Re: SQL/JSON path issues/questions - Mailing list pgsql-hackers

On Wed, Jun 19, 2019 at 10:14 PM Alexander Korotkov
<a.korotkov@postgrespro.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