lp:736358 Unexpected increased timestamp resolution with UNION

partial fix

mysql-test/r/func_time_hires.result

@@ -149,3 +149,21 @@ select * from t1;
 a
 2011-01-01 01:01:01.123456
 drop table t1;
+create table t1 (f1 timestamp(6));
+insert into t1 values ('2002-07-15 21:00:00');
+select time(f1) from t1;
+time(f1)
+21:00:00.000000
+select time(f1) from t1 union all select time(f1) from t1;
+time(f1)
+21:00:00.000000
+21:00:00.000000
+alter table t1 modify f1 varchar(100);
+select time(f1) from t1;
+time(f1)
+21:00:00
+select time(f1) from t1 union all select time(f1) from t1;
+time(f1)
+21:00:00.000000
+21:00:00.000000
+drop table t1;

mysql-test/t/func_time_hires.test

@@ -78,3 +78,19 @@ insert t1 values (now(6));
 select * from t1;
 drop table t1;
 
+#
+# lp:736358 Unexpected increased timestamp resolution with UNION
+#
+# timestamp(6) case is fixed:
+create table t1 (f1 timestamp(6));
+insert into t1 values ('2002-07-15 21:00:00');
+select time(f1) from t1;
+select time(f1) from t1 union all select time(f1) from t1;
+#alter table t1 modify f1 timestamp;
+#select time(f1) from t1;
+#select time(f1) from t1 union all select time(f1) from t1;
+# but the effect cannot be eliminated completely:
+alter table t1 modify f1 varchar(100);
+select time(f1) from t1;
+select time(f1) from t1 union all select time(f1) from t1;
+drop table t1;

sql/item_timefunc.h

@@ -699,6 +699,8 @@ public:
     collation.set(&my_charset_bin);
     maybe_null= 1;
     max_length= MAX_TIME_WIDTH;
+    if (decimals == NOT_FIXED_DEC)
+      decimals= args[0]->decimals;
     if (decimals && decimals != NOT_FIXED_DEC)
       max_length+= min(decimals, MAX_SEC_PART_DIGITS) + 1;
   }