Support alternate format for Utf8 unparsing (CHAR) (apache#11494)

* Add dialect param to use CHAR instead of TEXT for Utf8 unparsing for MySQL (apache#12)

* Configurable data type instead of flag for Utf8 unparsing

* Fix type in comment

datafusion/sql/src/unparser/dialect.rs

@@ -16,7 +16,7 @@
 // under the License.
 
 use regex::Regex;
-use sqlparser::keywords::ALL_KEYWORDS;
+use sqlparser::{ast, keywords::ALL_KEYWORDS};
 
 /// `Dialect` to use for Unparsing
 ///
@@ -45,6 +45,17 @@ pub trait Dialect {
     fn interval_style(&self) -> IntervalStyle {
         IntervalStyle::PostgresVerbose
     }
+
+    // The SQL type to use for Arrow Utf8 unparsing
+    // Most dialects use VARCHAR, but some, like MySQL, require CHAR
+    fn utf8_cast_dtype(&self) -> ast::DataType {
+        ast::DataType::Varchar(None)
+    }
+    // The SQL type to use for Arrow LargeUtf8 unparsing
+    // Most dialects use TEXT, but some, like MySQL, require CHAR
+    fn large_utf8_cast_dtype(&self) -> ast::DataType {
+        ast::DataType::Text
+    }
 }
 
 /// `IntervalStyle` to use for unparsing
@@ -103,6 +114,14 @@ impl Dialect for MySqlDialect {
     fn interval_style(&self) -> IntervalStyle {
         IntervalStyle::MySQL
     }
+
+    fn utf8_cast_dtype(&self) -> ast::DataType {
+        ast::DataType::Char(None)
+    }
+
+    fn large_utf8_cast_dtype(&self) -> ast::DataType {
+        ast::DataType::Char(None)
+    }
 }
 
 pub struct SqliteDialect {}
@@ -118,6 +137,8 @@ pub struct CustomDialect {
     supports_nulls_first_in_sort: bool,
     use_timestamp_for_date64: bool,
     interval_style: IntervalStyle,
+    utf8_cast_dtype: ast::DataType,
+    large_utf8_cast_dtype: ast::DataType,
 }
 
 impl Default for CustomDialect {
@@ -127,6 +148,8 @@ impl Default for CustomDialect {
             supports_nulls_first_in_sort: true,
             use_timestamp_for_date64: false,
             interval_style: IntervalStyle::SQLStandard,
+            utf8_cast_dtype: ast::DataType::Varchar(None),
+            large_utf8_cast_dtype: ast::DataType::Text,
         }
     }
 }
@@ -158,6 +181,14 @@ impl Dialect for CustomDialect {
     fn interval_style(&self) -> IntervalStyle {
         self.interval_style
     }
+
+    fn utf8_cast_dtype(&self) -> ast::DataType {
+        self.utf8_cast_dtype.clone()
+    }
+
+    fn large_utf8_cast_dtype(&self) -> ast::DataType {
+        self.large_utf8_cast_dtype.clone()
+    }
 }
 
 /// `CustomDialectBuilder` to build `CustomDialect` using builder pattern
@@ -179,6 +210,8 @@ pub struct CustomDialectBuilder {
     supports_nulls_first_in_sort: bool,
     use_timestamp_for_date64: bool,
     interval_style: IntervalStyle,
+    utf8_cast_dtype: ast::DataType,
+    large_utf8_cast_dtype: ast::DataType,
 }
 
 impl Default for CustomDialectBuilder {
@@ -194,6 +227,8 @@ impl CustomDialectBuilder {
             supports_nulls_first_in_sort: true,
             use_timestamp_for_date64: false,
             interval_style: IntervalStyle::PostgresVerbose,
+            utf8_cast_dtype: ast::DataType::Varchar(None),
+            large_utf8_cast_dtype: ast::DataType::Text,
         }
     }
 
@@ -203,6 +238,8 @@ impl CustomDialectBuilder {
             supports_nulls_first_in_sort: self.supports_nulls_first_in_sort,
             use_timestamp_for_date64: self.use_timestamp_for_date64,
             interval_style: self.interval_style,
+            utf8_cast_dtype: self.utf8_cast_dtype,
+            large_utf8_cast_dtype: self.large_utf8_cast_dtype,
         }
     }
 
@@ -235,4 +272,17 @@ impl CustomDialectBuilder {
         self.interval_style = interval_style;
         self
     }
+
+    pub fn with_utf8_cast_dtype(mut self, utf8_cast_dtype: ast::DataType) -> Self {
+        self.utf8_cast_dtype = utf8_cast_dtype;
+        self
+    }
+
+    pub fn with_large_utf8_cast_dtype(
+        mut self,
+        large_utf8_cast_dtype: ast::DataType,
+    ) -> Self {
+        self.large_utf8_cast_dtype = large_utf8_cast_dtype;
+        self
+    }
 }

datafusion/sql/src/unparser/expr.rs

@@ -1275,8 +1275,8 @@ impl Unparser<'_> {
             DataType::BinaryView => {
                 not_impl_err!("Unsupported DataType: conversion: {data_type:?}")
             }
-            DataType::Utf8 => Ok(ast::DataType::Varchar(None)),
-            DataType::LargeUtf8 => Ok(ast::DataType::Text),
+            DataType::Utf8 => Ok(self.dialect.utf8_cast_dtype()),
+            DataType::LargeUtf8 => Ok(self.dialect.large_utf8_cast_dtype()),
             DataType::Utf8View => {
                 not_impl_err!("Unsupported DataType: conversion: {data_type:?}")
             }
@@ -1936,4 +1936,34 @@ mod tests {
             assert_eq!(actual, expected);
         }
     }
+
+    #[test]
+    fn custom_dialect_use_char_for_utf8_cast() -> Result<()> {
+        let default_dialect = CustomDialectBuilder::default().build();
+        let mysql_custom_dialect = CustomDialectBuilder::new()
+            .with_utf8_cast_dtype(ast::DataType::Char(None))
+            .with_large_utf8_cast_dtype(ast::DataType::Char(None))
+            .build();
+
+        for (dialect, data_type, identifier) in [
+            (&default_dialect, DataType::Utf8, "VARCHAR"),
+            (&default_dialect, DataType::LargeUtf8, "TEXT"),
+            (&mysql_custom_dialect, DataType::Utf8, "CHAR"),
+            (&mysql_custom_dialect, DataType::LargeUtf8, "CHAR"),
+        ] {
+            let unparser = Unparser::new(dialect);
+
+            let expr = Expr::Cast(Cast {
+                expr: Box::new(col("a")),
+                data_type,
+            });
+            let ast = unparser.expr_to_sql(&expr)?;
+
+            let actual = format!("{}", ast);
+            let expected = format!(r#"CAST(a AS {identifier})"#);
+
+            assert_eq!(actual, expected);
+        }
+        Ok(())
+    }
 }