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Don Burleson Blog 







Oracle Concurrent statistics gathering

Oracle Database Tips by Donald BurlesonFebruary 20, 2015

Oracle 11g release 2 has introduced a new feature titled "concurrent statistics gathering.  This "concurrent statistics gathering" is recognition of "server consolidation", an industry move to large monolithic servers that contain 32 and 64 CPU's, enough processing horsepower to perform parallelized full-scan operations that are required by the dbms_stats utility.

Concurrent statistics gathering uses dbms_scheduler and advanced queuing in order to achieve high concurrency.  Oracle has consistently enhanced optimizer statistics gathering, introducing new algorithms in 11g to improve the sampling efficiency to increase the quality of the optimizer statistics, thereby improving the execution plans for the SQL workloads.

The dbms_stats utility has had a "degree" parameter for several releases to enable parallelized full-table and full-index scans, plus, dbms_stats has a way to analyze "cascade" an analyze to dependent tables, and analyze an entire schema or database with a single dbms_stats command.

However, this new high-concurrency model allows for dbms_stats to fully-saturate a large server, which is a typical goal during scheduled DBA downtime windows.  The goal is to keep all 64 processors running at full capacity to maximize throughput, and traditionally a DBA would perform "parallel parallelism" with dbms_stats.

Traditional parallel parallelism was done by nohupping multiple dbms_stats jobs for concurrent execution while each dbms_stats job contained the "degree" argument, used to have Oracle invoke parallel query on the target object.

Now with 11g release 2, we have this parallel parallelism functionality built-in to dbms_stats.  Oracle now has a global dbms_stats configuration parameter concurrent=true which enables concurrent optimizer statistics gathering.

exec dbms_stats.set_global_prefs('concurrent','true');

A new method for parallelizing CBO statistics refreshing

Instead of submitting parallel jobs at the OS level (e.g. the nohup command), 11gr2 concurrent optimizer statistics uses the dbms_scheduler mechanism to submit parallel jobs to perform the concurrent analysis.

Oracle has enhanced the dba views to allow you to monitor concurrent statistics gathering:

set lines 200;

   job_class like 'CONC%'
   state = 'RUNNING";

The dba_scheduler_running_jobs view has a new "elapsed time" column that allows you to monitor the elapsed time for all concurrent statistics gathering jobs.

  job_name like 'ST$%'

Note:  It's important to note that the vast majority of OLTP databases will see no major benefits from frequent re-analyze of optimizer statistics because these databases are relative static.  Large tables remain large and the distribution of values within columns remains such that new statistics will not make measurable improvements in SQL performance.

It's also important to note that most shops run multiple workloads, processing short OLTP transactions during the day and running batch-oriented jobs during off-hours.  For these databases, Oracle allows you to import and export optimizer statistics.

For complete information on improving SQL performance with optimizer statistics collection, see the book Advanced Oracle SQL Tuning: The Definitive Reference.

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