Typo fixes for "usefull" -> "useful".

Docs/myisam.txt

@@ -868,7 +868,7 @@ Space compression makes the index file smaller if the string column has a lot of
 Prefix compression helps if there are many strings with an identical prefix.
 
 In memory table characteristics 
-HEAP tables only exists in memory so they are lost if `mysqld' is taken down or crashes. But since they are *very* fast they are usefull as anyway. 
+HEAP tables only exists in memory so they are lost if `mysqld' is taken down or crashes. But since they are *very* fast they are useful as anyway. 
 
 The *MySQL* internal HEAP tables uses 100% dynamic hashing without overflow areas and don't have problems with delete. 
 

configure.in

@@ -2845,7 +2845,7 @@ do
 done
 AC_SUBST(sql_union_dirs)
 
-# Some usefull subst
+# Some useful subst
 AC_SUBST(CC)
 AC_SUBST(GXX)
 

mysql-test/mysql-stress-test.pl

@@ -1100,7 +1100,7 @@ mysql-stress-test.pl --stress-basedir=<dir> --stress-suite-basedir=<dir> --serve
       they specified in the list file. 
       
 --sleep-time=<time in seconds>
-  Delay between test execution. Could be usefull in continued testsing 
+  Delay between test execution. Could be useful in continued testsing 
   when one of instance of stress script perform periodical cleanup or
   recreating of some database objects
 
@@ -1109,7 +1109,7 @@ mysql-stress-test.pl --stress-basedir=<dir> --stress-suite-basedir=<dir> --serve
 
 --check-tests-file
   Check file with list of tests. If file was modified it will force to
-  reread list of tests. Could be usefull in continued testing for
+  reread list of tests. Could be useful in continued testing for
   adding/removing tests without script interruption 
 
 --mysqltest=/path/to/mysqltest binary

mysys/safemalloc.c

@@ -436,7 +436,7 @@ void TERMINATE(FILE *file, uint flag)
 /*
   Report where a piece of memory was allocated
 
-  This is usefull to call from withing a debugger
+  This is useful to call from withing a debugger
 */
 
 void sf_malloc_report_allocated(void *memory)

sql/mysqld.cc

@@ -6631,7 +6631,7 @@ log and this option does nothing anymore.",
    0, 0, 0, 0, 0},
 
   {"test-ignore-wrong-options", OPT_TEST_IGNORE_WRONG_OPTIONS,
-   "Ignore wrong enums values in command line arguments. Usefull only for test scripts",
+   "Ignore wrong enums values in command line arguments. Useful only for test scripts",
    (uchar**) &opt_ignore_wrong_options, (uchar**) &opt_ignore_wrong_options,
    0, GET_BOOL, NO_ARG, 0, 0, 0, 0, 0, 0},
   {"timed_mutexes", OPT_TIMED_MUTEXES,

sql/sql_select.cc

@@ -11812,7 +11812,7 @@ join_read_const_table(JOIN_TAB *tab, POSITION *pos)
   {
 #if !defined(DBUG_OFF) && defined(NOT_USING_ITEM_EQUAL)
     /*
-      This test could be very usefull to find bugs in the optimizer
+      This test could be very useful to find bugs in the optimizer
       where we would call this function with an expression that can't be
       evaluated yet. We can't have this enabled by default as long as
       have items like Item_equal, that doesn't report they are const but

sql/udf_example.c

@@ -205,7 +205,7 @@ char *is_const(UDF_INIT *initid, UDF_ARGS *args, char *result, unsigned long
 **		try to keep the error message less than 80 bytes long!
 **
 ** This function should return 1 if something goes wrong. In this case
-** message should contain something usefull!
+** message should contain something useful!
 **************************************************************************/
 
 #define MAXMETAPH 8