About the Object Use Count Cache
The object use count cache, or OUC cache, tracks object access use counts, UDI (UPDATE, DELETE, INSERT) counts, and statistics access counts for DML requests made against Teradata Database.
The system uses the object use count cache information for various purposes, including:
- Query optimization
- Performance monitoring
- Database object optimization
- Statistics management
Because only table-level insert and delete UDI counts are needed for cardinality estimation, Teradata Database records that information in the OUC cache as INTEGER data. Column-level update counts are only needed when the Optimizer is deciding whether a COLLECT STATISTICS request can be skipped because it fails to meet one or more USING thresholds, so column-level update counts are only retrieved when necessary.
When Teradata Database flushes the OUC cache, it writes its data to the dictionary table DBC.ObjectUsage. See Data Dictionary for the definition of the ObjectUsage table.
Populating the Object Use Count Cache
The following Parser components populate the object use count cache with both object use count and UDI data:
|Parser Component||Type of Information Populated in the OUC Cache|
The red tree is the Parser tree output of the Resolver and the Query Rewrite subsystem.
The white tree is the Parser tree output of the Optimizer.
The output of the Dispatcher is the AMP steps.
|Runtime data on the following step-level operations.
Flushing the OUC Cache to Disk
The OUC cache must be flushed periodically to avoid the accumulation of stale use counts in DBC.ObjectUsage. The following criteria trigger a flush of the OUC cache to disk:
- When a specific user-defined time interval is reached.
You can specify a time-based flushing interval using the DBS Control field ObjectUseCountCollectRate. Teradata Database flushes the OUC cache when either of the following events first occurs.
- The specified elapsed time since the last flush of the cache is reached.
- The cache is full.
See Utilities for more information about the ObjectUseCountCollectRate field.
- When a high amount of activity occurs on a set of database objects.
Flushing the cache based on high activity is necessary to avoid accumulating stale counts in DBC.ObjectUsage. Fresh use counts are important for determining when statistics need to be recollected.
High activity cache flushing is particularly useful after bulk load jobs where table cardinalities can change radically in a short period of time. Because of this, Teradata Database always flushes the OUC cache immediately after high activity processes like FastLoad and MultiLoad jobs complete.
- When the Optimizer makes a specific request to flush the cache.
This is referred to as on-demand flushing.