Troubleshooting SQLFluff Parsing Errors With Unreserved Keywords In Snowflake
Introduction
This article addresses a common issue encountered when using SQLFluff, a popular SQL linter and formatter, with Snowflake, a cloud-based data warehousing platform. Specifically, we will delve into parsing errors that arise when SQLFluff encounters unreserved keywords, such as limit, used as identifiers (e.g., column names or aliases) in Snowflake SQL queries. This problem has been observed in SQLFluff versions 3.1.0 and later, marking a shift from the behavior in version 3.0.x where such queries were parsed without issues. Understanding and resolving these parsing errors is crucial for maintaining code quality and consistency in Snowflake projects.
Understanding the Issue
SQLFluff's primary function is to analyze SQL code for syntax errors, style inconsistencies, and potential issues. It relies on a dialect-specific grammar to parse the SQL code. In the context of Snowflake, SQLFluff should ideally recognize Snowflake's SQL syntax rules, including the handling of unreserved keywords. An unreserved keyword is a word that has a specific meaning in SQL but can also be used as an identifier (e.g., a column name or alias) without causing a syntax error, if properly quoted or contextually disambiguated. However, a parsing error occurs when SQLFluff fails to correctly interpret the SQL code, often due to a mismatch between its grammar rules and the actual SQL syntax.
The core problem we're addressing is that SQLFluff, in versions 3.1.0 and later, sometimes misinterprets unreserved keywords like limit when used as identifiers in Snowflake queries. This leads to parsing errors, even though the queries are valid Snowflake SQL. For example, consider the following simple query:
select
limit as renamed
from sometable;
In this case, limit is used as an alias for a column. Snowflake allows this, as limit is an unreserved keyword. However, SQLFluff may incorrectly flag this as a parsing error. The error message typically indicates an "unparsable section" of the code, pinpointing the line where the unreserved keyword is used as an identifier. This issue disrupts the linting process and can hinder the adoption of SQLFluff in Snowflake projects.
Reproducing the Error
The error can be easily reproduced by linting a SQL file containing a query that uses an unreserved keyword as an identifier. Here's a step-by-step guide:
-
Create a SQL file: Create a new file, for example,
junk.sql, and add the following SQL query:select limit as renamed from sometable; -
Run SQLFluff: Execute the SQLFluff lint command in your terminal, specifying the file path:
sqlfluff lint -t raw premium/models/intermediate/crowbar/junk.sql -
Observe the error: You should see an output similar to the following, indicating a parsing error:
== [premium/models/intermediate/crowbar/junk.sql] FAIL L: 1 | P: 1 | PRS | Line 1, Position 1: Found unparsable section: 'select' L: 2 | P: 5 | PRS | Line 2, Position 5: Found unparsable section: 'limit as | renamed\nfrom sometable' WARNING: Parsing errors found and dialect is set to 'snowflake'. Have you configured your dialect correctly?
This error confirms that SQLFluff is failing to parse the query due to the use of the unreserved keyword limit as an identifier. This issue can be reproduced across different SQLFluff versions starting from 3.1.0 when configured to use the Snowflake dialect.
Analyzing the Root Cause
The root cause of this parsing error lies in how SQLFluff's Snowflake dialect grammar handles unreserved keywords. In older versions of SQLFluff (3.0.x and earlier), the grammar likely had a more permissive interpretation of identifiers, allowing unreserved keywords to be used without triggering errors. However, subsequent versions appear to have introduced stricter parsing rules, possibly to improve accuracy or adhere to specific SQL standards. This change, while potentially beneficial in some contexts, inadvertently affects valid Snowflake SQL code that leverages unreserved keywords as identifiers.
The Snowflake documentation explicitly states that certain keywords are unreserved and can be used as identifiers. This means that a valid Snowflake parser should be able to distinguish between the keyword's reserved meaning and its usage as an identifier based on the context. SQLFluff's parsing logic, in this case, seems to be overly strict, not recognizing the contextual difference and flagging the use of limit as an error.
This issue highlights the challenges in creating a SQL parser that is both strict enough to catch genuine errors and flexible enough to accommodate the nuances of different SQL dialects. Snowflake's SQL dialect, while adhering to general SQL principles, has its own specific rules and conventions, including the handling of unreserved keywords. SQLFluff needs to accurately reflect these dialect-specific rules to avoid false positives and ensure smooth integration with Snowflake projects.
Solutions and Workarounds
Several approaches can be taken to address this SQLFluff parsing error. These include:
1. Quoting Identifiers
The most direct solution is to quote the identifier that clashes with the unreserved keyword. In Snowflake, identifiers can be quoted using double quotes ("). By quoting the identifier, you explicitly tell SQLFluff (and the SQL engine) that you intend to use the word as an identifier, not as a keyword. For the example query, the corrected version would be:
select
"limit" as renamed
from sometable;
By enclosing limit in double quotes, we resolve the parsing error and ensure that SQLFluff correctly interprets the query. This is the recommended approach as it aligns with SQL standards and avoids ambiguity.
2. Configuring SQLFluff's Dialect
SQLFluff provides configuration options to customize its behavior for specific dialects. It might be possible to adjust the Snowflake dialect settings to be more lenient with unreserved keywords. However, this approach should be taken with caution, as it could potentially mask genuine syntax errors. Consult SQLFluff's documentation for details on dialect-specific configurations.
3. Using a Different Identifier
If possible, consider renaming the identifier to avoid using the unreserved keyword altogether. While this might not always be feasible, it can be a simple and effective solution in many cases. Choose a different name that is descriptive and doesn't conflict with any SQL keywords.
4. Downgrading SQLFluff (Temporary Workaround)
As a temporary workaround, you could downgrade to a SQLFluff version prior to 3.1.0, where this issue was not present. However, this is not a long-term solution, as you would miss out on bug fixes and new features in later versions. It's best to address the underlying issue by using one of the other solutions mentioned above.
5. Contributing to SQLFluff
If you encounter this issue and are comfortable with Python and SQL parsing concepts, consider contributing to the SQLFluff project. You could investigate the Snowflake dialect grammar and propose a fix that correctly handles unreserved keywords. This would benefit the entire SQLFluff community and ensure better support for Snowflake SQL.
Detailed Configuration (pyproject.toml)
The pyproject.toml file is used to configure SQLFluff's behavior. Here's an example configuration that includes settings relevant to the Snowflake dialect:
[tool.sqlfluff.core]
dialect = "snowflake"
templater = "dbt"
runaway_limit = 10
max_line_length = 135
indent_unit = "space"
[tool.sqlfluff.templater.dbt]
project_dir = "./premium"
[sqlfluff.templater.jinja]
load_macros_from_path = "premium/macros/"
apply_dbt_builtins = true
[tool.sqlfluff.indentation]
tab_space_size = 4
[tool.sqlfluff.layout.type.comma]
spacing_before = "touch:inline"
line_position = "trailing"
[tool.sqlfluff.rules.capitalisation.keywords]
extended_capitalisation_policy = "lower"
[tool.sqlfluff.rules.aliasing.table]
aliasing = "explicit"
[tool.sqlfluff.rules.aliasing.column]
aliasing = "explicit"
[tool.sqlfluff.rules.references.special_chars]
quoted_identifiers_policy = "aliases"
[tool.sqlfluff.rules.aliasing.expression]
allow_scalar = false
[tool.sqlfluff.rules.capitalisation.identifiers]
extended_capitalisation_policy = "lower"
[tool.sqlfluff.rules.capitalisation.functions]
extended_capitalisation_policy = "lower"
[tool.sqlfluff.rules.capitalisation.literals]
extended_capitalisation_policy = "lower"
[tool.sqlfluff.rules.references.keywords]
# Comma separated list of words to ignore for this rule
ignore_words = "type"
[tool.sqlfluff.rules.ambiguous.column_references]
group_by_and_order_by_style = "explicit"
This configuration sets the dialect to snowflake, configures DBT templating, and defines various rules for capitalization, aliasing, and indentation. While this configuration doesn't directly address the unreserved keyword issue, it provides a comprehensive example of how SQLFluff can be configured for Snowflake projects. You might need to explore additional dialect-specific settings within this configuration to fine-tune SQLFluff's parsing behavior.
Conclusion
Parsing errors caused by unreserved keywords in Snowflake SQL code can be a frustrating issue when using SQLFluff. However, by understanding the root cause and applying the solutions outlined in this article, you can effectively address these errors and ensure that SQLFluff correctly lints your Snowflake SQL code. Quoting identifiers is the most robust and recommended approach. Remember to keep your SQLFluff configuration up-to-date and consider contributing to the project if you encounter further issues or have suggestions for improvement. By addressing these issues, you can improve the quality and maintainability of your Snowflake projects.
