Harnessing the Power of Parallelism: PostgreSQL’s Parallel Labeling for Faster Queries

Parallel Labeling for Functions and Aggregation is a feature in PostgreSQL that allows certain functions and aggregations to be executed in parallel, taking advantage of multiple CPUs and cores to speed up query processing. This feature was introduced in PostgreSQL 9.6 and has been further improved in subsequent versions.

When a query includes a function or an aggregate that is parallel-safe (i.e., it produces the same results when executed in parallel), PostgreSQL can split the work across multiple parallel workers to process the data in parallel. This can significantly reduce the query execution time for large datasets and complex computations.

Here’s an explanation of how Parallel Labeling for Functions and Aggregation works, along with real-life examples and use cases:

  1. Parallel Aggregation: Parallel aggregation allows aggregations like SUMCOUNTAVG, etc., to be processed in parallel. Consider a scenario where you have a large sales table and want to calculate the total sales amount for each product category. With parallel aggregation, PostgreSQL can divide the table into chunks and distribute the computation of each category’s total sales to multiple parallel workers. This can lead to a considerable performance boost, especially for large datasets.


SELECT category, SUM(sales_amount) AS total_sales
FROM sales_table
GROUP BY category;

2. Parallel Functions: Parallel functions enable certain user-defined functions to be executed in parallel. These functions must be marked as parallel-safe and have appropriate implementation to ensure consistent results when executed in parallel. This feature is useful for complex calculations and data processing tasks.

Example: Let’s say you have a custom function that performs sentiment analysis on customer feedback. By enabling parallel execution for this function, you can process multiple feedback entries in parallel, improving overall performance.

3. PostgreSQL can use parallelism for hash join operations, which is particularly beneficial when joining large tables. Parallel hash joins divide the join operation among multiple parallel workers, each handling a portion of the data. This can significantly speed up join operations for large datasets.


FROM orders
JOIN customers ON orders.customer_id = customers.customer_id;

For large tables like orders and customers, parallel hash join can be used to distribute the join operation across multiple parallel workers, leading to faster query processing.

It’s important to note that enabling parallelism may not always lead to better performance. The decision to use parallelism depends on various factors such as the size of the dataset, hardware capabilities, and query complexity. PostgreSQL’s query planner will determine whether parallel execution is beneficial based on the query and system configuration.

In conclusion, Parallel Labeling for Functions and Aggregation is a powerful feature in PostgreSQL that enables the database to leverage multiple CPUs and cores for faster query execution. By carefully choosing which functions and aggregations can be executed in parallel and optimizing queries accordingly, PostgreSQL can achieve significant performance gains for certain workloads. However, it’s essential to analyze the specific use case and test the performance impact of parallelism before enabling it for a production environment.

About Shiv Iyer 330 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.