Better comments

sql/sql_select.cc

@@ -20736,6 +20736,24 @@ static void free_blobs(Field **ptr)
 }
 
 
+/*
+  @brief
+    Remove duplicates from a temporary table.
+
+  @detail
+    Remove duplicate rows from a temporary table. This is used for e.g. queries
+    like
+
+      select distinct count(*) as CNT from tbl group by col
+
+    Here, we get a group table with count(*) values. It is not possible to
+    prevent duplicates from appearing in the table (as we don't know the values
+    before we've done the grouping).  Because of that, we have this function to
+    scan the temptable (maybe, multiple times) and remove the duplicate rows
+
+    Rows that do not satisfy 'having' condition are also removed.
+*/
+
 static int
 remove_duplicates(JOIN *join, TABLE *table, List<Item> &fields, Item *having)
 {

sql/table.h

@@ -1072,6 +1072,12 @@ public:
   TABLE_LIST *pos_in_table_list;/* Element referring to this table */
   /* Position in thd->locked_table_list under LOCK TABLES */
   TABLE_LIST *pos_in_locked_tables;
+
+  /*
+    Not-null for temporary tables only. Non-null values means this table is
+    used to compute GROUP BY, it has a unique of GROUP BY columns.
+    (set by create_tmp_table)
+  */
   ORDER		*group;
   String	alias;            	  /* alias or table name */
   uchar		*null_flags;