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Chapter 2 of 7
The index
A full scan opened every drawer for one order. The team builds a card index on order number, sorted like a phone book.
This chapter: How does an index find one order in a few page reads?
About this chapter
You will learn
- An index is a sorted copy of a column
- A B-tree lookup reads one page per level
- Why the tree stays shallow
Words you’ll meet
- Index
- A sorted copy of one or more columns, each entry pointing at its row.
- B-tree
- The tree most databases use for indexes: a root page, levels of pages under it, sorted leaves.
- Leaf
- A page at the bottom of a B-tree, holding sorted keys and where each row lives.
Step 1 of 5
An is a sorted copy of one column, each entry pointing at its row. Larkspur’s is a .
Build the order-number index.
The SQL
CREATE INDEX ON orders (id);