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Parallel Server SQL lock hit

Oracle Tips by Burleson Consulting

The Data Warehouse Development Life Cycle

Parallelism And Oracle Data Warehousing


The V$SYSSTAT view can be used to determine whether lock converts are being performed too often. Excessive lock convert rates usually mean there is contention for a common resource within the database. This resource may be a commonly updated table. For example, inventory management systems often utilize one-of-a-kind (OOAK) rows. An OOAK row may be used to keep the order number of the last order, and all application tasks must increment this row when a new order is placed. This type of architecture forces each parallel instance to single-thread all requests for this resource. But how do we identify these types of database resources?

Just as the buffer hit ratio measures contention for data blocks, the lock hit ratio can be used to identify excessive lock conversion by the DLM. The lock hit ratio should generally be above 90 percent, and if it falls below 90 percent, you should look for sources of data contention. Here is the SQL to determine the lock hit ratio for Oracle parallel server:

(a.value - b.value)/(a.value)
v$sysstat a, v$sysstat b
SEE CODE DEPOT FOR FULL SCRIPT = ‘global lock converts (async)’;

This is an excerpt from "High Performance Data Warehousing", copyright 1997.

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