use cached decimal type for casts

enginetest/queries/queries.go

@@ -6967,6 +6967,24 @@ Select * from (
 			{"6"},
 		},
 	},
+	{
+		Query: "SELECT CASE WHEN COUNT( * ) THEN 10 * CAST(-19 AS SIGNED ) + CAST(82 AS DECIMAL) END;",
+		Expected: []sql.Row{
+			{"-108"},
+		},
+	},
+	{
+		Query: "SELECT CASE WHEN COUNT( * ) THEN 10.0 * CAST(2012 AS UNSIGNED) + CAST(82 AS CHAR) END;",
+		Expected: []sql.Row{
+			{20202.0},
+		},
+	},
+	{
+		Query: "SELECT CASE WHEN COUNT( * ) THEN 10.0 * CAST(1234 AS DATE) + CAST(82 AS CHAR) END;",
+		Expected: []sql.Row{
+			{nil},
+		},
+	},
 	{
 		Query:    "SELECT 2.0 + CAST(5 AS DECIMAL)",
 		Expected: []sql.Row{{"7.0"}},

sql/expression/div.go

@@ -389,6 +389,16 @@ func getFloatOrMaxDecimalType(e sql.Expression, treatIntsAsFloats bool) sql.Type
 					maxFrac = s
 				}
 			}
+		case *Convert:
+			if c.cachedDecimalType != nil {
+				p, s := GetPrecisionAndScale(c.cachedDecimalType)
+				if whole := p - s; whole > maxWhole {
+					maxWhole = whole
+				}
+				if s > maxFrac {
+					maxFrac = s
+				}
+			}
 		case *Literal:
 			if types.IsNumber(c.Type()) {
 				l, err := c.Eval(nil, nil)