What is cardinality hint in Oracle?
CARDINALITY(table n): This hint instructs Oracle to use n as the table, rather than rely on its own stats. You may need to use this hint with a global temporary table, for instance.
What is Optimizer hint in Oracle?
An Oracle hint provides directive to the optimizer in choosing an execution plan for the SQL statement being executed. The Oracle INDEX hint instructs the optimizer to use an index scan for the specified table. Note: The optimizer in Oracle 12c can override this type of hint.
What are optimizer hints?
Understanding Optimizer Hints. Hints let you make decisions usually made by the optimizer. As an application designer, you might know information about your data that the optimizer does not know. Hints provide a mechanism to direct the optimizer to choose a certain query execution plan based on the specific criteria.
How do you reduce cardinality in a query?
The easiest and the quickest step you can take to reduce cardinality is to change your query parameter setting. You can reduce the number of possible values in the Page dimension by filtering out dynamic session/customer ID variables in the query parameter settings.
What are optimizer hints give an example of their use?
Hints provide a mechanism to instruct the optimizer to choose a certain query execution plan based on the specific criteria. For example, you might know that a certain index is more selective for certain queries. Based on this information, you might be able to choose a more efficient execution plan than the optimizer.
How do you reduce cardinality?
How is cardinality calculated in Oracle?
The query optimizer will then assume a uniform distribution, so that the cardinality of each operation in the execution plan is calculated by dividing the total number of rows by the number of distinct values after the equality predicate has been applied. The number of rounded up and shown in the column CARDINALITY .
How do you optimize a join in Oracle?
As for simple statements, the optimizer must choose an access path to retrieve data from each table in the join statement. (See “Choosing Access Paths”.) To join each pair of row sources, Oracle must perform one of these operations: nested loops….Join Operations
- Nested Loops Join.
- Sort-Merge Join.
- Cluster Join.
- Hash Join.
When do you use the cardinality hint in Oracle?
The cardinality hint is used in two general cases, complex joins and dynamically created tables like global temporary tables (and possibly using materializations using the WITH clause): Here are some working examples of the cardinality hint:
Why do we need hints in Oracle optimizer?
Hints let you make decisions usually made by the optimizer. As an application designer, you might know information about your data that the optimizer does not know. Hints provide a mechanism to direct the optimizer to choose a certain query execution plan based on the specific criteria.
Is there a way to implement a dynamic cardinality hint without using dynamic SQL?
Dynamic sampling is not working for that. Is there a way to implement a dynamic cardinality hint without using dynamic SQL? It would be good to avoid the parsing step forced by the use of dynamic SQL. but, if you wanted the “dynamic cardinality” to be used – we’d have to hard parse it to see if a different plan were warranted.
Which is the correct way to use hints in Oracle?
The APPENDhint always follows the INSERTkeyword, and the PARALLELhint can follow the INSERTkeyword. The following syntax shows hints contained in both styles of comments that Oracle supports within a statement block. {DELETE|INSERT|MERGE|SELECT|UPDATE} /*+ hint [text] [hint[text]]…