From 026e5f35c5c9d7e559ccb476ce6da48fdf70a2a3 Mon Sep 17 00:00:00 2001
From: Sergei Grebnov <sergei.grebnov@gmail.com>
Date: Wed, 1 Apr 2026 14:01:34 +0300
Subject: [PATCH] fix(unparser): make BigQueryDialect more robust

---
 datafusion/sql/src/unparser/dialect.rs | 49 +++++++++++++++++++++++
 datafusion/sql/src/unparser/expr.rs    | 54 ++++++++++++++++++++++++++
 2 files changed, 103 insertions(+)

diff --git a/datafusion/sql/src/unparser/dialect.rs b/datafusion/sql/src/unparser/dialect.rs
index fa9d40c6c78a8..3ed6514b07f80 100644
--- a/datafusion/sql/src/unparser/dialect.rs
+++ b/datafusion/sql/src/unparser/dialect.rs
@@ -657,6 +657,55 @@ impl Dialect for BigQueryDialect {
         true
     }
 
+    fn supports_column_alias_in_table_alias(&self) -> bool {
+        false
+    }
+
+    fn float64_ast_dtype(&self) -> ast::DataType {
+        ast::DataType::Float64
+    }
+
+    fn utf8_cast_dtype(&self) -> ast::DataType {
+        ast::DataType::String(None)
+    }
+
+    fn large_utf8_cast_dtype(&self) -> ast::DataType {
+        ast::DataType::String(None)
+    }
+
+    fn int64_cast_dtype(&self) -> ast::DataType {
+        ast::DataType::Int64
+    }
+
+    fn timestamp_cast_dtype(
+        &self,
+        _time_unit: &TimeUnit,
+        _tz: &Option<Arc<str>>,
+    ) -> ast::DataType {
+        ast::DataType::Timestamp(None, TimezoneInfo::None)
+    }
+
+    fn date_field_extract_style(&self) -> DateFieldExtractStyle {
+        DateFieldExtractStyle::Extract
+    }
+
+    fn interval_style(&self) -> IntervalStyle {
+        IntervalStyle::SQLStandard
+    }
+
+    fn scalar_function_to_sql_overrides(
+        &self,
+        unparser: &Unparser,
+        func_name: &str,
+        args: &[Expr],
+    ) -> Result<Option<ast::Expr>> {
+        if func_name == "date_part" {
+            return date_part_to_sql(unparser, self.date_field_extract_style(), args);
+        }
+
+        Ok(None)
+    }
+
     fn timestamp_with_tz_to_string(&self, dt: DateTime<Tz>, unit: TimeUnit) -> String {
         // https://docs.cloud.google.com/bigquery/docs/reference/standard-sql/data-types#timestamp_type
         let format = match unit {
diff --git a/datafusion/sql/src/unparser/expr.rs b/datafusion/sql/src/unparser/expr.rs
index a4a231d9857c2..eef21983b4ac8 100644
--- a/datafusion/sql/src/unparser/expr.rs
+++ b/datafusion/sql/src/unparser/expr.rs
@@ -3469,4 +3469,58 @@ mod tests {
         }
         Ok(())
     }
+
+    #[test]
+    fn test_bigquery_dialect_overrides() -> Result<()> {
+        let bigquery_dialect: Arc<dyn Dialect> = Arc::new(BigQueryDialect::new());
+        let unparser = Unparser::new(bigquery_dialect.as_ref());
+
+        // date_field_extract_style: EXTRACT instead of date_part
+        let expr = Expr::ScalarFunction(ScalarFunction {
+            func: Arc::new(ScalarUDF::new_from_impl(
+                datafusion_functions::datetime::date_part::DatePartFunc::new(),
+            )),
+            args: vec![lit("YEAR"), col("date_col")],
+        });
+        let actual = format!("{}", unparser.expr_to_sql(&expr)?);
+        assert_eq!(actual, "EXTRACT(YEAR FROM `date_col`)");
+
+        // interval_style: SQL standard instead of PostgresVerbose
+        let expr = interval_year_month_lit("3 months");
+        let actual = format!("{}", unparser.expr_to_sql(&expr)?);
+        assert_eq!(actual, "INTERVAL '3' MONTH");
+
+        // float64_ast_dtype: FLOAT64 instead of DOUBLE
+        let expr = cast(col("a"), DataType::Float64);
+        let actual = format!("{}", unparser.expr_to_sql(&expr)?);
+        assert_eq!(actual, "CAST(`a` AS FLOAT64)");
+
+        // supports_column_alias_in_table_alias: false
+        assert!(!bigquery_dialect.supports_column_alias_in_table_alias());
+
+        // utf8_cast_dtype: STRING instead of VARCHAR
+        let expr = cast(col("a"), DataType::Utf8);
+        let actual = format!("{}", unparser.expr_to_sql(&expr)?);
+        assert_eq!(actual, "CAST(`a` AS STRING)");
+
+        // large_utf8_cast_dtype: STRING instead of TEXT
+        let expr = cast(col("a"), DataType::LargeUtf8);
+        let actual = format!("{}", unparser.expr_to_sql(&expr)?);
+        assert_eq!(actual, "CAST(`a` AS STRING)");
+
+        // int64_cast_dtype: INT64 instead of BIGINT
+        let expr = cast(col("a"), DataType::Int64);
+        let actual = format!("{}", unparser.expr_to_sql(&expr)?);
+        assert_eq!(actual, "CAST(`a` AS INT64)");
+
+        // timestamp_cast_dtype: TIMESTAMP (no WITH TIME ZONE)
+        let expr = cast(
+            col("a"),
+            DataType::Timestamp(TimeUnit::Microsecond, Some("+00:00".into())),
+        );
+        let actual = format!("{}", unparser.expr_to_sql(&expr)?);
+        assert_eq!(actual, "CAST(`a` AS TIMESTAMP)");
+
+        Ok(())
+    }
 }