Clarifying rownum order of operations in Oracle

Question

According to this link, when using rownum in a query, it is called in the following order of operations.

The FROM/WHERE clause goes first.
ROWNUM is assigned and incremented to each output row from the FROM/WHERE clause.
SELECT is applied.
GROUP BY is applied.
HAVING is applied.
ORDER BY is applied.

I want to know where the AND would be categorized on this list. Would it be evaluated at the same time as the WHERE? What if the WHERE has a rownum and the AND does not?

Answer 1

The AND has no role in this. When result set is being constructed, the rownum is assigned to the results before outermost ordering. Filtering on ROWNUM is a hard stop from feeding results up from deeper in the execution plan. Therefore for example a construct like where rownum > 5 returns no rows.

Hopefully this helps. If not, please elaborate in your question and/or explain why you are asking. There are alternatives that are sometimes better, such as row_number().

Answer 2

I just had an update not work that had where field_value is not null and rownum = 1. since rownum = 1 was for a row that had field_value = null that was the row it tired to return and then no action occurred in the update.

So with in the Where clause there is also an order of operation. need to put () around it or make it a sub query to be sure rownum is on the results of the other parts of the where cause.

I used a distinct instead of rownum = 1 for my case.