The kind of IO oracle used surprised me at first, so I did a test to verify. Of course I takes your word, but anyone could have some typo, and I hope one day I can catch one of your typo :).

I approve of you desire to test an verify – so I’m only slightly upset that you didn’t take my word for it ;)

The answer to your question is one that I often have to give to the many questions that people put to me about why Oracle does something in a certain – I don’t know, I didn’t write the specification or the code.

However, I can speculate. If you assume that subquery factoring exists to allow large temporary results sets to be reusable within a query, then Oracle may have decided that writing to temp then reading to the buffer cache would be less of a threat to memory than creating and keeping the result set in the PGA.

This, in many cases, is the difference between “with subquery” and deliberately introducing an inline non-mergeable view. (Although the materialized data can, of course, be used in multiple places in the same query, while the in-line view cannot).

I did a test and it really did “db file scattered reads” to read back. Why doesn’t oracle read back using “direct path read” which will bypass SGA and use no latch?