Support default precision and scale toCAST <EXPR> AS DECIMAL

datafusion/sql/src/planner.rs

@@ -2678,6 +2678,34 @@ mod tests {
         );
     }
 
+    #[test]
+    fn test_int_decimal_default() {
+        quick_test(
+            "SELECT CAST(10 AS DECIMAL)",
+            "Projection: CAST(Int64(10) AS Decimal(38, 10))\
+             \n  EmptyRelation",
+        );
+    }
+
+    #[test]
+    fn test_int_decimal_no_scale() {
+        quick_test(
+            "SELECT CAST(10 AS DECIMAL(5))",
+            "Projection: CAST(Int64(10) AS Decimal(5, 0))\
+             \n  EmptyRelation",
+        );
+    }
+
+    #[test]
+    fn test_int_decimal_scale_larger_precision() {
+        let sql = "SELECT CAST(10 AS DECIMAL(5, 10))";
+        let err = logical_plan(sql).expect_err("query should have failed");
+        assert_eq!(
+            r##"Internal("For decimal(precision, scale) precision must be less than or equal to 38 and scale can't be greater than precision. Got (5, 10)")"##,
+            format!("{:?}", err)
+        );
+    }
+
     #[test]
     fn select_column_does_not_exist() {
         let sql = "SELECT doesnotexist FROM person";

datafusion/sql/src/utils.rs

@@ -17,7 +17,7 @@
 
 //! SQL Utility Functions
 
-use arrow::datatypes::{DataType, DECIMAL_MAX_PRECISION};
+use arrow::datatypes::{DataType, DECIMAL_DEFAULT_SCALE, DECIMAL_MAX_PRECISION};
 use sqlparser::ast::Ident;
 
 use datafusion_common::{DataFusionError, Result, ScalarValue};
@@ -447,22 +447,26 @@ pub(crate) fn make_decimal_type(
     precision: Option<u64>,
     scale: Option<u64>,
 ) -> Result<DataType> {
-    match (precision, scale) {
-        (None, _) | (_, None) => Err(DataFusionError::Internal(format!(
-            "Decimal(precision, scale) must both be specified, got ({:?}, {:?})",
-            precision, scale
-        ))),
-        (Some(p), Some(s)) => {
-            // Arrow decimal is i128 meaning 38 maximum decimal digits
-            if (p as usize) > DECIMAL_MAX_PRECISION || s > p {
-                Err(DataFusionError::Internal(format!(
-                    "For decimal(precision, scale) precision must be less than or equal to 38 and scale can't be greater than precision. Got ({}, {})",
-                    p, s
-                )))
-            } else {
-                Ok(DataType::Decimal(p as usize, s as usize))
-            }
+    // postgres like behavior
+    let (precision, scale) = match (precision, scale) {
+        (Some(p), Some(s)) => (p as usize, s as usize),
+        (Some(p), None) => (p as usize, 0),
+        (None, Some(_)) => {
+            return Err(DataFusionError::Internal(
+                "Cannot specify only scale for decimal data type".to_string(),
+            ))
         }
+        (None, None) => (DECIMAL_MAX_PRECISION, DECIMAL_DEFAULT_SCALE),
+    };
+
+    // Arrow decimal is i128 meaning 38 maximum decimal digits
+    if precision > DECIMAL_MAX_PRECISION || scale > precision {
+        return Err(DataFusionError::Internal(format!(
+            "For decimal(precision, scale) precision must be less than or equal to 38 and scale can't be greater than precision. Got ({}, {})",
+            precision, scale
+        )));
+    } else {
+        Ok(DataType::Decimal(precision, scale))
     }
 }