Optimizer dynamic sampling
Optimizer dynamic sampling refers to the ability of the SQL optimizer to take a sample of rows from a table to calculate missing statistics. Dynamic sampling can be controlled with the OPTIMIZER_DYNAMIC_SAMPLING parameter or the DYNAMIC_SAMPLING hint.
Dynamic sampling levels
Some of the sampling levels that can be used;
- level 0 - do not use dynamic sampling
- level 1 - Sample all tables that have not been analyzed if the following criteria are met: (1) there is at least 1 unanalyzed table in the query; (2) this unanalyzed table is joined to another table or appears in a subquery or non-mergeable view; (3) this unanalyzed table has no indexes; (4) this unanalyzed table has more blocks than the number of blocks that would be used for dynamic sampling of this table. The number of blocks sampled is the default number of dynamic sampling blocks (32).
- level 2 (default)- Apply dynamic sampling to all unanalyzed tables. The number of blocks sampled is two times the default number of dynamic sampling blocks.
- level 3 - Any statement that meets level 2 criteria and any statement that has one or more expressions used in the where clause predicates e.g. Where substr(CUSTLASTNAME,1,3) or Where a + b =5
- level 4 - Apply dynamic sampling to all tables that meet Level 3 criteria, plus all tables that have single-table predicates that reference 2 or more columns. The number of blocks sampled is the default number of dynamic sampling blocks. For unanalyzed tables, the number of blocks sampled is two times the default number of dynamic sampling blocks.
- levels 5, 6, 7, 8, and 9 - Apply dynamic sampling to all tables that meet the previous level criteria using 2, 4, 8, 32, or 128 times the default number of dynamic sampling blocks respectively.
- level 10 - sample all blocks in the table