the tree and the insertion is superfluous. Otherwise, we must end up at a leaf node at the bottom level of the tree where 59 would be stored. In the above case, the leaf node contains 58, 60, 61, and room for a fourth key, so 59 is simply inserted in the leaf node in sorted order:
[58 59 60 61]Now we'll insert the key "77". The initial search leads us to the leaf node where 77 would be inserted, but the node is already full with 4 keys: 74, 75, 76, and 78. Adding another key would violate the rule that order 2 B-trees can't have more than 4 keys. Because of this "overflow" condition, the leaf node is split into two leaf nodes. The leftmost 2 keys are put in the left node, the rightmost 2 keys are put in the right node, and the middle key is "promoted" by inserting it into the parent node above the leaf. Here, inserting 77 causes the 74-75-76-78 node to be split into two nodes, and 76 is moved up to the parent node that contained 72 and 84:
Before inserting 77 After inserting 77 [ 72 84 . .] [ 72 76 84 .] | | | | | | | -+ | +- --+ | | +-- | | | | +----+ +------+ | | | v v v [74 75 76 78] [74 75 . .] [77 78 . .]In this case, the parent node contained only 2 keys (72 and 84), leaving room for 76 to be promoted and inserted. But if the parent node was also already full with 4 keys, then it too would have to split. Indeed, splitting may propagate all the way up to the root node. When the root splits, the B-tree grows in height by one level, and a new root with a single promoted key is formed. (A situation when an order n root node sometimes has fewer than n keys, just like the situation described earlier when the root node stores the very first key placed in the B-tree.)
B-trees shrink when keys are deleted. To delete a key, first perform the usual search operation to locate the node containing the key. (If the key isn't found, it isn't in the tree and can't be deleted.)
If the found key is not in a leaf, move it to a leaf by swapping the key with the logical "next" key. In a B-tree, the "next" key is always the first key in the leftmost leaf of the right subtree.
For example, in this B-tree we want to delete "37", which is not in a leaf. "xx" indicates key values that don't matter:
[ xx 37 xx xx ] | | +->[ xx xx xx xx ] | | +->[ xx xx xx xx ] | | +->[41 43 . .]We follow the pointer immediately to the right of 37 to find 37's right subtree, then follow the leftmost pointers in each subnode until we reach a leaf. The first key in the leaf is "41", the logical "next" key after 37 in the list of all keys in the tree. By swapping 37 and 41, we can move 37 to a leaf node to set up a deletion without violating the key order or pointer order of the overall B-tree.
Once the key we want is in a leaf, we can delete it. If at least n keys remain in the node, we're done, otherwise it is an "underflow", since every node (except the root) must have at least n keys.
If a node underflows, we may be able to "redistribute" keys by borrowing some from a neighboring node. For example, in the order 3 B-tree below, the key 67 is being deleted, which causes a node to underflow since it only has keys 66 and 88 left. So keys from the neighbor on the left are "shifted through" the parent node and redistributed so both leaf nodes end up with 4 keys:
Before deleting 67 After deleting 67 [ xx 55 xx ] [ xx 33 xx ] | | | | +--------+ +--------+ +--------+ +------+ | | | | v v v v [22 24 26 28 33 44] [66 67 88 . . .] [22 24 26 28 . .] [44 55 66 88 . .]But if the underflow node and the neighbor node have less than 2n keys to redistribute, the two nodes will have to be combined. For example, here key 52 is being deleted from the B-tree below, causing an underflow, and the neighbor node can't afford to give up any keys for redistribution. So one node is discarded, and the parent key moves down with the other keys to fill up a single node:
Before deleting 52 After deleting 52 [ 35 45 55 . ] [ 35 55 . . ] | | | | | | | -+ | | +- -+ | +- | | | +-----+ +---+ | | | | v v v [40 42 . .] [50 52 . .] [40 42 45 50]In the above case, moving the key 45 out of the parent node left two keys (35 and 55) remaining. But if the parent node only had n keys to begin with, then the parent node also would underflow when the parent key was moved down to combine with the leaf key. Indeed, underflow and the combining of nodes may propagate all the way up to the root node. When the root underflows, the B-tree shrinks in height by one level, and the nodes under the old root combine to form a new root.
The payoff of the B-tree insert and delete rules are that B-trees are always "balanced". Searching an unbalanced tree may require traversing an arbitrary and unpredictable number of nodes and pointers.
An unbalanced tree of 4 nodes A balanced tree of 4 nodes [ x x ] [ x x ] | | | | [ x x ] +------+ | +------+ | | | | [ x x ] [ x x ] [ x x ] [ x x ] | [ x x ]Searching a balanced tree means that all leaves are at the same depth. There is no runaway pointer overhead. Indeed, even very large B-trees can guarantee only a small number of nodes must be retrieved to find a given key. For example, a B-tree of 10,000,000 keys with 50 keys per node never needs to retrieve more than 4 nodes to find any key.