diff --git a/mysql-test/r/type_decimal.result b/mysql-test/r/type_decimal.result
index cd7902c8ae9..4e9dcf41256 100644
--- a/mysql-test/r/type_decimal.result
+++ b/mysql-test/r/type_decimal.result
@@ -822,4 +822,71 @@ this must not produce error 1048:
 select * from t1 where ua_invited_by_id not in (select ua_id from t1);
 ua_id	ua_invited_by_id
 drop table t1;
+DROP TABLE IF EXISTS t3;
+DROP TABLE IF EXISTS t4;
+CREATE TABLE t1( a NUMERIC, b INT );
+INSERT INTO t1 VALUES (123456, 40), (123456, 40);
+SELECT TRUNCATE( a, b ) AS c FROM t1 ORDER BY c;
+c
+123456
+123456
+SELECT ROUND( a, b ) AS c FROM t1 ORDER BY c;
+c
+123456
+123456
+SELECT ROUND( a, 100 ) AS c FROM t1 ORDER BY c;
+c
+123456.000000000000000000000000000000
+123456.000000000000000000000000000000
+CREATE TABLE t2( a NUMERIC, b INT );
+INSERT INTO t2 VALUES (123456, 100);
+SELECT TRUNCATE( a, b ) AS c FROM t2 ORDER BY c;
+c
+123456
+SELECT ROUND( a, b ) AS c FROM t2 ORDER BY c;
+c
+123456
+CREATE TABLE t3( a DECIMAL, b INT );
+INSERT INTO t3 VALUES (123456, 40), (123456, 40);
+SELECT TRUNCATE( a, b ) AS c FROM t3 ORDER BY c;
+c
+123456
+123456
+SELECT ROUND( a, b ) AS c FROM t3 ORDER BY c;
+c
+123456
+123456
+SELECT ROUND( a, 100 ) AS c FROM t3 ORDER BY c;
+c
+123456.000000000000000000000000000000
+123456.000000000000000000000000000000
+CREATE TABLE t4( a DECIMAL, b INT );
+INSERT INTO t4 VALUES (123456, 40), (123456, 40);
+SELECT TRUNCATE( a, b ) AS c FROM t4 ORDER BY c;
+c
+123456
+123456
+SELECT ROUND( a, b ) AS c FROM t4 ORDER BY c;
+c
+123456
+123456
+SELECT ROUND( a, 100 ) AS c FROM t4 ORDER BY c;
+c
+123456.000000000000000000000000000000
+123456.000000000000000000000000000000
+delete from t1;
+INSERT INTO t1 VALUES (1234567890, 20), (999.99, 5);
+Warnings:
+Note	1265	Data truncated for column 'a' at row 2
+show create table t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `a` decimal(10,0) default NULL,
+  `b` int(11) default NULL
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+select round(a,b) as c from t1 order by c;
+c
+1000
+1234567890
+DROP TABLE t1, t2, t3, t4;
 End of 5.0 tests
diff --git a/mysql-test/t/type_decimal.test b/mysql-test/t/type_decimal.test
index 12d4398dd57..807d1e6b01e 100644
--- a/mysql-test/t/type_decimal.test
+++ b/mysql-test/t/type_decimal.test
@@ -440,5 +440,42 @@ select * from t1 where ua_invited_by_id not in (select ua_id from t1);
 
 drop table t1;
 
---echo End of 5.0 tests
+#
+# Bug #30889: filesort and order by with float/numeric crashes server
+#
+--disable_warnings
+DROP TABLE IF EXISTS t3;
+DROP TABLE IF EXISTS t4;
+--enable_warnings
+CREATE TABLE t1( a NUMERIC, b INT );
+INSERT INTO t1 VALUES (123456, 40), (123456, 40);
+SELECT TRUNCATE( a, b ) AS c FROM t1 ORDER BY c;
+SELECT ROUND( a, b ) AS c FROM t1 ORDER BY c;
+SELECT ROUND( a, 100 ) AS c FROM t1 ORDER BY c;
 
+CREATE TABLE t2( a NUMERIC, b INT );
+INSERT INTO t2 VALUES (123456, 100);
+SELECT TRUNCATE( a, b ) AS c FROM t2 ORDER BY c;
+SELECT ROUND( a, b ) AS c FROM t2 ORDER BY c;
+
+CREATE TABLE t3( a DECIMAL, b INT );
+INSERT INTO t3 VALUES (123456, 40), (123456, 40);
+SELECT TRUNCATE( a, b ) AS c FROM t3 ORDER BY c;
+SELECT ROUND( a, b ) AS c FROM t3 ORDER BY c;
+SELECT ROUND( a, 100 ) AS c FROM t3 ORDER BY c;
+
+CREATE TABLE t4( a DECIMAL, b INT );
+INSERT INTO t4 VALUES (123456, 40), (123456, 40);
+SELECT TRUNCATE( a, b ) AS c FROM t4 ORDER BY c;
+SELECT ROUND( a, b ) AS c FROM t4 ORDER BY c;
+SELECT ROUND( a, 100 ) AS c FROM t4 ORDER BY c;
+
+delete from t1;
+INSERT INTO t1 VALUES (1234567890, 20), (999.99, 5);
+show create table t1;
+
+select round(a,b) as c from t1 order by c;
+
+DROP TABLE t1, t2, t3, t4;
+
+--echo End of 5.0 tests
diff --git a/sql/item_func.cc b/sql/item_func.cc
index 64574a9ad74..49fef6cdb3d 100644
--- a/sql/item_func.cc
+++ b/sql/item_func.cc
@@ -2000,6 +2000,7 @@ void Item_func_round::fix_length_and_dec()
   case DECIMAL_RESULT:
   {
     hybrid_type= DECIMAL_RESULT;
+    decimals_to_set= min(DECIMAL_MAX_SCALE, decimals_to_set);
     int decimals_delta= args[0]->decimals - decimals_to_set;
     int precision= args[0]->decimal_precision();
     int length_increase= ((decimals_delta <= 0) || truncate) ? 0:1;
@@ -2106,7 +2107,7 @@ my_decimal *Item_func_round::decimal_op(my_decimal *decimal_value)
   longlong dec= args[1]->val_int();
   if (dec > 0 || (dec < 0 && args[1]->unsigned_flag))
   {
-    dec= min((ulonglong) dec, DECIMAL_MAX_SCALE);
+    dec= min((ulonglong) dec, decimals);
     decimals= (uint8) dec; // to get correct output
   }
   else if (dec < INT_MIN)
diff --git a/sql/item_func.h b/sql/item_func.h
index b2ba57af817..4d7c96a18c1 100644
--- a/sql/item_func.h
+++ b/sql/item_func.h
@@ -236,9 +236,40 @@ public:
   my_decimal *val_decimal(my_decimal *);
   String *val_str(String*str);
 
+  /**
+     @brief Performs the operation that this functions implements when the
+     result type is INT.
+
+     @return The result of the operation.
+  */
   virtual longlong int_op()= 0;
+
+  /**
+     @brief Performs the operation that this functions implements when the
+     result type is REAL.
+
+     @return The result of the operation.
+  */
   virtual double real_op()= 0;
+
+  /**
+     @brief Performs the operation that this functions implements when the
+     result type is DECIMAL.
+
+     @param A pointer where the DECIMAL value will be allocated.
+     @return 
+       - 0 If the result is NULL
+       - The same pointer it was given, with the area initialized to the
+         result of the operation.
+  */
   virtual my_decimal *decimal_op(my_decimal *)= 0;
+
+  /**
+     @brief Performs the operation that this functions implements when the
+     result type is a string type.
+
+     @return The result of the operation.
+  */
   virtual String *str_op(String *)= 0;
   bool is_null() { update_null_value(); return null_value; }
 };