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docs: clarify infinite range values from data-type infinities

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The previous docs referenced these distinct ideas confusingly.

Reported-by: Eugen Konkov

Discussion: https://postgr.es/m/376945611.20191026161529@yandex.ru

Backpatch-through: 9.4
This commit is contained in:
Bruce Momjian 2019-12-27 14:33:30 -05:00
parent 786540085d
commit 4666da12d0
1 changed files with 17 additions and 20 deletions
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doc/src/sgml/rangetypes.sgml
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@ -131,29 +131,26 @@ SELECT isempty(numrange(1, 5));
<title>Infinite (Unbounded) Ranges</title>
<para>
The lower bound of a range can be omitted, meaning that all points less
than the upper bound are included in the range. Likewise, if the upper
bound of the range is omitted, then all points greater than the lower bound
are included in the range. If both lower and upper bounds are omitted, all
values of the element type are considered to be in the range.
The lower bound of a range can be omitted, meaning that all
values less than the upper bound are included in the range, e.g.,
<literal>(,3]</literal>. Likewise, if the upper bound of the range
is omitted, then all values greater than the lower bound are included
in the range. If both lower and upper bounds are omitted, all values
of the element type are considered to be in the range. Specifying a
missing bound as inclusive is automatically converted to exclusive,
e.g., <literal>[,]</literal> is converted to <literal>(,)</literal>.
You can think of these missing values as +/-infinity, but they are
special range type values and are considered to be beyond any range
element type's +/-infinity values.
</para>
<para>
This is equivalent to considering that the lower bound is <quote>minus
infinity</quote>, or the upper bound is <quote>plus infinity</quote>,
respectively. But note that these infinite values are never values of
the range's element type, and can never be part of the range. (So there
is no such thing as an inclusive infinite bound &mdash; if you try to
write one, it will automatically be converted to an exclusive bound.)
</para>
<para>
Also, some element types have a notion of <quote>infinity</quote>, but that
is just another value so far as the range type mechanisms are concerned.
For example, in timestamp ranges, <literal>[today,]</literal> means the same
thing as <literal>[today,)</literal>. But <literal>[today,infinity]</literal> means
something different from <literal>[today,infinity)</literal> &mdash; the latter
excludes the special <type>timestamp</type> value <literal>infinity</literal>.
Element types that have the notion of <quote>infinity</quote> can
use them as explicit bound values. For example, with timestamp
ranges, <literal>[today,infinity)</literal> excludes the special
<type>timestamp</type> value <literal>infinity</literal>,
while <literal>[today,infinity]</literal> include it, as does
<literal>[today,)</literal> and <literal>[today,]</literal>.
</para>
<para>