Some small accelerations ...

I intend to do some more accelerations in mysql_execute_command and
in SQLCOM_SELECT part for which I need to fix EXPLAIN with derived
tables plus few additional items. If this proves to be much I wil
just make it a new SCRUM task

sql/sql_lex.h

@@ -378,7 +378,7 @@ typedef struct st_lex
   uint param_count;
   bool drop_primary, drop_if_exists, local_file, olap;
   bool in_comment, ignore_space, verbose, simple_alter;
-  bool derived_tables;
+  bool derived_tables, describe;
   uint slave_thd_opt;
   CHARSET_INFO *charset;
 } LEX;

sql/sql_parse.cc

@@ -1335,8 +1335,7 @@ mysql_execute_command(THD *thd)
     TODO: solve problem with depended derived tables in subselects
   */
   if (lex->sql_command == SQLCOM_SELECT && 
-      (select_lex->options & SELECT_DESCRIBE) &&
-      lex->derived_tables)
+      lex->describe && lex->derived_tables)
   {
     if (!(explain_result= new select_send()))
     {
@@ -1405,8 +1404,6 @@ mysql_execute_command(THD *thd)
   case SQLCOM_SELECT:
   {
     select_result *result=lex->result;
-    if (select_lex->options & SELECT_DESCRIBE)
-      lex->exchange=0;
     if (tables)
     {
       res=check_table_access(thd,
@@ -1431,6 +1428,27 @@ mysql_execute_command(THD *thd)
     if (unit->select_limit_cnt == HA_POS_ERROR)
       select_lex->options&= ~OPTION_FOUND_ROWS;
 
+    if (!(res=open_and_lock_tables(thd,tables)))
+    {
+      if (lex->describe)
+      {
+	if (!explain_result)
+	  if (!(explain_result= new select_send()))
+	  {
+	    send_error(thd, ER_OUT_OF_RESOURCES);
+	    DBUG_VOID_RETURN;
+	  }
+	  else
+	    thd->send_explain_fields(explain_result);
+	fix_tables_pointers(select_lex);
+	res= mysql_explain_union(thd, &thd->lex.unit, explain_result);
+	MYSQL_LOCK *save_lock= thd->lock;
+	thd->lock= (MYSQL_LOCK *)0;
+	explain_result->send_eof();
+	thd->lock= save_lock;
+      }
+      else
+      {
 	if (!result)
 	{
 	  if ((result=new select_send()))
@@ -1454,35 +1472,10 @@ mysql_execute_command(THD *thd)
 	    break;
 	  }
 	}
-
-    if (!(res=open_and_lock_tables(thd,tables)))
-    {
-      if (select_lex->options & SELECT_DESCRIBE)
-      {
-	delete result; // we do not need it for explain
-	if (!explain_result)
-	  if (!(explain_result= new select_send()))
-	  {
-	    send_error(thd, ER_OUT_OF_RESOURCES);
-	    DBUG_VOID_RETURN;
-	  }
-	  else
-	    thd->send_explain_fields(explain_result);
-	fix_tables_pointers(select_lex);
-	res= mysql_explain_union(thd, &thd->lex.unit, explain_result);
-	MYSQL_LOCK *save_lock= thd->lock;
-	thd->lock= (MYSQL_LOCK *)0;
-	explain_result->send_eof();
-	thd->lock= save_lock;
-      }
-      else
-      {
 	query_cache_store_query(thd, tables);
 	res=handle_select(thd, lex, result);
       }
     }
-    else
-      delete result;
     break;
   }
   case SQLCOM_DO:
@@ -2913,7 +2906,7 @@ mysql_init_query(THD *thd)
   thd->free_list= 0;
   thd->lex.union_option= 0;
   thd->lex.select= &thd->lex.select_lex;
-  thd->lex.olap=0;
+  thd->lex.olap=thd->lex.describe=0;
   thd->lex.select->olap= UNSPECIFIED_OLAP_TYPE;
   thd->fatal_error= 0;				// Safety
   thd->total_warn_count=0;			// Warnings for this query

sql/sql_yacc.yy

@@ -2553,7 +2553,8 @@ procedure_item:
 
 select_var_list_init:
 	   {
-	      if (!(Lex->result= new select_dumpvar()))
+             LEX *lex=Lex;
+	     if (!lex->describe && (!(lex->result= new select_dumpvar())))
 	        YYABORT;
 	   }
 	   select_var_list 
@@ -2567,28 +2568,35 @@ select_var_list:
 select_var_ident:  '@' ident_or_text
            {
              LEX *lex=Lex;
-	     if ( ((select_dumpvar *)lex->result)->var_list.push_back((LEX_STRING*) sql_memdup(&$2,sizeof(LEX_STRING))))
+	     if (lex->result && ((select_dumpvar *)lex->result)->var_list.push_back((LEX_STRING*) sql_memdup(&$2,sizeof(LEX_STRING))))
 	       YYABORT;
 	   }
+           ;
 
 opt_into:
         INTO OUTFILE TEXT_STRING
 	{
 	  LEX *lex=Lex;
+	  if (!lex->describe)
+	  {
 	    if (!(lex->exchange= new sql_exchange($3.str,0)))
 	      YYABORT;
 	    if (!(lex->result= new select_export(lex->exchange)))
 	      YYABORT;
 	  }
+	}
 	opt_field_term opt_line_term
 	| INTO DUMPFILE TEXT_STRING
 	{
 	  LEX *lex=Lex;
+	  if (!lex->describe)
+	  {
 	    if (!(lex->exchange= new sql_exchange($3.str,1)))
 	      YYABORT;
 	    if (!(lex->result= new select_dump(lex->exchange)))
 	      YYABORT;
 	  }
+	}
         | INTO select_var_list_init
 	{
 	  current_thd->safe_to_cache_query=0;
@@ -3057,7 +3065,11 @@ describe:
 	}
 	opt_describe_column
 	| describe_command select
-          { Lex->select_lex.options|= SELECT_DESCRIBE; };
+          { 
+	    LEX *lex=Lex;
+	    lex->select_lex.options|= SELECT_DESCRIBE; 
+	    lex->describe=1;
+	  };
 
 
 describe_command: