包含oraclecardinality的词条

Oracle Cardinality

Introduction:

Oracle Cardinality is an important concept in the Oracle database management system. It refers to the estimated number of rows returned by a query or a specific operation in the execution plan. Understanding the cardinality helps in query optimization and performance tuning.

I. What is Cardinality?

A. Definition:

Cardinality is an estimation of the number of unique values in a column or the number of rows returned by a particular operation.

B. Importance:

Cardinality is a crucial factor in determining the most efficient execution plan for a query. It helps the optimizer select the best access path, join method, and join order, which ultimately affects the overall query performance.

II. Cardinality Estimation Methods:

A. Column Statistics:

1. Oracle maintains statistics on each column, including the number of distinct values and the number of rows in a table.

2. These statistics help in estimating the cardinality of a particular column or a combination of columns involved in the query predicate.

B. Histograms:

1. Histograms provide a more detailed distribution of data within a column.

2. They help in estimating the cardinality when column values are not uniformly distributed.

C. Join Cardinality:

1. Join cardinality estimation involves estimating the number of rows returned after joining two or more tables.

2. Oracle uses various techniques like dynamic sampling and join selectivity to estimate the join cardinality accurately.

III. Cardinality Feedback:

A. Definition:

Cardinality feedback is a feature introduced in Oracle 11g to improve the cardinality estimation accuracy.

B. How it works:

1. Oracle remembers the actual cardinality of a query after its first execution.

2. If subsequent executions return a different number of rows, Oracle adjusts the cardinality estimate accordingly.

C. Benefits:

1. Cardinality feedback helps Oracle adapt to changing data patterns, improving query execution plans.

2. It reduces the need for manual statistics gathering and query tuning.

IV. Cardinality and Query Performance Optimization:

A. Importance of Accurate Cardinality:

1. Accurate cardinality estimation leads to optimal execution plans, reducing overall response time.

2. Underestimating or overestimating cardinality can lead to inappropriate join methods, buffers, and sorting operations, degrading query performance.

B. Optimizing Cardinality Estimation:

1. Regularly gather and maintain statistics on tables and indexes.

2. Use histograms for columns with skewed or non-uniform data distributions.

3. Monitor and analyze query execution plans to identify and correct inaccuracies in cardinality estimation.

Conclusion:

Oracle Cardinality plays a significant role in query optimization and database performance tuning. By accurately estimating the number of rows returned by a query or an operation, Oracle can select the best execution plan, leading to faster and more efficient query processing. Regularly maintaining statistics, using histograms when necessary, and leveraging cardinality feedback can help improve the accuracy of cardinality estimation and optimize query performance.