Skip to content

Database Indexes


Why indexing

  • Faster reads and queries
  • slow queries are a common reason why websites are slow
    • slow websites -> less profit

Analogy for index


like tabs of a nice dictionary

links to the page, has no other info other than the first letter

Key vs Index

Index: data structure

  • keys aren't necessarily indexed
    • example: all the column fields form a key
    • key: list of columns that uniquely identify any row

Primary key is always indexed

What an index looks like



It will only have the data in the index

Downsides of an index

Downside to database indices

  1. Work to maintain the index
    1. reads are slower
    2. or you have to defrag/reindex
  2. CREATE INDEX is expensive
    1. especially on a large table
  3. Extra space
    1. b trees and hash tables
    2. some indexes can be huge, especially if the table is huge

Secondary indexing

Student(year, major)

sort by birthday

useful if your queries are like

FROM Student s
WHERE s.year = 2021 AND major = 'Computer Science'

Not useful if you have a query like

Data not sorted wrt secondary index

so it has to do a sequential scan

FROM Student s
WHERE major = 'Computer Science'

Date indexes

FROM orders
WHERE YEAR(created_at) = 2013;

can't use created_at index because of the index

psql solution

create an index on YEAR(created_at)

non psql solution

CREATE INDEX total_created_at_index ON orders(created_at, total;
-- ...
WHERE created_at BETWEEn January 1, 2013 AND December 31, 2013

Last update: 2022-09-23