2 posts tagged with "SQL optimization"

In the field of data analysis, correlated scalar subqueries are undoubtedly a double-edged sword: they are powerful, capable of solving many complex problems, while simultaneously presenting significant challenges to database optimizers. Currently, only commercial database giants like Oracle have achieved relatively outstanding performance in this area. Among domestic databases, only PolarDB provides some support for correlated subqueries. This article will not only introduce how PawSQL performs cost-based rewriting optimization for correlated scalar subqueries but also demonstrate how PawSQL identifies and merges multiple similar scalar subqueries in a query to further improve scalar subquery optimization performance. With PawSQL, you can experience Oracle-like rewriting optimization capabilities on databases like MySQL and PostgreSQL.

*## 🌟 Introduction to Correlated Scalar Subqueries

In the world of SQL, correlated scalar subqueries are a powerful tool that allows subqueries to depend on column values from the outer query. This is in stark contrast to non-correlated scalar subqueries that are independent of the outer query. Correlated scalar subqueries calculate results for each row by referencing columns from the outer query.

Example:

SELECT employee_name
FROM employees e
WHERE salary > (SELECT AVG(salary)
                 FROM employees
                 WHERE department_id = e.department_id);

In this example, the subquery calculates the average salary for each department and compares it with the salary in the main query, demonstrating the powerful functionality of correlated scalar subqueries.

🏎️ Challenges and Opportunities: From an Optimizer's Perspective​

Although correlated scalar subqueries are powerful, they present significant challenges to database optimizers:

• Repeated Calculations: On large datasets, subqueries may be recalculated repeatedly, impacting performance.
• High Computational Overhead: Complex calculations, such as aggregate functions, can lead to query performance degradation.
• Query Rewriting Difficulties: Transforming scalar subqueries into join operations or other forms is not always easy.
• Data Dependency: Optimization effectiveness depends on data distribution and table structure, requiring flexible optimizer responses.

For correlated scalar subqueries, while de-correlation does not always perform better than correlated subqueries, cost-based rewriting optimization strategies offer a new perspective. Currently, only a few databases like Oracle and PolarDB have implemented these advanced optimization techniques.

🚀 PawSQL: A New Realm of Correlated Scalar Subquery Optimization​

PawSQL optimizes correlated scalar subqueries through:

1. Cost-Based Rewriting: Supporting de-correlation of scalar subqueries in conditions and select columns.
2. Merge Rewriting: Optimizing multiple structurally similar scalar subqueries.

🎯 Case Study​

Original Query: The original query used two correlated subqueries, calculating the total order price and order count for each customer on a specific date. This structure is typically inefficient, as it requires repeated execution of two subqueries for each customer.

SELECT c_custkey,
  (SELECT SUM(o_totalprice)
    FROM ORDERS
    WHERE o_custkey = c_custkey AND o_orderdate = '2020-04-16') AS total,
  (SELECT COUNT(*)
    FROM ORDERS
    WHERE o_custkey = c_custkey AND o_orderdate = '2020-04-16') AS cnt
FROM CUSTOMER

Rewritten Query: PawSQL's optimization engine merges the two correlated subqueries into a derived table, then associates it with the main query through a left outer join.

SELECT c_custkey, SUM_ AS total, count_ AS cnt
FROM CUSTOMER LEFT OUTER JOIN (
    SELECT o_custkey, SUM(o_totalprice) AS SUM_, COUNT(*) AS count_
    FROM ORDERS
    WHERE o_orderdate = '2020-04-16'
    GROUP BY o_custkey) AS SQ ON o_custkey = c_custkey

Performance Improvement: After optimization, performance is expected to improve by 1,131.26%, a significant enhancement!

Execution Plan Improvements:

• Preliminary aggregation of orders table data significantly reduces the volume of data to be processed
• Eliminates repeated subquery executions by merging two subqueries into one
• Uses hash join to efficiently associate customer and aggregated orders data

This optimization case demonstrates the effectiveness of PawSQL's correlated scalar subquery rewriting technique. By merging multiple correlated subqueries into a single derived table and using an outer join, the optimizer can significantly reduce redundant calculations and data access.

🌐 About PawSQL​

PawSQL is dedicated to automatic and intelligent database performance optimization. The products provided by PawSQL include:

• PawSQL Cloud, an online automated SQL optimization tool that supports SQL auditing, intelligent query rewriting, cost-based index recommendations, suitable for database administrators and data application developers.
• PawSQL Advisor, an IntelliJ plugin that is suitable for data application developers and can be installed via the IDEA/DataGrip marketplace by searching for "PawSQL Advisor" by name.

Contact Us​

Email: service@pawsql.com

Website: https://www.pawsql.com

🌟 Introduction​

In the data-driven modern world, SQL query speed is key to rapid application response. Especially for complex queries involving window functions, performance bottlenecks can become an obstacle without appropriate index support. This article will explore how PawSQL significantly improves SQL query performance for window functions through intelligent index recommendations.

🔍 Case Analysis​

Through a practical case, we will demonstrate how PawSQL optimizes a query containing window functions.

📝 Original Query​

SELECT *
FROM (
    SELECT o.o_custkey, o.o_totalprice,
           RANK() OVER (PARTITION BY o.o_custkey ORDER BY o.o_totalprice) AS rn
    FROM orders AS o
    WHERE o.o_orderdate = '1996-06-20'
) AS A
WHERE A.rn = 1

This query aims to find the lowest order amount for each customer on June 20, 1996.

🎩 PawSQL's Optimization Techniques​

PawSQL conducted an in-depth analysis of the query and proposed the following optimization recommendations:

🌈 Secrets of Performance Improvement​

The optimization details page shows that the indexes recommended by PawSQL can improve query performance by approximately 5181.55%. How is this achieved?

1. Precise Index Matching​

The new index PAWSQL_IDX1878194728 perfectly matches the query requirements:

• o_orderdate as the first column supports rapid data filtering.
• The combination of o_custkey and o_totalprice provides support for window function partitioning and sorting.

2. Avoiding Sorting Operations​

Since the index is already sorted by o_custkey and o_totalprice, the database can directly utilize the index order, eliminating additional sorting steps.

3. The Power of Covering Indexes​

The new index contains all columns required by the query, achieving a "covering index". This means the database can retrieve all data directly from the index without accessing actual data pages, significantly reducing I/O operations.

4. Changes in Execution Plan​

Before Optimization:

• Uses Bitmap index scan and heap scan.
• Requires additional sorting operations.
• Execution time: 22.926 ms

After Optimization:

• Uses index-only scan.
• No additional sorting needed.
• Execution time reduced to 0.452 ms

📚 Best Practices​

1. Regular Analysis: Use PawSQL to periodically analyze your queries, especially complex queries containing window functions.
2. Balance Trade-offs: While the new index improves query performance, it also increases storage overhead and affects write performance. Real-world applications require careful consideration.
3. Remove Redundancy: Promptly clean up old indexes covered by the new index, such as odtidx in this example.
4. Holistic Perspective: Consider the query patterns of the entire application, and don't optimize individual queries at the expense of overall performance.

📈 Conclusion​

PawSQL demonstrates how to significantly improve SQL query performance for window functions through intelligent index recommendations. By creating indexes that precisely match query requirements, we can substantially reduce execution time and improve application response speed. In the big data era, such optimization not only enhances efficiency but also saves valuable computational resources.

Remember, database optimization is an ongoing process. Regularly using tools like PawSQL for analysis and optimization will help keep your application in peak performance condition.

🌐 About PawSQL​

PawSQL is dedicated to automatic and intelligent database performance optimization. The products provided by PawSQL include:

• PawSQL Cloud, an online automated SQL optimization tool that supports SQL auditing, intelligent query rewriting, cost-based index recommendations, suitable for database administrators and data application developers.
• PawSQL Advisor, an IntelliJ plugin that is suitable for data application developers and can be installed via the IDEA/DataGrip marketplace by searching for "PawSQL Advisor" by name.

Contact Us​

Email: service@pawsql.com

Website: https://www.pawsql.com