use max precision decimal for comparison

Author: James Cor

enginetest/queries/script_queries.go

@@ -616,6 +616,26 @@ FROM task_instance INNER JOIN job ON job.id = task_instance.queued_by_job_id INN
 				Query:    "SELECT '123.456' in (123.456);",
 				Expected: []sql.Row{{true}},
 			},
+			{
+				Query:    "SELECT 123.456 in (123.456);",
+				Expected: []sql.Row{{true}},
+			},
+			{
+				Query:    "SELECT 123.45 in (123.4);",
+				Expected: []sql.Row{{false}},
+			},
+			{
+				Query:    "SELECT 123.45 in (123.5);",
+				Expected: []sql.Row{{false}},
+			},
+			{
+				Query:    "SELECT '123.45a' in (123.5);",
+				Expected: []sql.Row{{false}},
+			},
+			{
+				Query:    "SELECT '123.45a' in (123.4);",
+				Expected: []sql.Row{{false}},
+			},
 			{
 				SkipResultCheckOnServerEngine:   true, // TODO: warnings do not make it to server engine
 				Query:                           "SELECT '123.456ABC' in (123.456);",

sql/types/utils.go

@@ -38,11 +38,8 @@ func GetCompareType(left, right sql.Type) sql.Type {
 		return LongBlob
 	}
 	if IsNumber(left) || IsNumber(right) {
-		if IsDecimal(left) {
-			return left
-		}
-		if IsDecimal(right) {
-			return right
+		if IsDecimal(left) || IsDecimal(right) {
+			return InternalDecimalType
 		}
 		if IsFloat(left) || IsFloat(right) {
 			return Float64