Fix precision loss when coercing date_part utf8 argument (#7846)

Author: Dandandan

datafusion/core/tests/sql/expr.rs

@@ -717,6 +717,25 @@ async fn test_extract_date_part() -> Result<()> {
         "date_part('nanosecond', to_timestamp('2020-09-08T12:00:12.12345678+00:00'))",
         "1.212345678e10"
     );
+
+    // Keep precision when coercing Utf8 to Timestamp
+    test_expression!(
+        "date_part('second', '2020-09-08T12:00:12.12345678+00:00')",
+        "12.12345678"
+    );
+    test_expression!(
+        "date_part('millisecond', '2020-09-08T12:00:12.12345678+00:00')",
+        "12123.45678"
+    );
+    test_expression!(
+        "date_part('microsecond', '2020-09-08T12:00:12.12345678+00:00')",
+        "12123456.78"
+    );
+    test_expression!(
+        "date_part('nanosecond', '2020-09-08T12:00:12.12345678+00:00')",
+        "1.212345678e10"
+    );
+
     Ok(())
 }
 

datafusion/expr/src/built_in_function.rs

@@ -1107,28 +1107,28 @@ impl BuiltinScalarFunction {
             }
             BuiltinScalarFunction::DatePart => Signature::one_of(
                 vec![
-                    Exact(vec![Utf8, Date32]),
-                    Exact(vec![Utf8, Date64]),
-                    Exact(vec![Utf8, Timestamp(Second, None)]),
+                    Exact(vec![Utf8, Timestamp(Nanosecond, None)]),
                     Exact(vec![
                         Utf8,
-                        Timestamp(Second, Some(TIMEZONE_WILDCARD.into())),
+                        Timestamp(Nanosecond, Some(TIMEZONE_WILDCARD.into())),
                     ]),
-                    Exact(vec![Utf8, Timestamp(Microsecond, None)]),
+                    Exact(vec![Utf8, Timestamp(Millisecond, None)]),
                     Exact(vec![
                         Utf8,
-                        Timestamp(Microsecond, Some(TIMEZONE_WILDCARD.into())),
+                        Timestamp(Millisecond, Some(TIMEZONE_WILDCARD.into())),
                     ]),
-                    Exact(vec![Utf8, Timestamp(Millisecond, None)]),
+                    Exact(vec![Utf8, Timestamp(Microsecond, None)]),
                     Exact(vec![
                         Utf8,
-                        Timestamp(Millisecond, Some(TIMEZONE_WILDCARD.into())),
+                        Timestamp(Microsecond, Some(TIMEZONE_WILDCARD.into())),
                     ]),
-                    Exact(vec![Utf8, Timestamp(Nanosecond, None)]),
+                    Exact(vec![Utf8, Timestamp(Second, None)]),
                     Exact(vec![
                         Utf8,
-                        Timestamp(Nanosecond, Some(TIMEZONE_WILDCARD.into())),
+                        Timestamp(Second, Some(TIMEZONE_WILDCARD.into())),
                     ]),
+                    Exact(vec![Utf8, Date64]),
+                    Exact(vec![Utf8, Date32]),
                 ],
                 self.volatility(),
             ),