Static current schema and state change trackers

Saves 2 new/delete per connection.

Part of MDEV-14984 - regression in connect performance

sql/session_tracker.cc

@@ -173,43 +173,6 @@ class Session_sysvars_tracker : public State_tracker
 };
 
 
-
-/**
-  Current_schema_tracker,
-
-  This is a tracker class that enables & manages the tracking of current
-  schema for a particular connection.
-*/
-
-class Current_schema_tracker : public State_tracker
-{
-public:
-  bool update(THD *thd, set_var *var);
-  bool store(THD *thd, String *buf);
-};
-
-/*
-  Session_state_change_tracker
-
-  This is a boolean tracker class that will monitor any change that contributes
-  to a session state change.
-  Attributes that contribute to session state change include:
-     - Successful change to System variables
-     - User defined variables assignments
-     - temporary tables created, altered or deleted
-     - prepared statements added or removed
-     - change in current database
-     - change of current role
-*/
-
-class Session_state_change_tracker : public State_tracker
-{
-public:
-  bool update(THD *thd, set_var *var);
-  bool store(THD *thd, String *buf);
-};
-
-
 /* To be used in expanding the buffer. */
 static const unsigned int EXTRA_ALLOC= 1024;
 
@@ -1379,8 +1342,10 @@ Session_tracker::Session_tracker()
   compile_time_assert((uint)SESSION_TRACK_always_at_the_end >=
                       (uint)SESSION_TRACKER_END);
 
-  for (int i= 0; i < SESSION_TRACKER_END; i++)
-    m_trackers[i]= NULL;
+  m_trackers[SESSION_SYSVARS_TRACKER]= 0;
+  m_trackers[CURRENT_SCHEMA_TRACKER]= &current_schema;
+  m_trackers[SESSION_STATE_CHANGE_TRACKER]= &state_change;
+  m_trackers[TRANSACTION_INFO_TRACKER]= 0;
 }
 
 
@@ -1402,10 +1367,6 @@ void Session_tracker::enable(THD *thd)
   deinit();
   m_trackers[SESSION_SYSVARS_TRACKER]=
     new (std::nothrow) Session_sysvars_tracker();
-  m_trackers[CURRENT_SCHEMA_TRACKER]=
-    new (std::nothrow) Current_schema_tracker;
-  m_trackers[SESSION_STATE_CHANGE_TRACKER]=
-    new (std::nothrow) Session_state_change_tracker;
   m_trackers[TRANSACTION_INFO_TRACKER]=
     new (std::nothrow) Transaction_state_tracker;
 

sql/session_tracker.h

@@ -88,8 +88,18 @@ class State_tracker
 
   void reset_changed() { m_changed= false; }
 
-  /** Called in the constructor of THD*/
-  virtual bool enable(THD *thd) { return update(thd, 0); }
+  /**
+    Called by THD::init() when new connection is being created
+
+    We may inherit m_changed from previous connection served by this THD if
+    connection was broken or client didn't have session tracking capability.
+    Thus we have to reset it here.
+  */
+  virtual bool enable(THD *thd)
+  {
+    reset_changed();
+    return update(thd, 0);
+  }
 
   /** To be invoked when the tracker's system variable is updated (ON_UPDATE).*/
   virtual bool update(THD *thd, set_var *var)= 0;
@@ -101,11 +111,49 @@ class State_tracker
   virtual void mark_as_changed(THD *thd, LEX_CSTRING *name);
 };
 
+
 bool sysvartrack_validate_value(THD *thd, const char *str, size_t len);
 bool sysvartrack_global_update(THD *thd, char *str, size_t len);
 uchar *sysvartrack_session_value_ptr(THD *thd, const LEX_CSTRING *base);
 
 
+/**
+  Current_schema_tracker,
+
+  This is a tracker class that enables & manages the tracking of current
+  schema for a particular connection.
+*/
+
+class Current_schema_tracker: public State_tracker
+{
+public:
+  bool update(THD *thd, set_var *var);
+  bool store(THD *thd, String *buf);
+};
+
+
+/*
+  Session_state_change_tracker
+
+  This is a boolean tracker class that will monitor any change that contributes
+  to a session state change.
+  Attributes that contribute to session state change include:
+     - Successful change to System variables
+     - User defined variables assignments
+     - temporary tables created, altered or deleted
+     - prepared statements added or removed
+     - change in current database
+     - change of current role
+*/
+
+class Session_state_change_tracker: public State_tracker
+{
+public:
+  bool update(THD *thd, set_var *var);
+  bool store(THD *thd, String *buf);
+};
+
+
 /**
   Session_tracker
 
@@ -130,22 +178,19 @@ class Session_tracker
   }
 
 public:
+  Current_schema_tracker current_schema;
+  Session_state_change_tracker state_change;
 
   Session_tracker();
-  ~Session_tracker()
-  {
-    deinit();
-  }
+  ~Session_tracker() { deinit(); }
 
   /* trick to make happy memory accounting system */
   void deinit()
   {
-    for (int i= 0; i < SESSION_TRACKER_END; i++)
-    {
-      if (m_trackers[i])
-        delete m_trackers[i];
-      m_trackers[i]= NULL;
-    }
+    delete m_trackers[SESSION_SYSVARS_TRACKER];
+    m_trackers[SESSION_SYSVARS_TRACKER]= 0;
+    delete m_trackers[TRANSACTION_INFO_TRACKER];
+    m_trackers[TRANSACTION_INFO_TRACKER]= 0;
   }
 
   void enable(THD *thd);

sql/sys_vars.cc

@@ -6128,8 +6128,7 @@ static bool update_session_track_schema(sys_var *self, THD *thd,
                                         enum_var_type type)
 {
   DBUG_ENTER("update_session_track_schema");
-  DBUG_RETURN(thd->session_tracker.get_tracker(CURRENT_SCHEMA_TRACKER)->
-              update(thd, NULL));
+  DBUG_RETURN(thd->session_tracker.current_schema.update(thd, NULL));
 }
 
 static Sys_var_mybool Sys_session_track_schema(
@@ -6172,8 +6171,7 @@ static bool update_session_track_state_change(sys_var *self, THD *thd,
                                               enum_var_type type)
 {
   DBUG_ENTER("update_session_track_state_change");
-  DBUG_RETURN(thd->session_tracker.get_tracker(SESSION_STATE_CHANGE_TRACKER)->
-              update(thd, NULL));
+  DBUG_RETURN(thd->session_tracker.state_change.update(thd, NULL));
 }
 
 static Sys_var_mybool Sys_session_track_state_change(