Add dialect param to use double precision for float64 in Postgres

datafusion/sql/src/unparser/dialect.rs

@@ -45,6 +45,12 @@ pub trait Dialect {
     fn interval_style(&self) -> IntervalStyle {
         IntervalStyle::PostgresVerbose
     }
+
+    // Does the dialect use DOUBLE PRECISION to represent Float64 rather than DOUBLE?
+    // E.g. Postgres uses DOUBLE PRECISION instead of DOUBLE
+    fn float64_ast_dtype(&self) -> sqlparser::ast::DataType {
+        sqlparser::ast::DataType::Double
+    }
 }
 
 /// `IntervalStyle` to use for unparsing
@@ -87,6 +93,10 @@ impl Dialect for PostgreSqlDialect {
     fn interval_style(&self) -> IntervalStyle {
         IntervalStyle::PostgresVerbose
     }
+
+    fn float64_ast_dtype(&self) -> sqlparser::ast::DataType {
+        sqlparser::ast::DataType::DoublePrecision
+    }
 }
 
 pub struct MySqlDialect {}
@@ -118,6 +128,7 @@ pub struct CustomDialect {
     supports_nulls_first_in_sort: bool,
     use_timestamp_for_date64: bool,
     interval_style: IntervalStyle,
+    float64_ast_dtype: sqlparser::ast::DataType,
 }
 
 impl Default for CustomDialect {
@@ -127,6 +138,7 @@ impl Default for CustomDialect {
             supports_nulls_first_in_sort: true,
             use_timestamp_for_date64: false,
             interval_style: IntervalStyle::SQLStandard,
+            float64_ast_dtype: sqlparser::ast::DataType::Double,
         }
     }
 }
@@ -158,6 +170,10 @@ impl Dialect for CustomDialect {
     fn interval_style(&self) -> IntervalStyle {
         self.interval_style
     }
+
+    fn float64_ast_dtype(&self) -> sqlparser::ast::DataType {
+        self.float64_ast_dtype.clone()
+    }
 }
 
 /// `CustomDialectBuilder` to build `CustomDialect` using builder pattern
@@ -179,6 +195,7 @@ pub struct CustomDialectBuilder {
     supports_nulls_first_in_sort: bool,
     use_timestamp_for_date64: bool,
     interval_style: IntervalStyle,
+    float64_ast_dtype: sqlparser::ast::DataType,
 }
 
 impl Default for CustomDialectBuilder {
@@ -194,6 +211,7 @@ impl CustomDialectBuilder {
             supports_nulls_first_in_sort: true,
             use_timestamp_for_date64: false,
             interval_style: IntervalStyle::PostgresVerbose,
+            float64_ast_dtype: sqlparser::ast::DataType::Double,
         }
     }
 
@@ -203,6 +221,7 @@ 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,
+            float64_ast_dtype: self.float64_ast_dtype,
         }
     }
 
@@ -235,4 +254,12 @@ impl CustomDialectBuilder {
         self.interval_style = interval_style;
         self
     }
+
+    pub fn with_float64_ast_dtype(
+        mut self,
+        float64_ast_dtype: sqlparser::ast::DataType,
+    ) -> Self {
+        self.float64_ast_dtype = float64_ast_dtype;
+        self
+    }
 }

datafusion/sql/src/unparser/expr.rs

@@ -1237,7 +1237,7 @@ impl Unparser<'_> {
                 not_impl_err!("Unsupported DataType: conversion: {data_type:?}")
             }
             DataType::Float32 => Ok(ast::DataType::Float(None)),
-            DataType::Float64 => Ok(ast::DataType::Double),
+            DataType::Float64 => Ok(self.dialect.float64_ast_dtype()),
             DataType::Timestamp(_, tz) => {
                 let tz_info = match tz {
                     Some(_) => TimezoneInfo::WithTimeZone,
@@ -1819,6 +1819,34 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn custom_dialect_float64_ast_dtype() -> Result<()> {
+        for (float64_ast_dtype, identifier) in [
+            (sqlparser::ast::DataType::Double, "DOUBLE"),
+            (
+                sqlparser::ast::DataType::DoublePrecision,
+                "DOUBLE PRECISION",
+            ),
+        ] {
+            let dialect = CustomDialectBuilder::new()
+                .with_float64_ast_dtype(float64_ast_dtype)
+                .build();
+            let unparser = Unparser::new(&dialect);
+
+            let expr = Expr::Cast(Cast {
+                expr: Box::new(col("a")),
+                data_type: DataType::Float64,
+            });
+            let ast = unparser.expr_to_sql(&expr)?;
+
+            let actual = format!("{}", ast);
+
+            let expected = format!(r#"CAST(a AS {identifier})"#);
+            assert_eq!(actual, expected);
+        }
+        Ok(())
+    }
+
     #[test]
     fn customer_dialect_support_nulls_first_in_ort() -> Result<()> {
         let tests: Vec<(Expr, &str, bool)> = vec![