Currently committed semantics
Under currently committed semantics, only committed data is returned to readers. However, readers do not wait for writers to release row locks. Instead, readers return data that is based on the currently committed version of data: that is, the version of the data before the start of the write operation.
Lock timeouts and deadlocks can occur under the cursor stability (CS) isolation level with row-level locking, especially with applications that are not designed to prevent such problems. Some high-throughput database applications cannot tolerate waiting on locks that are issued during transaction processing. Also, some applications cannot tolerate processing uncommitted data but still require non-blocking behavior for read transactions.
Currently committed semantics are turned on by default for new databases. You do not have to make application changes to take advantage of the new behavior. To override the default behavior, Set the cur_commit database configuration parameter to DISABLED. Overriding the behavior might be useful, for example, if applications require the blocking of writers to synchronize internal logic. During database upgrade from V9.5 or earlier, the cur_commit configuration parameter is set to DISABLED to maintain the same behavior as in previous releases. If you want to use currently committed on cursor stability scans, you need to set the cur_commit configuration parameter to ON after the upgrade.
Currently committed semantics apply only to read-only scans that do not involve catalog tables and internal scans that are used to evaluate or enforce constraints. Because currently committed semantics are decided at the scan level, the access plan of a writer might include currently committed scans. For example, the scan for a read-only subquery can involve currently committed semantics.
Because currently committed semantics obey isolation level semantics, applications running under currently committed semantics continue to respect isolation levels.
Currently committed semantics require increased log space for writers. Additional space is required for logging the first update of a data row during a transaction. This data is required for retrieving the currently committed image of the row. Depending on the workload, this can have an insignificant or measurable impact on the total log space used. The requirement for additional log space does not apply when cur_commit database configuration parameter is set to DISABLED.
- The target table object in a section that is to be used for data update or deletion operations does not use currently committed semantics. Rows that are to be modified must be lock protected to ensure that they do not change after they have satisfied any query predicates that are part of the update operation.
- A transaction that makes an uncommitted modification to a row forces the currently committed reader to access appropriate log records to determine the currently committed version of the row. Although log records that are no longer in the log buffer can be physically read, currently committed semantics do not support the retrieval of log files from the log archive. This affects only databases that you configure to use infinite logging.
- The following scans do not use currently committed semantics:
- Catalog table scans
- Scans that are used to enforce referential integrity constraints
- Scans that reference LONG VARCHAR or LONG VARGRAPHIC columns
- Range-clustered table (RCT) scans
- Scans that use spatial or extended indexes
- In a Db2® pureScale® environment, currently committed semantics only apply to lock conflicts between applications running on the same member. If the lock conflict is with an application on a remote member, the requester of the lock waits for the lock to be released before processing the row.
|Step||Application A||Application B|
|1||update T1 set col1 = ? where col2 = ?||update T2 set col1 = ? where col2 = ?|
|2||select col1, col3, col4 from T2 where col2 >= ?||select col1, col5, from T1 where col5 = ? and col2 = ?|
Under currently committed semantics, if one of the applications that is running a query in step 2 requires the data that is being updated by the other application, the first application does not wait for the lock to be released. As a result, a deadlock is impossible. The first application locates and uses the previously committed version of the data instead.