Database / Supabase Intermediate to Advanced Interview Questions

When should you use a read replica versus scaling the primary database's resources?

Scaling the primary (more CPU, RAM, or storage) helps when both reads and writes are the bottleneck, or when the workload doesn't tolerate any replication lag — for example, a checkout flow that must read back data it just wrote. It's also the simpler change, since it doesn't introduce a second connection target for your application to reason about.

A read replica makes more sense when reads vastly outnumber writes and the write path itself isn't the constraint — a content site serving thousands of read-only page views per write, or an internal analytics dashboard hammering the same tables users are actively writing to. Routing that read traffic to a replica keeps it from competing with production writes for the primary's resources, at the cost of accepting eventual consistency for anything read from the replica.

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When is scaling the primary the better choice over adding a replica? When you need to store more images

✗ Try again, that's not it.

When writes are also a bottleneck or reads can't tolerate any lag

✓ Correct! Well done.

When you want to disable Row Level Security

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When you need a new API key

✗ Try again, that's not it.

What's the trade-off of routing read traffic to a replica? Doubling the cost of every write

✗ Try again, that's not it.

Accepting eventual consistency due to replication lag

✓ Correct! Well done.

Disabling Realtime entirely

✗ Try again, that's not it.

Losing the ability to run any SELECT queries

✗ Try again, that's not it.

More Related questions...

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