Disallow ORDER BY In Aggregate Functions: DataFusion SQL

by SLV Team 57 views

Let's dive into a discussion about a specific feature request concerning Apache DataFusion and its SQL planner. This article will explore the proposal to disallow the use of ORDER BY clauses within the argument lists of ordered-set aggregate functions. We'll break down the problem, the proposed solution, and the reasoning behind it. So, buckle up, data enthusiasts, and let's get started!

The Issue: Unexpected Behavior with ORDER BY in Aggregate Functions

At the heart of this discussion lies the potential for unexpected behavior when using ORDER BY within the argument lists of ordered-set aggregate functions. To illustrate this, consider the following SQL queries:

select quantile_cont(col0, 0.75 order by col0) from values (1, 3), (2, 2), (3, 1) t(col0, col1);
select quantile_cont(0.75 order by col0) from values (1, 3), (2, 2), (3, 1) t(col0, col1);

The first query attempts to calculate the 0.75 quantile of col0, ordering the values by col0 within the quantile_cont function. The second query similarly tries to apply an ORDER BY clause within the function's argument list. The problem? This syntax isn't intended to be supported, and it can lead to confusion and errors.

Currently, the second query fails, but the error message isn't as clear as it could be. It fails because the system can't find a matching function signature, rather than explicitly stating that ORDER BY is disallowed in this context. Ideally, we want the SQL planner to catch these cases earlier in the process and provide a more informative error message, acting as a syntax error.

Why is this important? Explicitly disallowing ORDER BY within the argument lists of ordered-set aggregate functions enhances code clarity and prevents potential misinterpretations. It ensures that users adhere to the intended syntax and avoids unexpected or incorrect results. The goal is to make the SQL planner more robust and user-friendly, guiding users toward correct syntax and preventing errors.

Proposed Solution: Disallowing ORDER BY Syntax

The proposed solution is straightforward: explicitly disallow the ORDER BY syntax within the argument lists of ordered-set aggregate functions. This means that the SQL planner should recognize and reject queries like the ones shown above, raising a syntax error. This approach offers several advantages:

  • Clarity: It clearly communicates to users that this syntax is not supported.
  • Prevention: It prevents unexpected behavior and incorrect results that might arise from misinterpreting the syntax.
  • Early Error Detection: It catches errors early in the planning process, providing faster feedback to users.
  • Consistency: It ensures consistency in how aggregate functions are used, making the language more predictable.

By implementing this restriction, DataFusion can provide a more robust and user-friendly experience for its users. It helps to prevent common mistakes and guides users toward the correct syntax for aggregate functions.

Alternatives Considered: Why Disallowing is the Best Approach

While alternative solutions might exist, disallowing the syntax is considered the most effective approach in this case. One alternative might be to try and interpret the ORDER BY clause within the function, but this could lead to complex and potentially ambiguous behavior. It could also create inconsistencies with how other parts of the SQL language handle ORDER BY clauses.

Another alternative might be to simply ignore the ORDER BY clause and proceed with the function execution. However, this silent failure would be highly undesirable, as it could lead to incorrect results without any warning to the user. It's always better to explicitly reject invalid syntax than to silently ignore it.

Disallowing the syntax provides the clearest and most predictable behavior. It aligns with the intended usage of ordered-set aggregate functions and prevents potential errors and confusion. It ensures that users are using the functions correctly and getting the expected results.

Additional Context: Pull Request #17805

This discussion is related to Pull Request #17805 in the Apache DataFusion repository. Specifically, it stems from a comment made during the review process (issuecomment-3418476797). This pull request and the associated discussion highlight the ongoing efforts to improve DataFusion's SQL planner and ensure its robustness and user-friendliness.

The conversation around this issue demonstrates the importance of careful consideration of syntax and potential ambiguities in SQL. It also underscores the value of community involvement in identifying and addressing these issues.

Diving Deeper into Ordered-Set Aggregate Functions

To fully appreciate the context of this discussion, let's briefly delve into what ordered-set aggregate functions are and how they're typically used. Ordered-set aggregate functions are a special type of aggregate function that operate on a sorted set of input values. This sorting is crucial for the function's calculation, as it determines the order in which the values are processed.

Common examples of ordered-set aggregate functions include:

  • quantile_cont: Calculates the continuous quantile of a set of values.
  • percentile_cont: Another name for quantile_cont, often used interchangeably.
  • median: Calculates the median (50th percentile) of a set of values.

These functions are frequently used in statistical analysis and data reporting to understand the distribution of data and identify key percentile values.

The standard SQL syntax for these functions typically involves specifying the column or expression to be aggregated and, optionally, a percentile value. The sorting of the input values is usually handled outside the function's argument list, using an ORDER BY clause in the main query.

Example of Correct Usage:

SELECT quantile_cont(col0, 0.75) WITHIN GROUP (ORDER BY col0)
FROM values (1, 3), (2, 2), (3, 1) t(col0, col1);

In this example, the ORDER BY clause is correctly placed within the WITHIN GROUP clause, which is the standard way to specify the sorting order for ordered-set aggregate functions. This syntax clearly separates the aggregation logic from the sorting logic, making the query easier to understand and maintain.

Conclusion: Enhancing DataFusion's SQL Planner

In conclusion, the proposal to disallow ORDER BY within the argument lists of ordered-set aggregate functions is a valuable step toward enhancing DataFusion's SQL planner. By explicitly rejecting this syntax, DataFusion can prevent potential errors, improve code clarity, and provide a more consistent and user-friendly experience. This change aligns with the intended usage of these functions and ensures that users are guided toward the correct syntax.

This discussion also highlights the importance of community involvement and careful consideration of SQL syntax. By addressing potential ambiguities and inconsistencies, we can build more robust and reliable data processing systems. So, let's continue to explore ways to improve DataFusion and make it an even more powerful tool for data analysis!