re-fix vcols on demand, not always for every SELECT

sql/item.cc

@@ -5219,6 +5219,8 @@ bool Item_field::fix_fields(THD *thd, Item **reference)
   }
 #endif
   fixed= 1;
+  if (field->vcol_info)
+    fix_session_vcol_expr_for_read(thd, field, field->vcol_info);
   if (thd->variables.sql_mode & MODE_ONLY_FULL_GROUP_BY &&
       !outer_fixed && !thd->lex->in_sum_func &&
       thd->lex->current_select->cur_pos_in_select_list != UNDEF_POS &&
@@ -8231,6 +8233,7 @@ bool Item_default_value::fix_fields(THD *thd, Item **items)
   set_field(def_field);
   if (field->default_value)
   {
+    fix_session_vcol_expr_for_read(thd, field, field->default_value);
     if (thd->mark_used_columns != MARK_COLUMNS_NONE)
       field->default_value->expr_item->walk(&Item::register_field_in_read_map, 1, 0);
     IF_DBUG(def_field->is_stat_field=1,); // a hack to fool ASSERT_COLUMN_MARKED_FOR_WRITE_OR_COMPUTED

sql/sql_base.cc

@@ -4697,7 +4697,8 @@ static bool fix_all_session_vcol_exprs(THD *thd, TABLE_LIST *tables)
        table= table->next_global)
   {
     TABLE *t= table->table;
-    if (!table->placeholder() && t->s->vcols_need_refixing)
+    if (!table->placeholder() && t->s->vcols_need_refixing &&
+         table->lock_type >= TL_WRITE_ALLOW_WRITE)
     {
       if (table->security_ctx)
         thd->security_ctx= table->security_ctx;
@@ -4711,7 +4712,6 @@ static bool fix_all_session_vcol_exprs(THD *thd, TABLE_LIST *tables)
             fix_session_vcol_expr(thd, (*df)->default_value))
           goto err;
 
-      if (table->lock_type >= TL_WRITE_ALLOW_WRITE)
         for (Virtual_column_info **cc= t->check_constraints; cc && *cc; cc++)
           if (fix_session_vcol_expr(thd, (*cc)))
             goto err;

sql/table.cc

@@ -2543,7 +2543,11 @@ static bool fix_vcol_expr(THD *thd, Virtual_column_info *vcol)
   DBUG_RETURN(0);
 }
 
+/** rerun fix_fields for vcols that returns time- or session- dependent values
 
+    @note this is done for all vcols for INSERT/UPDATE/DELETE,
+    and only as needed for SELECTs.
+*/
 bool fix_session_vcol_expr(THD *thd, Virtual_column_info *vcol)
 {
   DBUG_ENTER("fix_session_vcol_expr");
@@ -2556,6 +2560,27 @@ bool fix_session_vcol_expr(THD *thd, Virtual_column_info *vcol)
 }
 
 
+/** invoke fix_session_vcol_expr for a vcol
+
+    @note this is called for generated column or a DEFAULT expression from
+    their corresponding fix_fields on SELECT.
+*/
+bool fix_session_vcol_expr_for_read(THD *thd, Field *field,
+                                    Virtual_column_info *vcol)
+{
+  DBUG_ENTER("fix_session_vcol_expr_for_read");
+  TABLE_LIST *tl= field->table->pos_in_table_list;
+  if (!tl || tl->lock_type >= TL_WRITE_ALLOW_WRITE)
+    DBUG_RETURN(0);
+  Security_context *save_security_ctx= thd->security_ctx;
+  if (tl->security_ctx)
+    thd->security_ctx= tl->security_ctx;
+  bool res= fix_session_vcol_expr(thd, vcol);
+  thd->security_ctx= save_security_ctx;
+  DBUG_RETURN(res);
+}
+
+
 /*
   @brief 
     Perform semantic analysis of the defining expression for a virtual column

sql/table.h

@@ -2620,6 +2620,8 @@ enum open_frm_error open_table_from_share(THD *thd, TABLE_SHARE *share,
                        uint ha_open_flags, TABLE *outparam,
                        bool is_create_table);
 bool fix_session_vcol_expr(THD *thd, Virtual_column_info *vcol);
+bool fix_session_vcol_expr_for_read(THD *thd, Field *field,
+                                    Virtual_column_info *vcol);
 Virtual_column_info *unpack_vcol_info_from_frm(THD *thd, MEM_ROOT *mem_root,
                                                TABLE *table,
                                                Field *field,