How does hints work in Oracle?
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.
What is No_merge hint in Oracle?
In other words, the no_merge hint prevents Oracle from re-writing the inner query (the in-line view) and the outer query into a merged view, which may be less efficient that addressing the inner query directly.
What is _optimizer_use_feedback?
_OPTIMIZER_USE_FEEDBACK It controls whether the query optimizer choose to use the feedback’s and produced more execution plan for compare. If it’s set to TRUE (the default), the feature is enabled. The cardinality feedback allows the SQL optimizer to learn from its mistakes.
What is cardinality hint?
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.
How do you reduce cardinality in a query?
Solution 1: Query Parameter Settings 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.
How do you use hint?
Hint sentence example
- Brennan offered a hint of information.
- The kind man’s face held a hint of sympathy.
- I sensed his surprise, and a hint of nervousness.
- If it was so simple, Andre wouldn’t be trying to drop a hint without betraying the confidence of Deidre.
- Landon took the hint and left.
What is cardinality feedback in Oracle?
Cardinality feedback was introduced in Oracle Database 11g Release 2 and was renamed Statistics Feedback in Oracle Database 12c. The purpose of this feature is to automatically improve plans for queries that are executed repeatedly, for which the optimizer does not estimate cardinalities in the plan properly.
What is Optimizer_adaptive_features?
OPTIMIZER_ADAPTIVE_FEATURES enables or disables all of the adaptive optimizer features, including adaptive plan (adaptive join methods and bitmap pruning), automatic re-optimization, SQL plan directives, and adaptive distribution methods.
What is Oracle cardinality?
CARDINALITY returns the number of elements in a nested table. The return type is NUMBER .
What is cardinality in index in Oracle?
Cardinality of an index means the number of distinct values in the column.
How do you handle high cardinality data?
Reducing Cardinality by using a simple Aggregating function Leave instances belonging to a value with high frequency as they are and replace the other instances with a new category which we will call other. Keep adding the frequency of these sorted (descending) unique values until a threshold is reached.
How do you check cardinality in SQL?
There is way to calculate cardinality in SQL statements which is : Select distinct Count(column_name) from Tablename; Definition of Database Cardinality for SQL Statements : The database cardinality is nothing but the uniqueness of values in SQL tables which helps to determine Query plan for performance optimization.
What are examples of hints?
The definition of a hint is a suggestion or small amount of something, or revealing just a bit of information. An example of a hint is someone smiling slightly suggesting that they are very happy. An example of a hint is a hot chocolate with just a slight flavor of chili.
What is cardinality feedback in Oracle Database 11g?
In Oracle Database 11g Cardinality Feedback is limited to just to individual index/table cardinalities and group by estimates. It’s possible that correcting only these estimates may not result in a better plan if the issue is in fact the join order or the join methods chosen.
Does Oracle use opt_estimate hint when invoking cardinality feedback?
A 10053 SQL trace shows that Oracle uses opt_estimate hint when invoking cardinality feedback. Cardinality feedback is controlled by a hidden parameter called _optimizer_use_feedback and _optimizer_extended_cursor_sharing_rel. Cardinality feedback can be enabled and disabled at the system or session level with alter system statements:
Can statistics feedback be used for cardinalities other than joins?
However, statistics feedback can still be used for cardinalities elsewhere in the plan. Cardinality produced by joins (in Oracle Database 12c when optimizer adaptive features/statistics = TRUE)
How do I enable or disable cardinality feedback?
Cardinality feedback can be enabled and disabled at the system or session level with alter system statements: The landmark book “Advanced Oracle SQL Tuning The Definitive Reference” is filled with valuable information on Oracle SQL Tuning.