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Thread: jsonb_path_query unexpected results when filter key / path does not exist..

jsonb_path_query unexpected results when filter key / path does not exist..

From
Ron Clarke
Date:

Currently using postgresql 14. This via pgAdmin 4 

If I execute a "jsonb_path_query" with an error in the filter expression, I get no results from the overall query instead of a null result from jsonb_path_query...

The example :-

2 json arrays, using array "a" to lookup values in array "b" with a common code...

This works fine, and gives me the expected result :-

SELECT
j_item as ordinal_num
, jr.j_row as item
, jr.j_row #>> '{a}' as item_code
, jsonb_build_object('x', jr.j_row #>> '{a}') as search_expression
,'[{"b":"A","id":1},{"b":"B","id":2},{"b":"C","id":3}]'::jsonb as item_ids
, jsonb_path_query('[{"b":"A","id":1},{"b":"B","id":2},{"b":"C","id":3}]'::jsonb,
                '$[*] ? (@.b == $x)'::jsonpath, jsonb_build_object('x', jr.j_row #>> '{a}')
                ) #>> '{id}' as xid
FROM jsonb_array_elements('[{"a":"A","first":"ann"}
                        ,{"a":"B","first":"bob"}
                        ,{"a":"C","first":"chris"}]'::jsonb) with ordinality as jr(j_row, j_item);

Successfully run. Total query runtime: 56 msec.
3 rows affected.

If I swap the filter of the jsonb_path_query expression from  (@.b == $x) to (@.a == $x) the whole select returns nothing and no errors are reported.  

SELECT
j_item as ordinal_num
, jr.j_row as item
, jr.j_row #>> '{a}' as item_code
, jsonb_build_object('x', jr.j_row #>> '{a}') as search_expression
,'[{"b":"A","id":1},{"b":"B","id":2},{"b":"C","id":3}]'::jsonb as item_ids
, jsonb_path_query('[{"b":"A","id":1},{"b":"B","id":2},{"b":"C","id":3}]'::jsonb,
                '$[*] ? (@.a == $x)'::jsonpath, jsonb_build_object('x', jr.j_row #>> '{a}')
                ) #>> '{id}' as xid
FROM jsonb_array_elements('[{"a":"A","first":"ann"}
                        ,{"a":"B","first":"bob"}
                        ,{"a":"C","first":"chris"}]'::jsonb) with ordinality as jr(j_row, j_item);

Successfully run. Total query runtime: 67 msec. 0 rows affected.

I appreciate that the filter is looking for an undefined key in the second example and so I would expect a null column. 

I guess this may have something to do with strict and lax options, but I would have only expected this to affect the column being created by jsonb_path_query and not to remove the whole result set. 

I also think it's likely that I've got something wrong!. 

I am trying to do set based stuff rather than looping through results, so I am expecting to depend upon a null values being returned where appropriate (i.e. the column returns null but the result still exists for other parts of the query)...

Thanks for looking at this, and let me know if it's intended behaviour.

Regards
Ron

Re: jsonb_path_query unexpected results when filter key / path does not exist..

From
"David G. Johnston"
Date:
On Wed, Nov 16, 2022 at 1:45 PM Ron Clarke <rclarkeai@gmail.com> wrote:
 
but I would have only expected this to affect the column being created by jsonb_path_query and not to remove the whole result set. 

Except that is exactly what happens when a set-returning function is placed into the target-list and it produces zero rows.

You should not place set-returning functions into the target-list.  They, like any other table, belong in the FROM clause.  The LATERAL modifier was created for just this purpose.

David J.