KITFORMA · EDITORIAL ANSWER
Why can two concurrent check-then-insert requests create duplicate rows?
KitForma editorial guide. A SELECT proving absence is only a momentary observation. Another transaction can insert before the following INSERT.
Step-by-step answer
A SELECT proving absence is only a momentary observation. Another transaction can insert before the following INSERT.
Enforce the business key with a database unique constraint and use an atomic conflict strategy such as PostgreSQL
INSERT ... ON CONFLICT. Test two connections attempting the same key simultaneously. Decide whether a conflict means reuse, update or an error; blindly overwriting existing data is not always correct. The uniqueness rule must match the real domain, including null and case behavior. Application checks can improve messages but should not be the only protection against duplicates.Sources and verification
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