What's the difference between Read Committed and Repeatable Read in practice?

Summary

Read Committed gives each statement a fresh view of committed data; Repeatable Read gives the whole transaction one snapshot. Multi-statement reports can see inconsistent data under Read Committed.

Longer Answer

• Postgres defaults to Read Committed; its Repeatable Read is snapshot isolation and raises serialization errors when a concurrently updated row is written.
• MySQL InnoDB defaults to Repeatable Read; plain reads use a snapshot, but locking reads and UPDATEs see the latest data — so read-modify-write can still lose updates.
• Neither prevents write skew; that needs locks, constraints or SERIALIZABLE.

Use Repeatable Read (read-only) for consistent reports; set isolation explicitly when correctness depends on it.

Key Points

• RC: per-statement snapshot
• RR: per-transaction snapshot
• Different defaults in Postgres and MySQL
• Neither stops write skew

Deep dive: Isolation Levels in Postgres vs MySQL