Maybe. I'm not going to pretend I fully understand the internals. Does
that mean the container contains ZSUncompressedBtreeItem as elements? Or
just the plain Datum values?
First, your reading of code and all the comments/questions so far have been highly encouraging. Thanks a lot for the same.
Container contains ZSUncompressedBtreeItem as elements. As for Item will have to store meta-data like size, undo and such info. We don't wish to restrict compressing only items from same insertion sessions only. Hence, yes doens't just store Datum values. Wish to consider it more tuple level operations and have meta-data for it and able to work with tuple level granularity than block level.
Definitely many more tricks can be and need to be applied to optimize storage format, like for fixed width columns no need to store the size in every item. Keep it simple is theme have been trying to maintain. Compression ideally should compress duplicate data pretty easily and efficiently as well, but we will try to optimize as much we can without the same.