MWL#17: Table elimination

- Moved table elimination code to sql/opt_table_elimination.cc
- Added comments 

.bzrignore:
  MWL#17: Table elimination
  - Moved table elimination code to sql/opt_table_elimination.cc
libmysqld/Makefile.am:
  MWL#17: Table elimination
  - Moved table elimination code to sql/opt_table_elimination.cc
sql/CMakeLists.txt:
  MWL#17: Table elimination
  - Moved table elimination code to sql/opt_table_elimination.cc
sql/Makefile.am:
  MWL#17: Table elimination
  - Moved table elimination code to sql/opt_table_elimination.cc

.bzrignore

@@ -1905,3 +1905,4 @@ sql/share/swedish
 sql/share/ukrainian
 libmysqld/examples/mysqltest.cc
 extra/libevent/event-config.h
+libmysqld/opt_table_elimination.cc

libmysqld/Makefile.am

@@ -76,7 +76,7 @@ sqlsources = derror.cc field.cc field_conv.cc strfunc.cc filesort.cc \
 	rpl_filter.cc sql_partition.cc sql_builtin.cc sql_plugin.cc \
 	sql_tablespace.cc \
 	rpl_injector.cc my_user.c partition_info.cc \
-	sql_servers.cc event_parse_data.cc
+	sql_servers.cc event_parse_data.cc opt_table_elimination.cc
 
 libmysqld_int_a_SOURCES= $(libmysqld_sources)
 nodist_libmysqld_int_a_SOURCES= $(libmysqlsources) $(sqlsources)

sql/CMakeLists.txt

@@ -73,7 +73,7 @@ ADD_EXECUTABLE(mysqld
                partition_info.cc rpl_utility.cc rpl_injector.cc sql_locale.cc
                rpl_rli.cc rpl_mi.cc sql_servers.cc
                sql_connect.cc scheduler.cc 
-               sql_profile.cc event_parse_data.cc
+               sql_profile.cc event_parse_data.cc opt_table_elimination.cc
                ${PROJECT_SOURCE_DIR}/sql/sql_yacc.cc
                ${PROJECT_SOURCE_DIR}/sql/sql_yacc.h
                ${PROJECT_SOURCE_DIR}/include/mysqld_error.h

sql/Makefile.am

@@ -121,7 +121,8 @@ mysqld_SOURCES =	sql_lex.cc sql_handler.cc sql_partition.cc \
                         event_queue.cc event_db_repository.cc events.cc \
 			sql_plugin.cc sql_binlog.cc \
 			sql_builtin.cc sql_tablespace.cc partition_info.cc \
-			sql_servers.cc event_parse_data.cc
+			sql_servers.cc event_parse_data.cc \
+                        opt_table_elimination.cc
 
 nodist_mysqld_SOURCES =	mini_client_errors.c pack.c client.c my_time.c my_user.c 
 

sql/item.cc

@@ -1915,17 +1915,22 @@ void Item_field::reset_field(Field *f)
   name= (char*) f->field_name;
 }
 
+
 bool Item_field::check_column_usage_processor(uchar *arg)
 {
   Field_processor_info* info=(Field_processor_info*)arg;
+
+  /* It is ok if this is a column of an allowed table: */
   if (used_tables() & ~info->allowed_tables)
     return FALSE;
 
   if (field->table == info->table)
   {
+    /* It is not ok to use columns that are not part of the key of interest: */
     if (!(field->part_of_key.is_set(info->keyno)))
        return TRUE;
-
+    
+    /* Find which key part we're using and mark it in needed_key_parts */
     KEY *key= &field->table->key_info[info->keyno];
     for (uint part= 0; part < key->key_parts; part++)
     {
@@ -1935,10 +1940,17 @@ bool Item_field::check_column_usage_processor(uchar *arg)
         break;
       }
     }
+    return FALSE;
   }
-  return FALSE;
+
+  /* 
+    We get here when this refers to a table that's neither the table of
+    interest, nor one of the allowed tables. 
+  */
+  return TRUE;
 }
 
+
 const char *Item_ident::full_name() const
 {
   char *tmp;

sql/item.h

@@ -731,7 +731,11 @@ public:
   virtual bool val_bool_result() { return val_bool(); }
   virtual bool is_null_result() { return is_null(); }
 
-  /* bit map of tables used by item */
+  /* 
+    Bitmap of tables used by item
+    (note: if you need to check dependencies on individual columns, check out
+     check_column_usage_processor)
+  */
   virtual table_map used_tables() const { return (table_map) 0L; }
   /*
     Return table map of tables that can't be NULL tables (tables that are
@@ -1013,7 +1017,7 @@ public:
   bool eq_by_collation(Item *item, bool binary_cmp, CHARSET_INFO *cs); 
 };
 
-
+/* Data for Item::check_column_usage_processor */
 typedef struct  
 {
   table_map allowed_tables;
@@ -1022,6 +1026,7 @@ typedef struct
   uint needed_key_parts;
 } Field_processor_info;
 
+
 class sp_head;
 
 

sql/item_subselect.cc

@@ -250,7 +250,7 @@ bool Item_subselect::walk(Item_processor processor, bool walk_subquery,
       if (lex->having && (lex->having)->walk(processor, walk_subquery,
                                              argument))
         return 1;
-      /* TODO: why doesn't this walk the OUTER JOINs' ON expressions */
+      /* TODO: why does this walk WHERE/HAVING but not ON expressions of outer joins? */
 
       while ((item=li++))
       {

sql/item_sum.h

@@ -351,9 +351,10 @@ public:
     Return bitmap of tables that are needed to evaluate the item.
 
     The implementation takes into account the used strategy: items resolved
-    at optimization phase report 0.
-    Items that depend on the number of rows only, e.g. COUNT(*) will report
-    zero, but will still false from const_item().
+    at optimization phase will report 0.
+    Items that depend on the number of join output records, but not columns
+    of any particular table (like COUNT(*)) will report 0 from used_tables(),
+    but will still return false from const_item().
   */
   table_map used_tables() const { return used_tables_cache; }
   void update_used_tables ();

sql/sql_select.h

@@ -28,6 +28,11 @@
 #include "procedure.h"
 #include <myisam.h>
 
+#define FT_KEYPART   (MAX_REF_PARTS+10)
+/* Values in optimize */
+#define KEY_OPTIMIZE_EXISTS		1
+#define KEY_OPTIMIZE_REF_OR_NULL	2
+
 typedef struct keyuse_t {
   TABLE *table;
   Item	*val;				/**< or value if no field */
@@ -51,6 +56,14 @@ typedef struct keyuse_t {
     NULL  - Otherwise (the source equality can't be turned off)
   */
   bool *cond_guard;
+  /*
+    TRUE  <=> This keyuse can be used to construct key access.
+    FALSE <=> Otherwise. Currently unusable KEYUSEs represent equalities
+              where one table column refers to another one, like this:
+                t.keyXpartA=func(t.keyXpartB)
+              This equality cannot be used for index access but is useful
+              for table elimination.
+  */
   bool usable;
 } KEYUSE;
 
@@ -734,9 +747,13 @@ bool error_if_full_join(JOIN *join);
 int report_error(TABLE *table, int error);
 int safe_index_read(JOIN_TAB *tab);
 COND *remove_eq_conds(THD *thd, COND *cond, Item::cond_result *cond_value);
+void set_position(JOIN *join,uint idx,JOIN_TAB *table,KEYUSE *key);
 
 inline bool optimizer_flag(THD *thd, uint flag)
 { 
   return (thd->variables.optimizer_switch & flag);
 }
 
+void eliminate_tables(JOIN *join, uint *const_tbl_count, 
+                      table_map *const_tables);
+