Respect precision when casting to datetime

enginetest/queries/queries.go

@@ -4246,48 +4246,36 @@ SELECT * FROM cte WHERE  d = 2;`,
 		Expected:              []sql.Row{{time.Date(2020, time.January, 1, 12, 34, 56, 0, time.UTC)}},
 	},
 	{
-		// TODO: implement precision for datetime casting
-		Skip:                  true,
 		Query:                 "select cast('2020-01-01 12:34:56.123456abc' as datetime(1));",
 		ExpectedWarning:       1292,
 		ExpectedWarningsCount: 1,
 		Expected:              []sql.Row{{time.Date(2020, time.January, 1, 12, 34, 56, 100000000, time.UTC)}},
 	},
 	{
-		// TODO: implement precision for datetime casting
-		Skip:                  true,
 		Query:                 "select cast('2020-01-01 12:34:56.123456abc' as datetime(2));",
 		ExpectedWarning:       1292,
 		ExpectedWarningsCount: 1,
 		Expected:              []sql.Row{{time.Date(2020, time.January, 1, 12, 34, 56, 120000000, time.UTC)}},
 	},
 	{
-		// TODO: implement precision for datetime casting
-		Skip:                  true,
 		Query:                 "select cast('2020-01-01 12:34:56.123456abc' as datetime(3));",
 		ExpectedWarning:       1292,
 		ExpectedWarningsCount: 1,
 		Expected:              []sql.Row{{time.Date(2020, time.January, 1, 12, 34, 56, 123000000, time.UTC)}},
 	},
 	{
-		// TODO: implement precision for datetime casting
-		Skip:                  true,
 		Query:                 "select cast('2020-01-01 12:34:56.123456abc' as datetime(4));",
 		ExpectedWarning:       1292,
 		ExpectedWarningsCount: 1,
 		Expected:              []sql.Row{{time.Date(2020, time.January, 1, 12, 34, 56, 123500000, time.UTC)}},
 	},
 	{
-		// TODO: implement precision for datetime casting
-		Skip:                  true,
 		Query:                 "select cast('2020-01-01 12:34:56.123456abc' as datetime(5));",
 		ExpectedWarning:       1292,
 		ExpectedWarningsCount: 1,
 		Expected:              []sql.Row{{time.Date(2020, time.January, 1, 12, 34, 56, 123460000, time.UTC)}},
 	},
 	{
-		// TODO: implement precision for datetime casting
-		Skip:                  true,
 		Query:                 "select cast('2020-01-01 12:34:56.123456abc' as datetime(6));",
 		ExpectedWarning:       1292,
 		ExpectedWarningsCount: 1,

sql/expression/comparison.go

@@ -222,12 +222,13 @@ func (c *comparison) castLeftAndRight(ctx *sql.Context, left, right interface{})
 	}
 
 	if types.IsTime(leftType) || types.IsTime(rightType) {
+		// TODO: We need to set to actual Datetime type
 		l, r, err := convertLeftAndRight(ctx, left, right, ConvertToDatetime)
 		if err != nil {
 			return nil, nil, nil, err
 		}
 
-		return l, r, types.DatetimeMaxPrecision, nil
+		return l, r, types.DatetimeDefaultPrecision, nil
 	}
 
 	// Rely on types.JSON.Compare to handle JSON comparisons
@@ -302,6 +303,7 @@ func (c *comparison) castLeftAndRight(ctx *sql.Context, left, right interface{})
 }
 
 func convertLeftAndRight(ctx *sql.Context, left, right interface{}, convertTo string) (interface{}, interface{}, error) {
+	// TODO: typeLength and typeScale should be the actual length and scale of the type to be converted
 	l, err := convertValue(ctx, left, convertTo, nil, 0, 0)
 	if err != nil {
 		return nil, nil, err

sql/expression/convert.go

@@ -341,7 +341,7 @@ func convertValue(ctx *sql.Context, val interface{}, castTo string, originType s
 		if !(isTime || isString || isBinary) {
 			return nil, nil
 		}
-		d, _, err := types.DatetimeDefaultPrecision.Convert(ctx, val)
+		d, _, err := types.MustCreateDatetimeType(sqltypes.Datetime, typeLength).Convert(ctx, val)
 		if err != nil {
 			if !sql.ErrTruncatedIncorrect.Is(err) {
 				return nil, err