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Interviews / Databases / What are indexes and how do they work in PostgreSQL?

Questions

What is the difference between `INNER JOIN`, `LEFT JOIN`, and `RIGHT JOIN`?
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What are window functions and how do you use them in PostgreSQL?
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What are common table expressions (CTEs) and how do you use them in PostgreSQL?
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What is the difference between `UNION` and `UNION ALL` in PostgreSQL?
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Your database query is slow. How do you optimize it?
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What are indexes and how do they work in PostgreSQL?
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What are transactions and why are they useful in PostgreSQL?
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How do you set up replication in PostgreSQL?
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How do you do a backup and restore of a PostgreSQL database?
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What is the `JSONB` data type and how do you use it in PostgreSQL?
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How do you do full-text search in PostgreSQL?
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What is database normalization and why is it important?
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What are indexes and how do they work in PostgreSQL?

conceptual Indexing Interactive Quiz Code Examples

The Scenario

You are a backend engineer at an e-commerce company. You are designing a new database schema for a new service. You need to decide which columns to index to optimize the performance of your queries.

The Challenge

Explain what indexes are in PostgreSQL and how they work. What are the different types of indexes, and what are the trade-offs between them?

Wrong Approach

A junior engineer might not be aware of indexes. They might just create a table without any indexes, which would lead to poor performance for large tables.

Right Approach

A senior engineer would know that indexes are a critical part of database performance. They would be able to explain what indexes are and how they work. They would also be able to explain the different types of indexes and would have a clear plan for how to use them to optimize the performance of their queries.

Step 1: Understand What Indexes Are

An index is a data structure that is used to speed up the process of finding rows in a table. It works by creating a copy of the indexed column(s) and storing them in a sorted order. This allows the database to quickly find the rows that match a WHERE clause without having to scan the entire table.

Step 2: The Different Types of Indexes

Index TypeDescription
B-TreeThe default index type in PostgreSQL. It is a self-balancing tree that is good for a wide variety of workloads.
HashCan only handle simple equality comparisons. Not recommended for most use cases.
GINDesigned for indexing composite values, such as arrays and JSONB.
GiSTDesigned for indexing geometric data and full-text search.

Step 3: Choose the Right Tool for the Job

Use CaseRecommended Index Type
Most use casesB-Tree
Indexing an array or JSONB columnGIN
Indexing geometric data or full-text searchGiST

Step 4: Code Examples

Here’s how we can create an index in PostgreSQL:

CREATE INDEX my_index ON my_table (my_column);

The Trade-offs of Using Indexes

ProsCons
Speeds up SELECT queries.Slows down INSERT, UPDATE, and DELETE queries.
Takes up disk space.

Because of these trade-offs, you should only create indexes on the columns that you frequently use in WHERE clauses.

Practice Question

You want to create an index on a JSONB column. Which of the following would be the most appropriate?