How Postgres Unique Constraints Can Cause Deadlock
A recent outage lead me to investigate Postgres unique constraints more deeply. Postgres implements unique constraints by creating a unique index – an index that can only contain unique values.1 It turns out that unique indices and concurrent transactions can interact in nasty and surprising ways. Before I get into the “why”, here are the implications:
When two transactions insert the same value into a unique index, one transaction will wait for the other transaction to finish before proceeding. Moreover, you can cause deadlock using only
- Avoid inserting duplicate keys into tables with unique constraints and relying on the unique constraint to save you. This is especially important if multiple transactions do this concurrently.
- Avoid inserting large numbers of records during the same transaction into a table with a unique constraint, especially if you broke rule...
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