How Oracle returns queries & using rownum

Question

Hi, I need to limit query results using rownum, but my question is regarding how Oracle retrieves its data.
Assume I have a table containing 500 records and my query without using rownum will retrieve 62 of those records. The actual query will use rownum and limit results to 20.
I have a JSP that will display those first 20. It has a simple feature for pagination, a "next" button that will retrieve the next results. So far simple enough, going forwards is easy. But how about going back.
I am trying to work out the SQL for using a "previous" button, so for example, say records 40-60 are being displayed, and say the first record on that page has a primary key value of 200, what should the SQL be to retrieve the previous 20 records using rownum ? I ask because if I do:
select * from tablename where pri_key < 200 and rownum <=20;
I know it will retrieve 20 records, but would the query fetch its results from the beginning of the table or would it count backwards from 200 ? If it is the first way, how could I word the SQL query to go backwards from 200 ? I am pretty sure it will start from the beginning and thats what I don't want. I know there are pagination tags for jsp out there but I want to do this myself.
Thanks
Kevin
Message was edited by:
MrVen

Answer 1

First, you need to understand that rownum is only assigned when a record is selected for output, it has nothing to do with any sort of internal order of the table (like insertion order). The same row can have a different rownum even for two invocations of the same query on the same data if the optimizer chooses a different access path. The only way to get a repeatable set of rownum is to sort the records by some field.
Second, any SQL statement you issue will be executed essentially in isolation. Oracle has no idea what query you executed previously. So, your query:
SELECT *
FROM tablename
WHERE pri_key < 200 and
      rownum <=20;will select a random set of 20 rows wih a primary key less than 200. It is likely to be the first (i.e. lowest values of the pk) rows, but that is not guaranteed.
The cannonical way to paginate rows is something like:
SELECT col1, col2, ...
FROM (SELECT col1, col2, ..., rownum rn
      FROM (SELECT col1, col2, ...
            FROM table
            WHERE <conditions>
            ORDER BY pk) ti
      WHERE rownum <= :maxrownum) to
WHERE rn >= :minrownumThe innermost query (with the <condition>) selects the appropriate rows and sorts them, in this case by the pk.
The middle query (rownum <- :maxrownum) discards the rows that would be after the last record you want to display. By using the maxrownum here, it gives Oracle information that allows it to do a fast sort on the inner query if possible.
In a fast sort, Oracle will get the first maxrownum rows from the query and sort them, then when it gets the next row, it checks if it is less than the highest value already found. If it is, it slots it into the correct place and discards the highest value, otherwise it discards that row.
The outermost query simply discards the records where the row number after sorting is less than the minimum value.
Another alternative may be:
SELECT col1, col2, ...
FROM (SELECT col1, col2, ...,
             ROW_NUMBER() OVER(ORDER BY pk) rn
      FROM table
      WHERE <conditions>)
WHERE rn BETWEEN :minrownum AND :maxrownumdepending on your database version, and your actual query.
Note tha pagination like this is expensive.
HTH
John
The outermost