"Fix" for BUG #2050 "10 to 1 performance drop with server 4.1.1".

Actually it is not a bug but right behavior observed as pefomance 
degradation after we have forced Item_field::fix_fields() to 
re-execute each time when we are executing prep stmt.

This patch implements small optimization which heals this bad 
behavior. We are caching field position in TABLE::field array in
Item's member and are using this position for speeding up field
lookups in fix_fields() in case of its re-execution.

BitKeeper/etc/logging_ok

@@ -26,6 +26,7 @@ bk@admin.bk
 bk@mysql.r18.ru
 carsten@tsort.bitbybit.dk
 davida@isil.mysql.com
+dlenev@brandersnatch.localdomain
 dlenev@build.mysql.com
 dlenev@mysql.com
 gerberb@ou800.zenez.com

sql/item.cc

@@ -105,7 +105,7 @@ Item_ident::Item_ident(const char *db_name_par,const char *table_name_par,
 		       const char *field_name_par)
   :changed_during_fix_field(0), db_name(db_name_par),
    table_name(table_name_par), field_name(field_name_par),
-   depended_from(0)
+   cached_field_index(-1), depended_from(0)
 {
   name = (char*) field_name_par;
 }
@@ -117,6 +117,7 @@ Item_ident::Item_ident(THD *thd, Item_ident *item)
    db_name(item->db_name),
    table_name(item->table_name),
    field_name(item->field_name),
+   cached_field_index(item->cached_field_index),
    depended_from(item->depended_from)
 {}
 

sql/item.h

@@ -262,6 +262,11 @@ public:
   const char *db_name;
   const char *table_name;
   const char *field_name;
+  /* 
+    Cached value of index for this field in table->field array, used by prep. 
+    stmts for speeding up their re-execution, -1 if index value is not known.
+  */
+  int cached_field_index;
   st_select_lex *depended_from;
   Item_ident(const char *db_name_par,const char *table_name_par,
 	     const char *field_name_par);

sql/mysql_priv.h

@@ -558,7 +558,8 @@ extern const Field *not_found_field;
 Field *find_field_in_tables(THD *thd, Item_ident *item, TABLE_LIST *tables,
 			    TABLE_LIST **where, bool report_error);
 Field *find_field_in_table(THD *thd,TABLE *table,const char *name,uint length,
-			   bool check_grant,bool allow_rowid);
+                           bool check_grant,bool allow_rowid,
+                           int *cached_field_index_ptr);
 #ifdef HAVE_OPENSSL
 #include <openssl/des.h>
 struct st_des_keyblock

sql/sql_acl.cc

@@ -2194,8 +2194,10 @@ int mysql_table_grant(THD *thd, TABLE_LIST *table_list,
       DBUG_RETURN(-1);
     while ((column = column_iter++))
     {
+      int unused_field_idx= -1;
       if (!find_field_in_table(thd,table,column->column.ptr(),
-			       column->column.length(),0,0))
+                               column->column.length(),0,0,
+                               &unused_field_idx))
       {
 	my_error(ER_BAD_FIELD_ERROR, MYF(0),
                  column->column.c_ptr(), table_list->alias);

sql/sql_base.cc

@@ -1790,33 +1790,44 @@ bool rm_temporary_table(enum db_type base, char *path)
 #define WRONG_GRANT (Field*) -1
 
 Field *find_field_in_table(THD *thd,TABLE *table,const char *name,uint length,
-			   bool check_grants, bool allow_rowid)
+                           bool check_grants, bool allow_rowid, 
+                           int *cached_field_index_ptr)
 {
-  Field *field;
-  if (table->name_hash.records)
-  {
-    if ((field=(Field*) hash_search(&table->name_hash,(byte*) name,
-				    length)))
-      goto found;
-  }
+  Field **field_ptr= 0, *field;
+  int cached_field_index= *cached_field_index_ptr;
+
+  if (cached_field_index >= 0 && cached_field_index < table->fields &&
+      !my_strcasecmp(system_charset_info, 
+                     table->field[cached_field_index]->field_name, name))
+      field_ptr= table->field + cached_field_index;
+  else if (table->name_hash.records)
+    field_ptr= (Field**)hash_search(&table->name_hash,(byte*) name,
+                                    length);
   else
   {
-    Field **ptr;
-    if (!(ptr=table->field))
+    if (!(field_ptr=table->field))
       return (Field *)0;
-    while ((field = *ptr++))
+    while (*field_ptr)
     {
-      if (!my_strcasecmp(system_charset_info, field->field_name, name))
-	goto found;
+      if (!my_strcasecmp(system_charset_info, (*field_ptr)->field_name, name))
+        break;
+      ++field_ptr;
     }
   }
-  if (allow_rowid &&
-      !my_strcasecmp(system_charset_info, name, "_rowid") &&
-      (field=table->rowid_field))
-    goto found;
-  return (Field*) 0;
 
- found:
+  if (field_ptr && *field_ptr)
+  {
+    *cached_field_index_ptr= field_ptr - table->field;
+    field= *field_ptr;
+  }
+  else
+  {
+    if (!allow_rowid ||
+        my_strcasecmp(system_charset_info, name, "_rowid") ||
+        !(field=table->rowid_field))
+      return (Field*) 0;
+  }
+
   if (thd->set_query_id)
   {
     if (field->query_id != thd->query_id)
@@ -1895,7 +1906,7 @@ find_field_in_tables(THD *thd, Item_ident *item, TABLE_LIST *tables,
 	Field *find=find_field_in_table(thd,tables->table,name,length,
 					test(tables->table->grant.
 					     want_privilege),
-					1);
+					1, &(item->cached_field_index));
 	if (find)
 	{
 	  (*where)= tables;
@@ -1952,7 +1963,7 @@ find_field_in_tables(THD *thd, Item_ident *item, TABLE_LIST *tables,
 
     Field *field=find_field_in_table(thd,tables->table,name,length,
 				     test(tables->table->grant.want_privilege),
-				     allow_rowid);
+				     allow_rowid, &(item->cached_field_index));
     if (field)
     {
       if (field == WRONG_GRANT)

sql/table.cc

@@ -30,11 +30,11 @@ static void fix_type_pointers(const char ***array, TYPELIB *point_to_type,
 static uint find_field(TABLE *form,uint start,uint length);
 
 
-static byte* get_field_name(Field *buff,uint *length,
+static byte* get_field_name(Field **buff,uint *length,
 			    my_bool not_used __attribute__((unused)))
 {
-  *length= (uint) strlen(buff->field_name);
-  return (byte*) buff->field_name;
+  *length= (uint) strlen((*buff)->field_name);
+  return (byte*) (*buff)->field_name;
 }
 
 /*
@@ -479,7 +479,7 @@ int openfrm(const char *name, const char *alias, uint db_stat, uint prgflag,
     if (outparam->timestamp_field == reg_field)
       outparam->timestamp_field_offset=i;
     if (use_hash)
-      (void) my_hash_insert(&outparam->name_hash,(byte*) *field_ptr); // Will never fail
+      (void) my_hash_insert(&outparam->name_hash,(byte*) field_ptr); // Will never fail
   }
   *field_ptr=0;					// End marker
 

sql/table.h

@@ -65,7 +65,8 @@ struct st_table {
   handler *file;
   Field **field;			/* Pointer to fields */
   Field_blob **blob_field;		/* Pointer to blob fields */
-  HASH	name_hash;			/* hash of field names */
+  /* hash of field names (contains pointers to elements of field array) */
+  HASH	name_hash;
   byte *record[2];			/* Pointer to records */
   byte *default_values;          	/* Default values for INSERT */
   byte *insert_values;                  /* used by INSERT ... UPDATE */