Re: SQL/JSON path issues/questions

On 6/21/19 8:04 PM, Alexander Korotkov wrote:
> On Wed, Jun 19, 2019 at 10:14 PM Alexander Korotkov
> <a(dot)korotkov(at)postgrespro(dot)ru> wrote:
>>> While I have no objections to the proposed fixes, I think we can further
>>> improve patch 0003 and the text it refers to.
>>> In attempt to clarify jsonpath docs and address the concern that ? is
>>> hard to trace in the current text, I'd also like to propose patch 0004.
>>> Please see both of them attached.
>> Thank you for your editing. I'm going to commit them as well.
>>
>> But I'm going to commit your changes separately from 0003 I've posted
>> before. Because 0003 fixes factual error, while you're proposing set
>> of grammar/style fixes.
> I made some review of these patches. My notes are following:
>
> <para>
> - See also <xref linkend="functions-aggregate"/> for the aggregate
> - function <function>json_agg</function> which aggregates record
> - values as JSON, and the aggregate function
> - <function>json_object_agg</function> which aggregates pairs of values
> - into a JSON object, and their <type>jsonb</type> equivalents,
> + See also <xref linkend="functions-aggregate"/> for details on
> + <function>json_agg</function> function that aggregates record
> + values as JSON, <function>json_object_agg</function> function
> + that aggregates pairs of values into a JSON object, and their
> <type>jsonb</type> equivalents,
> <function>jsonb_agg</function> and <function>jsonb_object_agg</function>.
> </para>
>
> This part is not directly related to jsonpath, and it has been there
> for a long time. I'd like some native english speaker to review this
> change before committing this.
>
> <para>
> - Expression inside subscript may consititue an integer,
> - numeric expression or any other <literal>jsonpath</literal> expression
> - returning single numeric value. The <literal>last</literal> keyword
> - can be used in the expression denoting the last subscript in an array.
> - That's helpful for handling arrays of unknown length.
> + The specified <replaceable>index</replaceable> can be an integer,
> + as well as a numeric or <literal>jsonpath</literal> expression that
> + returns a single integer value. Zero index corresponds to the first
> + array element. To access the last element in an array, you can use
> + the <literal>last</literal> keyword, which is useful for handling
> + arrays of unknown length.
> </para>
>
> I think this part requires more work. Let's see what cases do we have
> with examples:
>
> 1) Integer: '$.ar[1]'
> 2) Numeric: '$.ar[1.5]' (converted to integer)
> 3) Some numeric expression: '$.ar[last - 1]'
> 4) Arbitrary jsonpath expression: '$.ar[$.ar2.size() + $.num - 2]'
>
> In principle, it not necessary to divide 3 and 4, or divide 1 and 2.
> Or we may don't describe cases at all, but just say it's a jsonpath
> expression returning numeric, which is converted to integer.
>
> Also, note that we do not necessary *access* last array element with
> "last" keyword. "last" keyword denotes index of last element in
> expression. But completely different element might be actually
> accessed.
>
> ------
> Alexander Korotkov
> Postgres Professional: http://www.postgrespro.com
> The Russian Postgres Company
>
Hi Alexander,

Thank you for the catch! Please see the modified version of patch 0004
attached.

As for your comment on patch 0003, since I'm not a native speaker, I can
only refer to a recent discussion in pgsql-docs mailing list that seems
to support my view on a similar issue:

https://www.postgresql.org/message-id/9484.1558050957%40sss.pgh.pa.us

--
Liudmila Mantrova
Technical writer at Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company