B-Tree
Main Memory Secondary Memory ( RAM ) ( disks ) • B-Tree : • 性質： • 典型之 B-Tree 運用情況： 在 B-Tree 中若 node x 有 x.n keys, 則 x 含有 x.n+1 個 children. M D H Q T X B C J K L F G N P R S Y Z V W 資料量龐大無法全部存在 main memory. 處理 B-Tree 之 algorithm 只將部分之資料 copy 至 main memory.
B-Tree T 之定義： T：rooted tree 並滿足下列： 1. 對每一 node x 含下列之 fields • x.n：node x 所含之 key 數. • 依遞增排列. • x.leaf 為 true 若 x 為 leaf; false 若 x 不為 leaf. 2. 若 x 為一 internal node 則 x 有 x.n+1 個 pointers x.C1 , x.C2 ,… , x.Cx.n+1指向某 children. 若 x 為一 leaf. 其 x.Ci沒有定義. 3.若 x.keyi為 x.Ci所指之 subtree 中任一 key 則 x key1 key2 x.C3 x.C1 x.C2 K2 K3 K1
4. 每一 leaf 之高度相同. 5. t：minimum degree of the B-tree. a. node( 除 root ) 有 internal node ( 除 root ) 有 若 則 root 至少有一 key. b. node( 至多 ) 有 internal node ( 除 root ) 有 a node is FULL, if it contains exactly 2t-1 keys.
If , then for any n-key B-tree T of height h and minimum degree • Thm： Proof： 1 1 2 t-1 t-1 t t 2t t-1 t-1 t-1 t-1 t t t t 2t2
convention： • Root of the B-tree is always in main memory. • Any nodes that are passed as parameters must already have had a DISK_READ operation performed on them. • Operations： • Searching a B-Tree. • Creating an empty B-tree. • Splitting a node in a B-tree. • Inserting a key into a B-tree. • Deleting a key from a B-tree.
B-Tree-Search(x,k)： • Algorithm： • Total CPU time： B-Tree-Search(x,k) { }
B-Tree-Create(T)： • Algorithm： • time： B-Tree-Create(T) { }
P Q R T U V T3 T4 T1 T2 T7 T8 T5 T6 • B-Tree-Split-Child(x,i)： • Splitting a node in a B-Tree： Keyi-1[x] Keyi [x] x t=4 … N S W … … N W W … y=Ci[x] z=Ci+1[x] y=Ci[x] P Q R S T U V full T3 T4 T7 T8 T1 T2 T5 T6 Splitting a full node y ( have 2t-1 keys ) around its median key y.keyt into 2 nodes having (t-1) keys each.
Algorithm： B-Tree-Split-Child(x, i) • z=Allocate-Node() • y=x.Ci • z.leaf = y.leaf • z.n = t-1 • for j=1 to t-1 z.keyj= y.keyj+t • if not y.leaf • for j= 1 to t z.Cj=y.Cj+t • y.n = t-1 • for j = x.n +1 downto i+1 x.Cj+1=x.Cj • x.Cj+1 = z • for j = x.n down to i x.keyj+1= x.keyj • x.keyi=y.keyt • x.n=x.n+1 • DISK-WRITE(y); DISK-WRITE(z); DISK-WRITE(x);
A D F L N P T3 T4 T1 T2 T7 T8 T5 T6 • B-Tree-Insert(T,k)： • Insert a key in a B-Tree： Root[T] t=4 S Root[T] H r r A D F H L N P T3 T4 T7 T8 T1 T2 T5 T6
B-Tree-Insert(T, k) • r=T.root • if r.n == 2t-1 • s=Allocate-Node() • T.root = s • s.leaf = FLASE • s.n = 0 • s.c1 = r • B-Tree-Split-Child(s, 1) • B-Tree-Insert-Nonfull(s. k) • else B-Tree-Insert-Nonfull(r. k)
B-Tree-Insert-Nonfull(x, k) • i = x.n • if x.leaf • While i >=1 and k < x.keyi • x.keyi+1 =x.keyi • i=i-1 • x.keyi+1 = k • x.n = x.n + 1 • DISK-Write(x) • else while i >=1 and k < x.keyi • i = i-1 • i = i+1 • DISKRead(x.Ci) • if x.ci.n == 2t-1 • B-Tree-Split-Child(x, i) • if k > x.keyi • i = i+1 • B-Tree-Insert-Nonfull(x.Ci, k)
t=3 (a) Initial tree • Example：Inserting keys into a B-Tree. G M P X A C D E J K N O R S T U V Y Z (b) B inserted G M P X G M P T X A B C D E J K N O R S T U V Y Z R S U V (c) Q inserted G M P T X A B C D E J K N O Q R S U V Y Z
(d) L insert P G M T X A B C D E J K L N O Q R S U V Y Z (e) F insert P C G M T X A B J K L N O Q R S U V Y Z D E F
( x has t keys ) x • Deleting a key from a B-Tree： 1. K is in x and x is a leaf： 2. K is in x and x is an internal node： a. b. c. if both y,z has t-1 keys. Merge y,z and k into y. Recursively delete k from y. K delete k from x. x Recursively delete k’ and replace k by k’ in x. K y k’ x K z k’ x K z y t-1 t-1 2t-1 K
x Ci[x] 3. If K is not in internal node x： a. If Ci[x] has only t-1 keys but has a sibling with t keys b. If Ci[x] and all of Ci[x]’s siblings have t-1 keys, merge ci with one sibling. k is in this subtree. x • Move a key from x down to Ci[x]. • Move a key from Ci[x]’s sibling to x. • Move an appropriate child to Ci[x] from its sibling. Ci[x] t-1 t x 0 Ci[x] t-1 keys t-1 keys 2t-1 keys 0
t=3 (a) Initial tree P • Example：Deleting a key from a B-Tree. C G M T X A B J K L N O Q R S U V Y Z D E F (b) F delete：case 1 P C G M T X A B J K L N O Q R S U V Y Z D E (c) M delete：case 2a P C G L T X A B J K N O Q R S U V Y Z D E
(d) G deleted：case 2c P C L T X A B N O D E J K Q R S U V Y Z (e) D deleted：case 3b C L P T X N O Q R S U V Y Z A B E J K (e’) tree shrinks in height C L P T X N O Q R S U V Y Z A B E J K (f) B delete：case 3a E L P T X N O Q R S U V Y Z A C J K
