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Document behavior of the .** jsonpath accessor in the lax mode

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When the .** jsonpath accessor handles the array, it selects both array and
each of its elements.  When using lax mode, subsequent accessors automatically
unwrap arrays.  So, the content of each array element may be selected twice.

Even though this behavior is counterintuitive, it's correct because everything
works as designed.  This commit documents it.

Backpatch to 12 where the jsonpath language was introduced.

Reported-by: Thomas Kellerer
Bug: #16828
Discussion: https://postgr.es/m/16828-2b0229babfad2d8c%40postgresql.org
Discussion: https://postgr.es/m/CAPpHfdtS-nNidT%3DEqZbAYOPcnNOWh_sd6skVdu2CAQUGdvpT8Q%40mail.gmail.com
Author: Alexandex Korotkov, revised by Tom Lane
Reviewed-by: Alvaro Herrera, Thomas Kellerer, Tom Lane
Backpatch-through: 12
This commit is contained in:
Alexander Korotkov 2021-01-29 15:27:55 +03:00
parent 2592be8be5
commit b41645460a
1 changed files with 18 additions and 0 deletions
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18
doc/src/sgml/func.sgml
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@ -16277,6 +16277,24 @@ strict $.track.segments[*].location
</programlisting>
</para>
<para>
The <literal>.**</literal> accessor can lead to surprising results
when using the lax mode. For instance, the following query selects every
<literal>HR</literal> value twice:
<programlisting>
lax $.**.HR
</programlisting>
This happens because the <literal>.**</literal> accessor selects both
the <literal>segments</literal> array and each of its elements, while
the <literal>.HR</literal> accessor automatically unwraps arrays when
using the lax mode. To avoid surprising results, we recommend using
the <literal>.**</literal> accessor only in the strict mode. The
following query selects each <literal>HR</literal> value just once:
<programlisting>
strict $.**.HR
</programlisting>
</para>
</sect3>
<sect3 id="functions-sqljson-path-operators">