On 04/23/2017 12:44 PM, Simon Riggs wrote:
On 23 April 2017 at 09:17, Dean Rasheed <dean.a.rash...@gmail.com> wrote:
On 23 April 2017 at 03:37, Bruce Momjian <br...@momjian.us> wrote:
In looking at the new multi-column statistics "dependency" option in
Postgres 10, I am quite confused by the term "dependency".  Wouldn't
"correlation" be clearer and less confusing as "column dependency"
already means something else.

I also asked that exactly that question...

Actually, the terms "dependency" and "correlation" are both quite
broad terms that cover a whole range of other different things, and
hence could be misleading. The precise term for this is "functional
dependency" [1], so if anything, the option name should be
"functional_dependencies" or some shortening of that, keeping a part
of each of those words.

...and got that answer also.

For us "functional dependency" would sound like something to do with
functions (e.g. CREATE FUNCTION), so just "dependency" appears to me
to be the best term for this.

Not really. Functional dependency is a term well-defined in relational algebra, particularly in definition of normal forms. It has nothing to do with functions, and I'm sure it's not the only possible "ambiguity".

I actually considered functional_dependency when working on this, and I think it might be a better choice, but seemed a bit longish.

There are multiple statistics for dependency stored, hence
"dependencies". I don't like it, but its the best term I can see at

That is a good point, actually. It should probably be plural.


Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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