What is cardinality in Oracle database?
CARDINALITY returns the number of elements in a nested table. The return type is NUMBER .
What is cardinality estimate Oracle?
What is a Cardinality Estimate? A cardinality estimate is the estimated number of rows, the optimizer believes will be returned by a specific operation in the execution plan.
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 cardinality and selectivity in Oracle?
Cardinality = total no of rows * selectivity Selectivity affects the sorting cost. If the returning rows will be so many, it’s sort cost will be high. So it will be considered while creating an execution plan. Cardinality is used to determine to join, sort and filter costs.
What is Explain plan in Oracle?
The EXPLAIN PLAN statement displays execution plans chosen by the Oracle optimizer for SELECT , UPDATE , INSERT , and DELETE statements. A statement’s execution plan is the sequence of operations Oracle performs to run the statement. The row source tree is the core of the execution plan.
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 selectivity and cardinality?
selectivity = number of rows accessed/total number of rows = 10/10 = 1 (100% of the rows were accessed) cardinality = number of rows accessed = 10.
What is low cardinality and high cardinality in Oracle?
High and Low Database Cardinality Definition It’s more common to simply talk about “high” and “low” cardinality. A lot of distinct values is high cardinality; a lot of repeated values is low cardinality.
What is the difference between cost and cardinality in SQL?
Cardinality is the estimated number of rows the step will return. Cost is the estimated amount of work the plan will do. A higher cardinality => you’re going to fetch more rows => you’re going to do more work => the query will take longer.
What is the plan_Table Table?
The PLAN_TABLE is automatically created as a public synonym to a global temporary table. This temporary table holds the output of EXPLAIN PLAN statements for all users. PLAN_TABLE is the default sample output table into which the EXPLAIN PLAN statement inserts rows describing execution plans.
Why does Oracle recommend changing the name of the plan_Table Table?
Oracle recommends that you drop and rebuild your local PLAN_TABLE table after upgrading the version of the database because the columns might change. This can cause scripts to fail or cause TKPROF to fail, if you are specifying the table. If you do not want to use the name PLAN_TABLE, create a new synonym after running the catplan.sql script.
What is EXPLAIN PLAN in Oracle SQL?
Oracle Database SQL Language Reference for the syntax of the EXPLAIN PLAN statement The EXPLAIN PLAN statement displays execution plans chosen by the optimizer for SELECT, UPDATE, INSERT, and DELETE statements. A statement execution plan is the sequence of operations that the database performs to run the statement.