Bug #49756 Rows_examined is always 0 in the slow query log for

update statements
      
Only SELECT statements report any examined rows in the slow
log. Slow UPDATE, DELETE and INSERT statements report 0 rows
examined, unless the statement has a condition including a
SELECT substatement.
      
This patch adds counting of examined rows for the UPDATE and
DELETE statements. An INSERT ... VALUES statement will still 
not report any rows as examined.

sql/sql_class.h

@@ -1716,8 +1716,15 @@ public:
   */
   ha_rows    sent_row_count;
 
-  /*
-    number of rows we read, sent or not, including in create_sort_index()
+  /**
+    Number of rows read and/or evaluated for a statement. Used for
+    slow log reporting.
+
+    An examined row is defined as a row that is read and/or evaluated
+    according to a statement condition, including in
+    create_sort_index(). Rows may be counted more than once, e.g., a
+    statement including ORDER BY could possibly evaluate the row in
+    filesort() before reading it for e.g. update.
   */
   ha_rows    examined_row_count;