Which statement is efficient?

Question

Which statement is efficient?
1.  SELECT MSG_ID,IP_ADDRESS,DATETIME INTO V_MSG_ID,V_IP_ADDRESS,V_DATETIME
       FROM (SELECT * FROM INCOMING_MSG_IDX ORDER BY LAST_CHECKTIME)
         WHERE STATUS = 4 AND MSG_TYPE >= LOW_MSG_TYPE AND MSG_TYPE <=
            UP_MSG_TYPE AND ROWNUM <=1;
2.   SELECT MSG_ID,IP_ADDRESS,DATETIME INTO V_MSG_ID,V_IP_ADDRESS,V_DATETIME
         FROM INCOMING_MSG_IDX
           WHERE STATUS = 4 AND MSG_TYPE  BETWEEN  LOW_MSG_TYPE AND
             UP_MSG_TYPE AND ROWNUM =1
                  ORDER BY LAST_CHECKTIME

Answer 1

jetq wrote:
Which statement is efficient?If more than one row satisfies your filter predicates in the WHERE clause then your question is irrelevant since the two statements are not equal.
The first one obeys the ORDER and returns the first entry ordered by LAST_CHECKTIME that passes the predicates.
The second one returns an arbitrary row that passes the predicates, because ROWNUM is applied before the ORDER BY, and therefore the two statements are not equal.
Regards,
Randolf
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