Storage CMSC 461 Michael Wilson

Database storage  At some point, database information must be stored in some format  It’d be impossible to store hundreds of thousands/millions of rows in memory  Numerous ways we could accomplish this  We have to take a few things into consideration

Storage concerns  Insertion efficiency  When dealing with large amounts of data, it will become more and more of a problem to deal with inserting data depending on how you insert  Retrieval efficiency  Similarly, a larger index of data to search will also result in problems  Space  Make sure our data structure doesn’t take up a large amount of disk space

Storage structures  Arrays?  Hash map?

B-tree  Generalization of a binary search tree (BST)  Can have more than two children  Non-leaf nodes have several keys  Each key defines the bounds of the children of a node  num keys = num children – 1  Nodes contain keys and are paired with values  All leaves must be at the same depth

B-tree  Number of possible children in the tree is the order of the tree (Knuth’s definition)  Can have a minimum number of keys that must be in a node  Typically choose the maximum number of keys to be twice the minimum number  This helps with balancing  A number of keys less than the minimum is called an underflow

B-tree  Non-leaf node with 3 children  Non-leaf node has keys k 1 and k 2 such that k 1 < k 2  All keys less than k 1 will be in the child to the left of k 1  All keys in between k 1 and k 2 are in the child between k 1 and k 2  All keys greater than k 2 are in the child to the right of k 2

B-tree example

Insertion  Insert into the most appropriate leaf  If the node isn’t full, no problem – insert in the proper order (ordered keys)  If the node is full, we need to split

Splitting  A node splits when we try to insert a value into it and it is full  Take the list of numbers from the appropriate node and pick a median from that list  Remove it and store it in a value x  Make two new leaf nodes from the existing list  Left node – all values less than x  Right node – all values greater than x  Insert x into the parent node of the two new nodes and attach them appropriately

Splitting note  When inserting into the parent node, the two new child nodes stay at the same level  A B tree only grows in height from the root

Deletion  Deletion is more complicated  Two cases  Deleting from a leaf node  Deleting values from a leaf  Deleting from an internal node  Deleting a separator value

Deleting from a leaf node  If the value can be deleted and the node will not underflow, then delete it  Otherwise, the node is deficient  We must do work to rebalance the tree

Rotation (stealing from your siblings!)  You may remember this from red black trees  Similar, but not quite the same here  If a deficient node has a right sibling and it has keys to spare, rotate left  If a deficient node has a left sibling and it has keys to spare, rotate right

Rotating left  Rotate left  Copy the separator between the deficient node and it’s right sibling to the end of the deficient node  Replace the separator with the lowest value from the right sibling

Rotating right  Rotate right  Copy the separator between the deficient node and it’s left sibling to the end of the deficient node  Replace the separator with the lowest value from the left sibling

Third case  What if neither sibling has keys to spare?  Third case:  We merge two siblings together  Pick a sibling (any sibling!)  Doesn’t matter which  Refer to them as the left node and right node

Merging siblings (stealing from your parents!)  Copy the separator between the two nodes from the parent to the left node  Move all elements from the right node to the left  Remove the separator from the parent and remove the right node  If the parent was the root and it now has no elements, replace the root with the new node that was just created  If the parent is now underflowing, rebalance using this method

Deleting from an internal node (stealing from children!)  This is pretty simple  The value to be deleted is a separator  Pull the highest value from the left child or the lowest value from the right child and replace the separator, deleting it from the child it was taken from