Re: [DOCS] Window function docs

On Thu, Apr  6, 2017 at 06:13:44PM -0400, Tom Lane wrote:
> Bruce Momjian <bruce@momjian.us> writes:
> > As part of writing a talk about window functions, I have done some
> > cleanups of the window function docs, attached, that I would like to
> > apply to head.
>
> I think this change is just wrong:
>
>      aggregate function, but the <literal>OVER</> clause causes it to be
> -    treated as a window function and computed across an appropriate set of
> -    rows.)
> +    treated as a window function and computed across the entire partition.)
>     </para>
>
> avg() is going to be evaluated across the current frame, not the whole
> partition.  I don't mind trying to be more precise than "appropriate set",
> but it doesn't help to be apparently precise but wrong.

Good point.  I have adjusted the text to say "window frame".

> Not really convinced by the s/regular/normal/g changes.  That doesn't seem
> to clarify much either.  Maybe it would help to define "normal" near here.
> There's a definition in func.sgml (that they are not ordered-set or
> hypothetical-set aggregates) but that's too far away.

Yeah, I went back and forth on that, and it seems like a mine-field.
The problem is that SUM() is both a non-window aggregate, and a window
function, and there isn't a clear terminology for non-window aggregates,
so changed the patch to just call them that.  Hopefully the dual nature
of this is now clearer.

> In this bit:
>
> -   definition.  Rows that are not distinct in the <literal>ORDER BY</>
> -   ordering are said to be <firstterm>peers</>; the four ranking functions
> -   are defined so that they give the same answer for any two peer rows.
> +   definition.  Rows that are not distinct based on <literal>ORDER BY</>
> +   ordering are said to be <firstterm>peers</>.  The four ranking
> +   functions (includes <function>cume_dist</>) are defined so that they
> +   give the same answer for any two peer rows.
>
> "based on" seems little better than the previous wording.  Maybe it'd be
> better to say "Rows that are not distinct when considering only the
> <literal>ORDER BY</> columns are said to be ..."

OK, I used your wording.

>     sometimes also <function>nth_value</>.  You can redefine the frame by
> -   adding a suitable frame specification (<literal>RANGE</> or
> -   <literal>ROWS</>) to the <literal>OVER</> clause.
> -   See <xref linkend="syntax-window-functions"> for more information
> -   about frame specifications.
> +   modifying the frame specification;  see <xref
> +   linkend="syntax-window-functions"> for more information about frame
> +   specifications.
>    </para>
>
> Don't really like this change; it makes it vaguer for no apparent benefit.

Reverted.  I had changed it because it is more the frame definition that
we want to highlight here, not ROW vs. RANGE, but there is no clearer
way to specify that clause.

> +  <para>
> +   <function>cume_dist</> computes the ratio of partition rows that
> +   are less than or equal to the current row and its peers, while
> +   <function>percent_rank</> computes the ratio of lesser partition rows,
> +   assuming the current row is does not exist in the partition.
> +  </para>
>
> The first one probably ought to be "computes the fraction of partition
> rows".  The second definition seems nonsensical --- ratio to what?  Or
> probably better, fraction of what?  (And "is does not" needs help too.)

Updated.

>      the user.  Also, any built-in or user-defined normal aggregate function
> -    can be used as a window function.  Ordered-set aggregates presently
> -    cannot be used as window functions, however.
> +    can be used as a window function.  However, ordered-set aggregate
> +    cannot presently be used as window functions.
>     </para>
>
> Grammar (don't lose the "s" in ordered-set aggregates).

Thanks for the review.  Updated patch attached.

--
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

+ As you are, so once was I.  As I am, so you will be. +
+                      Ancient Roman grave inscription +