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테스트 : 데이타 650만개, fexid 는 인덱스가 걸려있습니다.

첫번째
mysql> select count(DISTINCT fexid) from tablename;
1 row in set (14.79 sec)

mysql> select count(*) from tablename group by ;
432 rows in set (2.43 sec)

mysql> select count(1) from ( select count(1) from tablename group by fexid ) ZET;
1 row in set (2.36 sec)

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두번째
mysql 서버를 종료 후 재 시작

mysql> select count(*) from tablename group by fexid;
432 rows in set (2.60 sec)

mysql> select count(DISTINCT fexid) from tablename;
1 row in set (2.91 sec)

mysql> select count(1) from ( select count(1) from tablename group by fexid ) ZET;
1 row in set (2.14 sec)

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세번째
mysql 서버를 종료 후 재 시작

mysql> select count(1) from ( select count(1) from tablename group by fexid ) ZET;
1 row in set (2.56 sec)

mysql> select count(*) from tablename group by fexid;
432 rows in set (2.09 sec)

mysql> select count(DISTINCT fexid) from tablename;
1 row in set (3.00 sec)