How to Avoid Repetitive Sub Queries in PostgreSQL for Efficient Data Retrieval

Discover how to streamline your PostgreSQL queries by eliminating repetitive sub queries, enhancing performance and simplifying your SQL code.
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How to Avoid Repetitive Sub Queries in PostgreSQL for Efficient Data Retrieval

When optimizing SQL queries, particularly in PostgreSQL, repetition can introduce unnecessary complexity and slowdowns. A common issue arises when using sub queries multiple times within a query, which not only clutters the code but also strains performance. In this guide, we will tackle a specific query issue involving repetitive sub queries and demonstrate how to improve it using Common Table Expressions (CTEs).

The Problem

Consider the following PostgreSQL query that fetches video data based on user subscriptions and their personal video history:

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In this query, the repeated sub query fetching video_id from PersonalInfo_history is both repetitive and inefficient. We need to find a way to avoid this redundancy.

The Solution: Using Common Table Expressions (CTEs)

To address this problem, we can leverage CTEs. CTEs allow us to define a temporary result set that can be referenced in subsequent parts of the query. This not only simplifies your code but also enhances query performance by executing the sub query just once.

Step 1: Define the CTE

First, we create a CTE called personal_history that retrieves the video IDs for the specified user:

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Step 2: Rewrite the Main Query Using the CTE

Next, we rewrite the main query to utilize this CTE, making it more concise and efficient:

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Further Optimization with Additional CTEs

For those looking to take it a step further, consider creating another CTE to streamline video data handling. Here's an enhanced version that combines video data retrieval with the history check:

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In this final version, we use a left join to mark whether each video is in the user's personal history (is_in), allowing us to filter more easily while maintaining clear, concise code.

Conclusion

By utilizing Common Table Expressions in PostgreSQL, we can eliminate repetitive sub queries, leading to cleaner, more efficient SQL code. This approach not only simplifies query structure but also enhances performance, making it easier to maintain and understand. If you're grappling with similar issues in your PostgreSQL queries, consider applying these strategies for clearer and more optimized code.