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Doc: clarify handling of duplicate elements in array containment tests.

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The array <@ and @> operators do not worry about duplicates: if every
member of array X matches some element of array Y, then X is contained
in Y, even if several members of X get matched to the same Y member.
This was not explicitly stated in the docs though, so improve matters.

Discussion: https://postgr.es/m/156614120484.1310.310161642239149585@wrigleys.postgresql.org
This commit is contained in:
Tom Lane 2019-09-23 12:37:04 -04:00
parent 096d34c3ba
commit 0652c3992e
1 changed files with 15 additions and 4 deletions
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19
doc/src/sgml/func.sgml
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@ -12591,14 +12591,14 @@ SELECT NULLIF(value, '(none)') ...
<row>
<entry> <literal>@&gt;</literal> </entry>
<entry>contains</entry>
<entry><literal>ARRAY[1,4,3] @&gt; ARRAY[3,1]</literal></entry>
<entry><literal>ARRAY[1,4,3] @&gt; ARRAY[3,1,3]</literal></entry>
<entry><literal>t</literal></entry>
</row>
<row>
<entry> <literal>&lt;@</literal> </entry>
<entry>is contained by</entry>
<entry><literal>ARRAY[2,7] &lt;@ ARRAY[1,7,4,2,6]</literal></entry>
<entry><literal>ARRAY[2,2,7] &lt;@ ARRAY[1,7,4,2,6]</literal></entry>
<entry><literal>t</literal></entry>
</row>
@ -12641,8 +12641,10 @@ SELECT NULLIF(value, '(none)') ...
</table>
<para>
Array comparisons compare the array contents element-by-element,
using the default B-tree comparison function for the element data type.
The array ordering operators (<literal>&lt;</literal>,
<literal>&gt;=</literal>, etc) compare the array contents
element-by-element, using the default B-tree comparison function for
the element data type, and sort based on the first difference.
In multidimensional arrays the elements are visited in row-major order
(last subscript varies most rapidly).
If the contents of two arrays are equal but the dimensionality is
@ -12653,6 +12655,15 @@ SELECT NULLIF(value, '(none)') ...
number of dimensions or subscript ranges were different.)
</para>
<para>
The array containment operators (<literal>&lt;@</literal>
and <literal>@&gt;</literal>) consider one array to be contained in
another one if each of its elements appears in the other one.
Duplicates are not treated specially, thus <literal>ARRAY[1]</literal>
and <literal>ARRAY[1,1]</literal> are each considered to contain the
other.
</para>
<para>
See <xref linkend="arrays"> for more details about array operator
behavior. See <xref linkend="indexes-types"> for more details about