Introduction

In the world of databases, cardinality refers to the distinctiveness or uniqueness of the values in a column of a table. It describes the number of unique values in a column and plays a crucial role in database design and query optimization. In this article, we will explore the concept of cardinality and its significance in databases.

How Cardinality is Determined

The cardinality of a column is determined by the number of unique values it contains. A column with high cardinality has a large number of distinct values, while a column with low cardinality has a small number of distinct values. For example, a column representing a unique identifier or a primary key will typically have high cardinality, as each value is expected to be unique. On the other hand, a column representing gender will have low cardinality since it typically contains a limited number of options (e.g., male, female).

Importance of Cardinality

Cardinality is one of the factors that the query optimizer takes into consideration when creating execution plans for queries. It helps the optimizer estimate the selectivity of a query, which is the percentage of rows in a table that match a particular condition. By understanding the cardinality, the optimizer can make informed decisions on the most efficient way to retrieve and join data.

Optimizing Cardinality

To optimize cardinality, it’s important to ensure that the statistics maintained by the database are up-to-date. These statistics include information on the distribution and cardinality of column values. Regularly updating statistics helps the query optimizer make accurate cardinality estimations, leading to better query plans.

In some cases, manual intervention may be required to adjust the cardinality of a column. For example, if the database mistakenly estimates a high cardinality for a column, it may choose a less efficient execution plan. In such cases, using techniques like histogram creation or creating composite indexes can help improve cardinality estimates.

Conclusion

Cardinality is a fundamental concept in database management systems. It provides insight into the uniqueness and distribution of values within a column, helping the query optimizer make informed decisions. By understanding cardinality and optimizing it where necessary, database administrators and developers can improve query performance and overall database efficiency.

#database #cardinality