feat: add custom_string_literal_override to unparser Dialect trait#20590
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alamb merged 5 commits intoapache:mainfrom Mar 16, 2026
Merged
feat: add custom_string_literal_override to unparser Dialect trait#20590alamb merged 5 commits intoapache:mainfrom
custom_string_literal_override to unparser Dialect trait#20590alamb merged 5 commits intoapache:mainfrom
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kosiew
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Mar 12, 2026
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@goldmedal
Thanks for working on this.
| /// | ||
| /// For example, MSSQL requires non-ASCII strings to use national string | ||
| /// literal syntax (`N'datafusion資料融合'`). | ||
| fn custom_string_literal_override(&self, _s: &str) -> Option<ast::Expr> { |
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Since this only affects scalar UTF8 literal unparsing today, a narrower name or a helper scoped to scalar string literals may make the API easier to understand and extend. Perhaps string_literal_to_sql – to keep the focus on literals, not “any scalar”?
datafusion/sql/src/unparser/expr.rs
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| let unparser = Unparser::new(&dialect); | ||
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| let expr = | ||
| Expr::Literal(ScalarValue::Utf8(Some("national string".to_string())), None); |
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The regression test currently exercises ScalarValue::Utf8, but the change also covers Utf8View and LargeUtf8.
Adding tests for each would make the coverage line up with the implementation.
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Thanks @kosiew for reviewing. All the comments has been addressed. |
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Thanks -- looks good to me @kosiew and @goldmedal |
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Which issue does this PR close?
Rationale for this change
When unparsing queries targeting databases like MSSQL, non-ASCII string literals need special handling. MSSQL requires the
N'...'(national string literal) prefix for strings containing Unicode characters. Currently the unparser always emits single-quoted strings with no way for dialects to customize this behavior.What changes are included in this PR?
custom_string_literal_overridemethod to theDialecttrait with a default implementation returningNone(no override).Utf8,Utf8View, andLargeUtf8match arms inscalar_value_to_sqland route them through the new dialect hook.Are these changes tested?
Yes. A test-only
MsSqlDialectis defined in the test module to verify:Nprefix)N'...')Nprefix regardless of content)It's used by Wren AI in production for a while: Canner#8
Are there any user-facing changes?
Yes. The
Dialecttrait gains a new methodcustom_string_literal_override. This is a non-breaking change since the method has a default implementation. Dialect implementors can override it to customize string literal unparsing.