Bug#31700: thd->examined_row_count not incremented for 'const' type queries

UNIQUE (eq-ref) lookups result in table being considered as a "constant" table.
Queries that consist of only constant tables are processed in do_select() in a
special way that doesn't invoke evaluate_join_record(), and therefore doesn't
increase the counters join->examined_rows and join->thd->row_count.

The patch increases these counters in this special case.

NOTICE:
This behavior seems to contradict what the documentation says in Sect. 5.11.4:
"Queries handled by the query cache are not added to the slow query log, nor
are queries that would not benefit from the presence of an index because the
table has zero rows or one row."

No test case in 5.0 as issue shows only in slow query log, and other counters
can give subtly different values (with regard to counting in create_sort_index(),
synthetic rows in ROLLUP, etc.).


sql/sql_class.h:
  add documentation for some variables
sql/sql_select.cc:
  Don't forget const tables when counting read records!

sql/sql_class.h

@@ -1368,9 +1368,20 @@ class THD :public Statement,
 
   ulonglong  limit_found_rows;
   ulonglong  options;           /* Bitmap of states */
-  longlong   row_count_func;	/* For the ROW_COUNT() function */
-  ha_rows    cuted_fields,
-             sent_row_count, examined_row_count;
+  longlong   row_count_func;    /* For the ROW_COUNT() function */
+  ha_rows    cuted_fields;
+
+  /*
+    number of rows we actually sent to the client, including "synthetic"
+    rows in ROLLUP etc.
+  */
+  ha_rows    sent_row_count;
+
+  /*
+    number of rows we read, sent or not, including in create_sort_index()
+  */
+  ha_rows    examined_row_count;
+
   /*
     The set of those tables whose fields are referenced in all subqueries
     of the query.
@@ -1403,7 +1414,11 @@ class THD :public Statement,
   /* Statement id is thread-wide. This counter is used to generate ids */
   ulong      statement_id_counter;
   ulong	     rand_saved_seed1, rand_saved_seed2;
-  ulong      row_count;  // Row counter, mainly for errors and warnings
+  /*
+    Row counter, mainly for errors and warnings. Not increased in
+    create_sort_index(); may differ from examined_row_count.
+  */
+  ulong      row_count;
   long	     dbug_thread_id;
   pthread_t  real_id;
   uint	     tmp_table, global_read_lock;

sql/sql_select.cc

@@ -10339,6 +10339,15 @@ do_select(JOIN *join,List<Item> *fields,TABLE *table,Procedure *procedure)
       error= (*end_select)(join,join_tab,0);
       if (error == NESTED_LOOP_OK || error == NESTED_LOOP_QUERY_LIMIT)
 	error= (*end_select)(join,join_tab,1);
+
+      /*
+        If we don't go through evaluate_join_record(), do the counting
+        here.  join->send_records is increased on success in end_send(),
+        so we don't touch it here.
+      */
+      join->examined_rows++;
+      join->thd->row_count++;
+      DBUG_ASSERT(join->examined_rows <= 1);
     }
     else if (join->send_row_on_empty_set())
     {