Re: MVCC snapshot timing

Bruce Momjian wrote
> We just want to get across the MVCC concept in the intro --- we cover
> the snapshots later in the document.

I just think we're being too vague here; and we are covering them in the
intro with the use of "some point in the past".

IMO, the main point regarding MVCC is that every change in the system
creates a new record and causes a prior record to be invalidated at a
point-in-time.  The combination of these two things increases concurrency
since you can create new records while people are still using the old ones.
One consequence, though, is that it is necessary for the user to decide at
what point in the timeline they want to view the database.

Does this sound right?

The current (and modified) intro indeed covers these two points so it really
comes down to style.

My current gut feel is the documentation (generally speaking) does a good
job of describing the mechanics of the system but, in some areas, could use
more detail as to why and also the various implications of those mechanics
[1].  Bringing those up in the intro gives the reader additional context so
that when they get into the "how" detail sections they can more quickly link
the mechanics with the problem they are meant to solve.  Thus introducing
the more specific "snapshot" concept in the intro provides the context for
when they are reading why isolation levels exist.

[1] Not that I'm proactively looking; but when questions arise regarding the
docs I try and put myself in the person's shoes and find not that the docs
are incorrect but that they could be improved - which is just a part of our
reality).

My $0.02

David J.





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