Source

https://www.postgresql.org/docs/current/indexes.html

TL;DR (by Claude Sonnet 3.5)

• An index in PostgreSQL is a data structure that improves the speed of data retrieval operations on a database table. It works similarly to an index in a book, allowing the database to find data without scanning the entire table.

How do indexes work?

• B-tree structure: By default, PostgreSQL uses B-tree indexes. These are balanced tree structures optimized for a wide range of queries.
• Sorted order: The index keeps data sorted based on the indexed column(s).
• Faster lookups: When a query involves the indexed column, PostgreSQL can use the index to quickly locate the relevant rows instead of scanning the entire table.

Benefits of these specific indexes:

• Faster queries: Queries that filter or join on user_id or provider_user_id will be much quicker.
• Improved performance for foreign key relationships: If user_id is a foreign key to a user's table, joins between oauth_accounts and the user’s table will be more efficient.
• Quicker lookups for OAuth operations: Finding accounts by provider_user_id (which likely stores the user's ID from the OAuth provider) will be faster.

Considerations:

• Write performance: While indexes speed up reads, they can slightly slow down writes (INSERT, UPDATE, DELETE operations) because the index must be updated along with the table.
• Storage space: Indexes require additional disk space.
• Maintenance: PostgreSQL will automatically maintain these indexes, but they may need to be rebuilt occasionally for optimal performance.
• Query planner: PostgreSQL's query planner will decide when to use these indexes based on the specific query and table statistics.

When to use indexes:

• On columns frequently used in WHERE clauses
• On columns used for joining tables