pgsql: Doc: improve documentation about composite-value usage.

On Tue, Nov 22, 2016 at 3:56 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:

Doc: improve documentation about composite-value usage.

Create a section specifically for the syntactic rules around whole-row
variable usage, such as expansion of "foo.*". This was previously
documented only haphazardly, with some critical info buried in
unexpected places like xfunc-sql-composite-functions. Per repeated
questions in different mailing lists.

​Tom,​

​I found it notable that you choose to introduce the OFFSET 0 hack instead
of writing a LATERAL query in the "optimization failure" example.​

SELECT (m).* FROM (SELECT myfunc(x) AS m FROM some_table OFFSET 0) ss;
​

instead of​

SELECT (m).* FROM some_table, LATERAL myfunc(some_table.x) m

​​Skipping or having a different example in 9.2 seems worth it in order to
introduce the now preferred way of writing such queries.

Or maybe in addition, so that some familiarity with the hack is gained
should the reader encounter it in the wild.

David J.

"David G. Johnston" <david.g.johnston@gmail.com> writes:

​I found it notable that you choose to introduce the OFFSET 0 hack instead
of writing a LATERAL query in the "optimization failure" example.​
SELECT (m).* FROM (SELECT myfunc(x) AS m FROM some_table OFFSET 0) ss;
instead of​
SELECT (m).* FROM some_table, LATERAL myfunc(some_table.x) m

Meh ... I'm not really sure that the latter is any less of a hack for this
purpose, or that we'd never optimize it into the unwanted form. LATERAL
is a good way to fix the weird semantics of set-returning functions in the
tlist, but this is not that case.

regards, tom lane

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