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Dynamic Sampling: Use,Levels,10g-11g behavior,

We all know that Oracle Optimizer use object statistics to determine the cost of accessing that object. What if, if that object has missed statistics or the columns or the

Hence Oracle introduced Dynamic sampling in 9i and has changed significantly through 11g.

Definition:-

Dynamic sampling augments missing or insufficient optimizer statistics. Using dynamic sampling the optimizer can improve plans by making better estimates for predicate selectivity. Dynamic sampling can supplement statistics such as table block counts, applicable index block counts, table cardinalities (estimated number of rows), and relevant join column statistics.

To make use of it:-

Dynamic sampling is enabled in the database by default (level 2). You can disable the feature by setting the initialization parameter OPTIMIZER_DYNAMIC_SAMPLING=0.

Dynamic Sampling Levels:-

The dynamic sampling level controls both when dynamic sampling is triggered, and the size of the sample that the optimizer uses to gather the statistics. You can set the dynamic sampling level using either the OPTIMIZER_DYNAMIC_SAMPLING initialization parameter or a SQL statement hint.

 

Level Description Blocks Sampled
0 Disable Dynamic Sampling 0
1 Perform Dynamic Sampling only if
- A table has missing statistics
- The table is involved in a join (in the same quer block)
- There is no index on this table
- This table has more blocks below the high water mark than will be sample
32
“_optimizer_dyn_
smp_blks”
=32
2 Perform Dynamic Sampling for all tables with missing statistics 64
3 Perform Dynamic Sampling for all tables that:
- Meet level 2 criteria
- – Or use an expression that leads the optimizer to guess (assume default hard-coded selectivity) for example a predicate like SUBSTR(COL, 1, 10) = ‘BLA’
64 (32)
4 Perform Dynamic Sampling for all tables that:
- Meet level 3 criteria
- Or have more than one predicate applied to it (complex predicate / possibly correlated column values)
64 (32)
5 The same as level 4 64
6 The same as level 4 128
7 The same as level 4 256
8 The same as level 4 1024
9 The same as level 4 4096
10 The same as level 4 4294967295

 

Different ways to set Dynamic Sampling?

Source:- http://jonathanlewis.wordpress.com/2010/02/23/dynamic-sampling/

You can set the parameter optimizer_dynamic_sampling to a level between 0 (no sample) and 10 (“sample” whole table).

You can use the dynamic_sampling() hint in two different ways in an SQL statement.

  • If you use the hint in the “cursor-level” form: /*+ dynamic_sampling ({level}) */ e.g. /*+ dynamic_sampling(4) */, this is equivalent to setting the parameter optimizer_dynamic_sampling to that level for the duration of that query.
  • If you use the hint in the “table-level” form /*+ dynamic_sampling({alias} {level}) */ e.g. /*+ dynamic_sampling(ord 4) */ you are directing Oracle to sample a specific table unconditionally. In this form, the sample size for levels 1 to 9 is 2level * “basic sample size” (which is set to 32 blocks by default by parameter _optimizer_dyn_smp_blks.) You can have multiple hints of this form in a single query, one for each table you want sampled.

How to know whether the statement is using Dynamic sampling technique:-

(” – dynamic sampling used for this statement”). in explain plan note.

10g and 11 Behavior

In 10g, the dynamic sampling level will be based on the parameter setting for all types of statements

In 11g, the level of dynamic sampling will be decided by optimizer automatically. This will be useful when you have a complex queries with parallelism enabled and you dont have idea about the statistics of the same where you cannot control it with any other feature like cardinality feedback etc,  But in some cases it may gives you undesired results

I have read somewhere, additional levels of dynamic sampling is possible in 11g, i.e above 10, I will post that reference when I able to catch that.

The above all is for my reference post of all about dynamic sampling from the different sources. hope this helps.

References:-

Permalink to Plan stability through Upgrade to 11G-Why is my plan changed–Auto Adjusted Dynamic

https://blogs.oracle.com/optimizer/entry/dynamic_sampling_and_its_impact_on_the_optimizer

-Thanks

Suresh

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