Cardinality in PostgreSQL: The Key to Query Performance Optimization

Cardinality in PostgreSQL and Its Impact on Query Performance

In PostgreSQL, cardinality refers to the estimated number of distinct values in a column or the estimated number of rows returned by a query. Accurate cardinality estimation is crucial for the query optimizer to make informed decisions about query plans and execution strategies. In this explanation, we’ll delve into cardinality in PostgreSQL, its significance, and its impact on query performance, illustrated with a practical real-life dataset example.

Understanding Cardinality

Cardinality estimation is PostgreSQL’s way of approximating the number of unique values or rows that match a specific condition. The query optimizer uses cardinality estimates to choose the most efficient query plan, which includes selecting the appropriate join methods, access methods (e.g., index scans or sequential scans), and join orders.

Factors Affecting Cardinality Estimation

Several factors influence cardinality estimation in PostgreSQL:

  1. Statistics: PostgreSQL maintains statistics for each column in a table. These statistics include the number of distinct values (distinct cardinality) and the total number of rows (total cardinality). Statistics play a pivotal role in estimating cardinality accurately.
  2. Comparison Operators: The type of comparison operator used in a WHERE clause condition (e.g., equality, range, LIKE) affects cardinality estimation differently. PostgreSQL takes into account the selectivity of the operator to estimate cardinality.
  3. Histograms: Histograms provide a more detailed distribution of values in a column. PostgreSQL uses histograms to estimate cardinality, especially when dealing with skewed data distributions.
  4. Join Cardinality: When joining multiple tables, the optimizer estimates join cardinality to determine how many rows will be returned by the join. This estimation considers the cardinality of the joined columns and join conditions.
  5. Aggregate Functions: When aggregate functions like COUNT, SUM, AVG, or DISTINCT are used in queries, cardinality estimation affects the size of the intermediate result sets.

Impact on Query Performance

Accurate cardinality estimation directly impacts query performance. Here’s how:

  1. Query Plan Selection: The query optimizer relies on cardinality estimates to choose the best execution plan. If the estimates are inaccurate, PostgreSQL might select suboptimal plans, leading to slower query execution.
  2. Join Order: In multi-table queries, the optimizer decides the order in which tables are joined based on cardinality estimates. Incorrect estimates can lead to poor join order decisions.
  3. Memory Usage: In-memory sorts and joins rely on cardinality estimates. If the estimates are far from reality, PostgreSQL may allocate too much or too little memory, affecting overall performance.
  4. I/O Operations: Accurate cardinality estimation helps minimize I/O operations. If the estimates are incorrect, PostgreSQL might read more data from disk than necessary, increasing query execution times.

Real-Life Data Set Example

Consider a real-life dataset of an e-commerce platform. You want to retrieve a list of customers who made purchases in the last month. The orders table contains order details, including customer IDs. To estimate the cardinality accurately, PostgreSQL relies on statistics about the customer_id column.

— Query to retrieve customers who made purchases in the last month
SELECT DISTINCT customer_id
FROM orders
WHERE order_date >= NOW() – INTERVAL ‘1 month’;

In this example, PostgreSQL uses cardinality estimation to predict how many distinct customer IDs match the condition. If the cardinality estimate is accurate, the optimizer can choose an efficient query plan, ensuring minimal resource usage and fast query execution.

Conclusion

Cardinality estimation is a critical aspect of PostgreSQL query optimization. Accurate estimates are essential for the query optimizer to make informed decisions, resulting in optimal query performance. Understanding how PostgreSQL estimates cardinality and its impact on query execution is vital for database administrators and developers to fine-tune and optimize their database systems effectively.

About Shiv Iyer 443 Articles
Open Source Database Systems Engineer with a deep understanding of Optimizer Internals, Performance Engineering, Scalability and Data SRE. Shiv currently is the Founder, Investor, Board Member and CEO of multiple Database Systems Infrastructure Operations companies in the Transaction Processing Computing and ColumnStores ecosystem. He is also a frequent speaker in open source software conferences globally.