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Locking & Deadlocks
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What does SELECT ... FOR UPDATE do in PostgreSQL?
AIt executes the SELECT as an UPDATE statement and returns the new row values
BIt acquires an exclusive row-level lock on the selected rows, preventing other transactions from modifying or locking them until the current transaction commits or rolls back
CIt creates an immutable snapshot of the rows for use in the current transaction without locking them
DIt marks the rows as read-only for the duration of the current session
What is the optimistic locking pattern using a version column?
AIssuing LOCK TABLE orders IN EXCLUSIVE MODE before every UPDATE statement
BWrapping every read-modify-write cycle in a SERIALIZABLE transaction
CUsing a updated_at TIMESTAMP column and retrying if the timestamp is more recent than expected
DAdding a version INT column; on UPDATE, including WHERE version = <read_version> in the clause and incrementing the version — if 0 rows are affected, a conflict was detected and the operation must be retried
What is the primary use case for SELECT ... FOR UPDATE SKIP LOCKED in PostgreSQL?
ASkipping rows that are locked by the current transaction to avoid self-deadlock
BPermanently skipping locked rows for the duration of the current session
CImplementing a high-throughput job queue where multiple workers concurrently claim and process tasks without blocking each other
DReading rows without acquiring any locks, bypassing isolation guarantees
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