The Leaf Nodes of an Sql Index
The primary purpose of an index is to provide an ordered representation of the indexed data. It is, however, not possible to store the data sequentially because an insert statement would need to move the following entries to make room for the new one. Moving large amounts of data is very time-consuming so the insert statement would be very slow. The solution to the problem is to establish a logical order that is independent of physical order in memory.
The data structure is called a doubly linked list because each node refers to the preceding and the following node. It enables the database to read the index forwards or backwards as needed. It is thus possible to insert new entries without moving large amounts of data—it just needs to change some pointers.
Databases use doubly linked lists to connect the so-called index leaf nodes. Each leaf node is stored in a database block or page; that is, the database’s smallest storage unit. All index blocks are of the same size—typically a few kilobytes.
Unlike the index, the table data is stored in a heap structure and is not sorted at all. There is neither a relationship between the rows stored in the same table block nor is there any connection between the blocks.