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Intermediate Topic 20 of 51 Databases

Indexes

Auxiliary structures speeding up lookups.

How it works

An index is an auxiliary data structure, most often a B-tree, that lets the database find rows without scanning the entire table, much like a book index. Indexes can be on one or several columns, unique, partial, covering with included columns, or special types such as hash, GIN, GiST and full-text indexes. They speed up lookups, joins, sorting and constraints, but each index uses storage and must be updated on every write.

Step-by-step walkthrough

  1. Without an index the database scans every row.
  2. An index is a sorted structure like a B-tree.
  3. The query searches the index in O(log n) steps.
  4. It jumps straight to the matching rows.
  5. Indexes speed reads but slow writes and use space.

Where it is used

Indexing foreign keys, columns used in WHERE and JOIN, and sort columns turns slow table scans into fast lookups for web apps and reports.

Common pitfall

Too many indexes slow inserts and updates and waste space, while missing ones cause full scans. Column order in a composite index matters, since it supports leftmost prefixes only. Functions on indexed columns or low-selectivity columns may make them unusable or useless. Verify with execution plans and drop unused indexes.

Where it fits in the course

Indexes is topic 20 of 51 in Databases, rated Intermediate. It follows Views. The next topic is Normalization.

Practice and explore

How to study this topic

  1. Read the explanation above and restate the definition in your own words.
  2. Open the interactive lesson and answer its mini quiz.
  3. Read the common pitfall, then write a short example that avoids it.
  4. Check your understanding in the Databases quiz, and revisit the topic if you miss a question.

Frequently asked questions

What is Indexes?
Auxiliary structures speeding up lookups. It belongs to Databases and is an intermediate-level topic.
Where do people use Indexes?
Indexing foreign keys, columns used in WHERE and JOIN, and sort columns turns slow table scans into fast lookups for web apps and reports.
What is the most common mistake with Indexes?
Too many indexes slow inserts and updates and waste space, while missing ones cause full scans.
What should I learn before this topic?
Start with Views, the topic just before it in Databases.
How can I practice it?
Answer the mini quiz in the interactive lesson, then take the Databases quiz.
Is Indexes hard to learn?
It is an Intermediate topic. Be comfortable with the Beginner topics in this area first, then read the common pitfall carefully, since it is where most mistakes happen.
How long does it take to learn Indexes?
Most learners can read the explanation and finish the interactive lesson in under an hour. Fluency comes from applying the idea, so plan a few short practice sessions with the quiz.
Which topics are related to Indexes?
Views, Normalization, Subqueries are closely connected. The full list is in the related topics section of this page.
What should I study after Indexes?
Continue with Normalization, the next topic in Databases.

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