Simplify test for updateable TABLE_CATEGORY's

- Re-numbered enum_table_category to make some tests easier
- Moved TABLE_CATEGORY_INFORMATION to be first CATEGORY of virtual tables
- Don't take MDL locks for not updateable table category's

sql/sql_base.cc

@@ -2038,8 +2038,8 @@ bool open_table(THD *thd, TABLE_LIST *table_list, Open_table_context *ot_ctx)
     tc_add_table(thd, table);
   }
 
-
-  if (!(flags & MYSQL_OPEN_HAS_MDL_LOCK))
+  if (!(flags & MYSQL_OPEN_HAS_MDL_LOCK) &&
+      table->s->table_category < TABLE_CATEGORY_INFORMATION)
   {
     /*
       We are not under LOCK TABLES and going to acquire write-lock/

sql/sql_parse.cc

@@ -1180,10 +1180,8 @@ static bool wsrep_tables_accessible_when_detached(const TABLE_LIST *tables)
 {
   for (const TABLE_LIST *table= tables; table; table= table->next_global)
   {
-    TABLE_CATEGORY c;
     LEX_CSTRING db= table->db, tn= table->table_name;
-    c= get_table_category(&db, &tn);
-    if (c != TABLE_CATEGORY_INFORMATION && c != TABLE_CATEGORY_PERFORMANCE)
+    if (get_table_category(&db, &tn)  < TABLE_CATEGORY_INFORMATION)
       return false;
   }
   return true;

sql/table.h

@@ -394,28 +394,6 @@ enum enum_table_category
   */
   TABLE_CATEGORY_SYSTEM=3,
 
-  /**
-    Information schema tables.
-    These tables are an interface provided by the system
-    to inspect the system metadata.
-    These tables do *not* honor:
-    - LOCK TABLE t FOR READ/WRITE
-    - FLUSH TABLES WITH READ LOCK
-    - SET GLOBAL READ_ONLY = ON
-    as there is no point in locking explicitly
-    an INFORMATION_SCHEMA table.
-    Nothing is directly written to information schema tables.
-    Note that this value is not used currently,
-    since information schema tables are not shared,
-    but implemented as session specific temporary tables.
-  */
-  /*
-    TODO: Fixing the performance issues of I_S will lead
-    to I_S tables in the table cache, which should use
-    this table type.
-  */
-  TABLE_CATEGORY_INFORMATION=4,
-
   /**
     Log tables.
     These tables are an interface provided by the system
@@ -436,7 +414,33 @@ enum enum_table_category
     The server implementation perform writes.
     Log tables are cached in the table cache.
   */
-  TABLE_CATEGORY_LOG=5,
+  TABLE_CATEGORY_LOG=4,
+
+  /*
+    Types below are read only tables, not affected by FLUSH TABLES or
+    MDL locks.
+  */
+  /**
+    Information schema tables.
+    These tables are an interface provided by the system
+    to inspect the system metadata.
+    These tables do *not* honor:
+    - LOCK TABLE t FOR READ/WRITE
+    - FLUSH TABLES WITH READ LOCK
+    - SET GLOBAL READ_ONLY = ON
+    as there is no point in locking explicitly
+    an INFORMATION_SCHEMA table.
+    Nothing is directly written to information schema tables.
+    Note that this value is not used currently,
+    since information schema tables are not shared,
+    but implemented as session specific temporary tables.
+  */
+  /*
+    TODO: Fixing the performance issues of I_S will lead
+    to I_S tables in the table cache, which should use
+    this table type.
+  */
+  TABLE_CATEGORY_INFORMATION=5,
 
   /**
     Performance schema tables.
@@ -460,6 +464,7 @@ enum enum_table_category
   */
   TABLE_CATEGORY_PERFORMANCE=6
 };
+
 typedef enum enum_table_category TABLE_CATEGORY;
 
 TABLE_CATEGORY get_table_category(const LEX_CSTRING *db,