What do the fields in monitoring tables mean?

Here are some facts provided by Vlad Khorsun:

What is the difference between page_read and page_fetch?

- page_read is when page read from disk, i.e. cache miss.
- page_fetch is when page found in cache, i.e. cache hit.

What is page_mark?

- someone has intention to change page contents.

What is record_backout?

- Undoing of not needed primary record version. It can be in case of transaction rollback or undo of savepoint if some statement under this savepoint has failed.

What is record_purge?

- Removing chain of unneded record versions, leaving primary record version
(and maybe few back-versions) on its place as it was created by transaction with number not less than OAT.

What is record_expunge?

- Removing whole record versions chain as record was deleted by transaction
with number less than OAT.

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