select for update skip locked hibernate tutorial

If you explicitly request a SERIALIZABLE READ ONLY DEFERRABLE transaction, it will block until it can establish this fact. In fact, READ ONLY transactions will often be able to establish that fact at startup and avoid taking any predicate locks. A READ ONLY transaction may be able to release its SIRead locks before completion, if it detects that no conflicts can still occur which could lead to a serialization anomaly.

The particular locks acquired during execution of a query will depend on the plan used by the query, and multiple finer-grained locks (e.g., tuple locks) may be combined into fewer coarser-grained locks (e.g., page locks) during the course of the transaction to prevent exhaustion of the memory used to track the locks. These will show up in the pg_locks system view with a mode of SIReadLock. Predicate locks in PostgreSQL, like in most other database systems, are based on data actually accessed by a transaction. In the case of SELECT FOR UPDATE and SELECT FOR SHARE, this means it is the updated version of the row that is locked and returned to the client. If so, the second updater proceeds with its operation using the updated version of the row. The search condition of the command (the WHERE clause) is re-evaluated to see if the updated version of the row still matches the search condition. If the first updater commits, the second updater will ignore the row if the first updater deleted it, otherwise it will attempt to apply its operation to the updated version of the row. If the first updater rolls back, then its effects are negated and the second updater can proceed with updating the originally found row. In this case, the would-be updater will wait for the first updating transaction to commit or roll back (if it is still in progress).

However, such a target row might have already been updated (or deleted or locked) by another concurrent transaction by the time it is found. UPDATE, DELETE, SELECT FOR UPDATE, and SELECT FOR SHARE commands behave the same as SELECT in terms of searching for target rows: they will only find target rows that were committed as of the command start time.

Select for update skip locked hibernate tutorial