The SQL Unparser Does Not Correctly Handle `UNION`

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Describe the Bug

The DataFusion SQL Unparser is a crucial component of the DataFusion system, responsible for converting the internal logical plan of a query into a human-readable SQL string. However, it has been observed that the unparser does not correctly handle queries that use the UNION operator rather than UNION ALL. This can lead to incorrect results when the unparsed SQL is executed.

To understand the issue, let's consider a simple example. Suppose we have two tables, footable and bartable, and we want to combine their contents into a single result set, eliminating any duplicate rows. We can achieve this by using the UNION operator, as shown below:

SELECT col1 FROM footable
UNION
SELECT col1 FROM bartable

In this case, the DataFusion system correctly handles the query by adding a LogicalPlan::Distinct node as the parent of the LogicalPlan::Union node. This ensures that the final result set contains only unique rows.

However, when we unpars the query using the DataFusion SQL Unparser, we get the following result:

SELECT col1 FROM footable
UNION ALL
SELECT col1 FROM bartable

As we can see, the unparser has incorrectly replaced the UNION operator with UNION ALL, which will cause duplicate rows to be included in the final result set. This is clearly not the expected behavior.

To Reproduce

To reproduce this issue, we can follow these steps:

  1. Parse the following query into a DataFusion LogicalPlan:
SELECT j1_string AS col1, j1_id AS id FROM j1
UNION
SELECT j2_string AS col1, j2_id AS id FROM j2
UNION
SELECT j3_string AS col1, j3_id AS id FROM j3
  1. Immediately unpars the LogicalPlan and note that it unpars to a UNION ALL instead of a UNION.

Expected Behavior

The expected behavior is that the UNION operator is correctly preserved in the unparsed SQL from a LogicalPlan that adds a Distinct node directly above a Union node. In other words, the unparser should not replace the UNION operator with UNION ALL, even if the LogicalPlan contains a Distinct node.

Additional Context

Fortunately, this issue has already been fixed, and a pull request will be submitted shortly to address the problem. However, it's essential to understand the root cause of the issue and how it can be avoided in the future.

Why is this a problem?

The incorrect handling of UNION by the DataFusion SQL Unparser can lead to incorrect results when the unparsed SQL is executed. This can have significant consequences, especially in production environments where data accuracy is critical.

How can we prevent this issue?

To prevent this issue, we need to ensure that the DataFusion SQL Unparser correctly handles the UNION operator. This can be achieved by implementing a more robust unparser that takes into account the presence of a Distinct node in the LogicalPlan.

Conclusion

In conclusion, the DataFusion SQL Unparser does not correctly handle the UNION operator, leading to incorrect results when the unparsed SQL is executed. This issue has already been fixed, and a pull request will be submitted shortly to address the problem. However, it's essential to understand the root cause of the issue and how it can be avoided in the future.

Recommendations

Based on our analysis, we recommend the following:

  1. Implement a more robust unparser: The DataFusion SQL Unparser should be modified to correctly handle the UNION operator, even if the LogicalPlan contains a Distinct node.
  2. Test the unparser thoroughly: The unparser should be thoroughly tested to ensure that it correctly handles various scenarios, including the presence of a Distinct node.
  3. Provide clear documentation: The DataFusion documentation should clearly explain the behavior of the SQL Unparser, including its handling of the UNION operator.

Q&A

Q: What is the issue with the DataFusion SQL Unparser?

A: The DataFusion SQL Unparser does not correctly handle the UNION operator, leading to incorrect results when the unparsed SQL is executed.

Q: What is the expected behavior of the unparser?

A: The expected behavior is that the UNION operator is correctly preserved in the unparsed SQL from a LogicalPlan that adds a Distinct node directly above a Union node.

Q: Why is this a problem?

A: The incorrect handling of UNION by the DataFusion SQL Unparser can lead to incorrect results when the unparsed SQL is executed. This can have significant consequences, especially in production environments where data accuracy is critical.

Q: How can we prevent this issue?

A: To prevent this issue, we need to ensure that the DataFusion SQL Unparser correctly handles the UNION operator. This can be achieved by implementing a more robust unparser that takes into account the presence of a Distinct node in the LogicalPlan.

Q: What are the recommendations for fixing this issue?

A: Based on our analysis, we recommend the following:

  1. Implement a more robust unparser: The DataFusion SQL Unparser should be modified to correctly handle the UNION operator, even if the LogicalPlan contains a Distinct node.
  2. Test the unparser thoroughly: The unparser should be thoroughly tested to ensure that it correctly handles various scenarios, including the presence of a Distinct node.
  3. Provide clear documentation: The DataFusion documentation should clearly explain the behavior of the SQL Unparser, including its handling of the UNION operator.

Q: Is this issue already fixed?

A: Yes, this issue has already been fixed, and a pull request will be submitted shortly to address the problem.

Q: What are the implications of this issue?

A: The implications of this issue are significant, as it can lead to incorrect results when the unparsed SQL is executed. This can have serious consequences, especially in production environments where data accuracy is critical.

Q: How can we ensure that this issue does not happen again?

A: To ensure that this issue does not happen again, we need to implement a more robust unparser that correctly handles the UNION operator, even if the LogicalPlan contains a Distinct node. We also need to thoroughly test the unparser to ensure that it correctly handles various scenarios.

Q: What are the benefits of fixing this issue?

A: The benefits of fixing this issue are significant, as it will ensure that the DataFusion SQL Unparser provides accurate and reliable results, even in complex scenarios involving the UNION operator.

Q: How can we get involved in fixing this issue?

A: If you are interested in getting involved in fixing this issue, you can contribute to the DataFusion project by submitting a pull request with a fix for the issue. You can also provide feedback and suggestions on how to improve the unparser.

Conclusion

In conclusion, the DataFusion SQL Unparser does not correctly handle the UNION operator, leading to incorrect results when unparsed SQL is executed. This issue has already been fixed, and a pull request will be submitted shortly to address the problem. However, it's essential to understand the root cause of the issue and how it can be avoided in the future.

Recommendations

Based on our analysis, we recommend the following:

  1. Implement a more robust unparser: The DataFusion SQL Unparser should be modified to correctly handle the UNION operator, even if the LogicalPlan contains a Distinct node.
  2. Test the unparser thoroughly: The unparser should be thoroughly tested to ensure that it correctly handles various scenarios, including the presence of a Distinct node.
  3. Provide clear documentation: The DataFusion documentation should clearly explain the behavior of the SQL Unparser, including its handling of the UNION operator.

By following these recommendations, we can ensure that the DataFusion SQL Unparser provides accurate and reliable results, even in complex scenarios involving the UNION operator.