SELECT COUNT(*) with an ORDER BY

Question

Will the following two queries be executed in the same way?

SELECT COUNT(*) from person ORDER BY last_name;

and

SELECT COUNT(*) from person;

Either way they should display the same results, so I was curious if the ORDER BY just gets ignored.

The reason I am asking is because I am displaying a paginated table where I will get 20 records at a time from the database and then firing a second query that counts the total number of records. I want to know if I should use the same criteria that the first query used, or if I should be removing all sorting from the criteria?

Answer 1

According to the execution plan, the two queries are different. For example, the query:

select count(*) from USER

Will give me:

INDEX (FAST FULL SCAN)  3.0 3   453812  3457        1   TPMDBO  USER_PK FAST FULL SCAN  INDEX (UNIQUE)  ANALYZED

As you can see, we hit USER_PK which is the primary key of that table.

If I sort by a non-indexed column:

select count(*) from USER ORDER BY FIRSTNAME --No Index on FIRSTNAME

I'll get:

TABLE ACCESS (FULL) 19.0    19  1124488 3457    24199   1   TPMDBO  USER    FULL    TABLE   ANALYZED    1

Meaning we did a full table scan (MUCH higher node cost)

If I sort by the primary key (which is already index,) Oracle is smart enough to use the index to do that sort:

INDEX (FAST FULL SCAN)  3.0 3   453812  3457    13828   1   TPMDBO  USER_PK FAST FULL SCAN  INDEX (UNIQUE)  ANALYZED

Which looks very similar to the first execution plan.

So, the answer to your question is absolutely not - they are not the same. However, ordering by an index that Oracle is already seeking anyway will probably result in the same query plan.

Answer 2

Of course not. Unless last name is the primary key and you are already ordered by that.

Answer 3

The Oracle query optimizer actually does perform a sort (I verified this looking at the explain plan) for the first version, but since both queries only return one row, the performance difference will be very small.

EDIT:

Mike's answer is correct. The performance difference can possibly be significant.