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Database / Supabase Intermediate to Advanced Interview Questions

What is the difference between a custom access token hook and a raw JWT claim?

A raw JWT claim is simply a key-value pair Supabase Auth includes in every issued token by default — things like the user's ID (sub), role, and expiry, generated the same way for every user with no customization point.

A custom access token hook is a function (Postgres or Edge Function) that Supabase Auth calls right before issuing a token, letting you inject additional claims based on your own data — for example, looking up a user's subscription tier or organization ID and adding it to the JWT so that value is available to Row Level Security policies and the client without an extra database round trip on every request.

What does a custom access token hook let you do that default claims don't?
When is the access token hook invoked?

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More Related questions...

How does Supabase expose custom Postgres functions as callable RPC endpoints? What is the difference between a SECURITY DEFINER and a SECURITY INVOKER Postgres function? How does pg_cron enable scheduled jobs directly inside a Supabase Postgres database? What is the difference between a read replica and the primary database in a Supabase project? When should you use a read replica versus scaling the primary database's resources? What is the difference between daily backups and Point-in-Time Recovery in Supabase? How does Supabase's database branching feature isolate schema changes per git branch? When would you choose a preview branch over testing migrations directly in staging? What is the difference between a custom access token hook and a raw JWT claim? Why should you avoid embedding sensitive business data directly inside JWT claims? How does Supabase support anonymous sign-ins, and how do you convert one to a permanent account? What is the difference between linking an OAuth identity and creating a brand-new user account? How is Storage object access controlled compared to table-level Row Level Security? What is the difference between resumable (TUS) uploads and standard uploads in Supabase Storage? How does Supabase's image transformation feature resize images without a separate CDN service? What is the difference between LISTEN/NOTIFY and Supabase Realtime's Postgres Changes channel? What is the difference between schema-per-tenant and RLS-based multi-tenancy? Why is EXPLAIN ANALYZE useful before optimizing a slow Supabase query? What is the difference between JSONB and a fully normalized relational schema in Postgres? When should you use a JSONB column instead of creating additional relational tables? How does wrapping auth.uid() in a subquery improve Row Level Security performance? Why do we use SECURITY DEFINER functions when RLS would otherwise block a needed operation? How does pg_net let Postgres make asynchronous HTTP calls without blocking a transaction? Explain the lifecycle of a Point-in-Time Recovery (PITR) backup in Supabase? How does a custom access token Auth Hook let you enrich a user's JWT with custom claims? How does Supabase enforce multi-factor authentication (MFA) at the session level? Why doesn't disabling RLS on a table make it invisible in the auto-generated API docs? Explain the internal working of Supabase Vault for storing encrypted secrets in Postgres? Why should you store third-party API keys in Vault instead of a plain table column? When should you choose LISTEN/NOTIFY over Supabase Realtime for internal service communication? How can you optimize a multi-tenant schema design using Row Level Security instead of schema-per-tenant? How do you troubleshoot a Postgres query that performs well in the SQL editor but slowly through the REST API? Explain the execution flow of a private Realtime broadcast channel authorized by Row Level Security? Why do private Realtime channels need their own RLS-style authorization check? How does Supabase's connection string differ between the direct connection, session pooler, and transaction pooler? Why doesn't the transaction pooler support prepared statements the way a direct connection does? How can you optimize zero-downtime schema migrations for a table already receiving production traffic? What happens when a long-running migration locks a table that's still receiving live writes? How does Supabase Studio's SQL editor differ from running migrations through the CLI in a CI pipeline? Which is better for production schema changes, editing directly in Studio or CLI-managed migrations, and why?
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