What is High Water Mark?

Hello everyone, can you please tell me what is High Water Mark? Thank you.

The biggest size any thing has been is a hi-water mark. Often used with respect to rollback segments and tables. E.G. If the table is 75M and once held enough records to get the 65M, even if there are no records now in the table (due to deletes), the HWM is still 65m.

It's isgnificant since when you perform a full table scan it reads up to the high water mark... even if there isn't any data. This is often why a count(1) will perform better than a count(*).

Terry