Transaction read command fetching of single Data Items

Hello, I am a Student of the TU-Darmstadt (Germany), and I would like to
run some tests with Postgres on SSDs.

I need to know at which position a Transaction executes a read on a Data
Item in the source code.
(Backround) I want to evaluate a certain worst case scenario:
When a Transaction "t" reads a Data Item "x" I want to fetch all the
versions "xNew" which are newer than the version of x that t read, if
they exist.
I just want to fetch and immediately forget them, even if that results
in additional reads on the storage drive
[it is even feasable at this point to fetch all Versions of x instead
of just the newer ones]
but I don't know where I should place the code.

I looked at some classes of the backend, but I wasn't able to find the
exact place.
I would be grateful for any hints you could give me.

Best regards, I appreciate your efforts
Robert Gottstein