Skip rows that cannot be immediately locked. By default, SELECT FOR UPDATE queries on rows that are already locked by an active transaction must wait for the transaction to finish. Wait policies determine how a SELECT FOR UPDATE statement handles conflicts with locks held by other active transactions. Note that in CockroachDB, the FOR NO KEY UPDATE locking strength is identical to the FOR UPDATE locking strength. Lock the rows returned by the SELECT statement, such that other transactions trying to access the rows must wait for the transaction to finish. Note that CockroachDB always ensures serializability, regardless of the specified locking strength. Specifying FOR SHARE/ FOR KEY SHARE does not cause CockroachDB to use shared locks over the rows retrieved by a statement. This syntax is a no-op, allowed for PostgreSQL compatibility. Locking strength dictates the row-level locking behavior on rows retrieved by a SELECT statement. To eliminate the need for application-level retry logic, in addition to SELECT FOR UPDATE your application also needs to use a driver that implements automatic retry handling.ĬockroachDB does not support the FOR SHARE or FOR KEY SHARE locking strengths.įOR NO KEY UPDATE KEY SHARE OF table_name_list SKIP LOCKED NOWAITįor the full SELECT statement syntax documentation, see Selection Queries. These errors can also arise due to time uncertainty. Note that using SELECT FOR UPDATE does not completely eliminate the chance of serialization errors, which use the SQLSTATE error code 40001, and emit error messages with the string restart transaction. By preventing thrashing, CockroachDB also prevents transaction retries that would otherwise occur due to contention.Īs a result, using SELECT FOR UPDATE leads to increased throughput and decreased tail latency for contended operations. These other transactions are effectively put into a queue based on when they tried to read the value of the locked rows.īecause this queueing happens during the read operation, the thrashing that would otherwise occur if multiple concurrently executing transactions attempt to SELECT the same data and then UPDATE the results of that selection is prevented. It works by locking the rows returned by a selection query, such that other transactions trying to access those rows are forced to wait for the transaction that locked the rows to finish. The SELECT FOR UPDATE statement is used to order transactions by controlling concurrent access to one or more rows of a table.
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