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Docs: add example clarifying use of nested JSON containment.

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Show how this can be used in practice to make queries simpler and more
flexible.  Also, draw an explicit contrast to the existence operator,
which doesn't work that way.

Peter Geoghegan and Tom Lane
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Tom Lane 2015-10-29 18:54:35 -04:00
parent c5130e8ee0
commit 23937a4253
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doc/src/sgml/json.sgml
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@ -349,6 +349,34 @@ SELECT '"foo"'::jsonb ? 'foo';
need to be searched linearly.
</para>
<tip>
<para>
Because JSON containment is nested, an appropriate query can skip
explicit selection of sub-objects. As an example, suppose that we have
a <structfield>doc</> column containing objects at the top level, with
most objects containing <literal>tags</> fields that contain arrays of
sub-objects. This query finds entries in which sub-objects containing
both <literal>"term":"paris"</> and <literal>"term":"food"</> appear,
while ignoring any such keys outside the <literal>tags</> array:
<programlisting>
SELECT doc-&gt;'site_name' FROM websites
WHERE doc @&gt; '{"tags":[{"term":"paris"}, {"term":"food"}]}';
</programlisting>
One could accomplish the same thing with, say,
<programlisting>
SELECT doc-&gt;'site_name' FROM websites
WHERE doc-&gt;'tags' @&gt; '[{"term":"paris"}, {"term":"food"}]';
</programlisting>
but that approach is less flexible, and often less efficient as well.
</para>
<para>
On the other hand, the JSON existence operator is not nested: it will
only look for the specified key or array element at top level of the
JSON value.
</para>
</tip>
<para>
The various containment and existence operators, along with all other
JSON operators and functions are documented