Database / Supabase Intermediate to Advanced Interview Questions

What is the difference between a read replica and the primary database in a Supabase project?

The primary database is the single instance that accepts writes (INSERT, UPDATE, DELETE); a read replica is a continuously updated copy that only serves read (SELECT) queries, kept in sync via Postgres streaming replication.

Because replication is asynchronous, a replica can lag the primary by a small amount, meaning a read issued immediately after a write on the primary might not yet reflect that change if it hits a replica. Read replicas exist to offload read-heavy traffic (dashboards, reporting queries, high-traffic public pages) away from the primary, and can also be placed in a different region to reduce read latency for geographically distributed users.

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What type of query does a read replica handle? Read-only SELECT queries

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Write queries only

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Realtime WebSocket connections only

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Schema migrations only

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Why might a read immediately after a write not reflect that change on a replica? Reads are cached forever

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Replication is asynchronous and can lag slightly behind the primary

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Replicas never receive any data

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Writes are blocked on the primary until replicas catch up

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