ISSUE:
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For UNION of selects, rows examined by the query will be sum
of rows examined by individual select operations and rows
examined for union operation. The value of session level
global counter that is used to count the rows examined by a
select statement should be accumulated and reset before it
is used for next select statement. But we have missed to
reset the same. Because of this examined row count of a
select query is accounted more than once.

SOLUTION:
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In union reset the session level global counter used to
accumulate count of examined rows after its value is saved.

mysql-test/r/union.result:
  Expected output of testcase added.
mysql-test/t/union.test:
  Test to verify examined row count of Union operations.
sql/sql_union.cc:
  Reset the value of thd->examined_row_count after
  accumulating the value.