Sunday, November 8, 2009

Dynamic Sampling in Oracle

I started my career with oracle 7.0 and i remember, in oracle7.x/8.x version, optimizer use Rule based optimizer if one or all of the tables does not have any statistics in the join query. In oracle9i R2, dynamic sampling query is introduced and it has ability to sample the table and collect the better statistics dynamically during the hard parse. This sampling takes place only at hard parse time and is used to dynamically generate better statistics for the optimizer to use, hence the name dynamic sampling.

Oracle optimizer use the default statistic values when table does not have any statistics and dynamic sampling query feature is disabled. The default statistics values are well documented in Performance Tuning Guide.

How do we set Dynamic sampling parameter? There are couple of ways, we can set the parameter.

1. We can set this in session level or instance level.
2. Dynamic sampling query hint in query level. (Hint)

Connected to:
Oracle Database 10g Enterprise Edition Release - Production
With the Partitioning, OLAP and Data Mining options

SQL> alter session set OPTIMIZER_DYNAMIC_SAMPLING = 2;

Session altered.


System altered.

SQL> select /*+ dynamic_sampling(emp 0) */ * from scott.emp;

When does dynamic sampling is useful?
1. It is useful when table data is drastically changing and statistic becomes stale very often.
2. Global temporary table is good place to use dynamic sampling feature.

Dynamic sampling Levels? Optimizer Dynamic Sampling has 11 levels between 0 to 10. What are these levels? The answer is pretty straight forward and it is clearly documented in Oracle Performance Tuning Guide. In Oracle9i R2, the default setting for dynamic sampling level is 1. At this setting, the optimizer will tend to use dynamic sampling only in some cases when a table is unanalyzed. This includes the case of global temporary tables without statistics but doesn’t ensure that dynamic sampling will be used against all unanalyzed tables. In Oracle Database 10g Release 1 and above, the default setting is 2. This setting ensures that the optimizer will have a good estimate whenever it optimizes a query that accesses a table with no statistics.

The source of this Article is Asktom. Please read this link and this link has very pretty good information about Dynamic sampling.

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