Re: SQL/JSON: JSON_TABLE - Mailing list pgsql-hackers

Attached 42th version of the patches rebased onto current master.


Changes from the previous version:* added EXISTS PATH columns* added DEFAULT clause for FORMAT JSON columns* added implicit FORMAT JSON for columns of json[b], array and composite types

čt 21. 11. 2019 v 17:31 odesílatel Nikita Glukhov <n.gluhov@postgrespro.ru> napsal:

On 17.11.2019 13:35, Pavel Stehule wrote:
I found:

a) Oracle & MySQL (Oracle) supports EXISTS clause, this implementation not. 
I think should be useful support this clause too. 

SELECT * FROM JSON_TABLE('...', '...' COLUMNS x INT EXISTS PATH ...

EXISTS PATH clause can be emulated with jsonpath EXISTS() predicate:

=# SELECT *   FROM JSON_TABLE('{"a": 1}', '$'                  COLUMNS (                    a bool PATH 'exists($.a)',                    b bool PATH 'exists($.b)'                  ));a | b 
---+---t | f
(1 row)

But this works as expected only in lax mode.  In strict mode EXISTS() returns 
Unknown that transformed into SQL NULL:

=# SELECT *   FROM JSON_TABLE('{"a": 1}', '$'                   COLUMNS (                    a bool PATH 'strict exists($.a)',                    b bool PATH 'strict exists($.b)'                  ));a | b 
---+---t | 
(1 row)

There is no easy way to return false without external COALESCE(),
DEFAULT false ON ERROR also does not help.  

So, I think it's worth to add EXISTS PATH clause to our implementation.

There is a question how to map boolean result to other data types. 

Now, boolean result can be used in JSON_TABLE columns of bool, int4, text, 
json[b], and other types which have CAST from bool:

SELECT * 
FROM JSON_TABLE('{"a": 1}', '$'               COLUMNS (                 a int PATH 'exists($.a)',                 b text PATH 'exists($.b)'               ));a |   b 
---+-------1 | false
(1 row)

EXISTS PATH columns were added.  Only column types having CASTS 
from boolean type are accepted.

Example:

SELECT * 
FROM JSON_TABLE( '{"foo": "bar"}', '$'  COLUMNS (    foo_exists boolean EXISTS PATH '$.foo',    foo int EXISTS,    err text EXISTS PATH '$ / 0' TRUE ON ERROR  )
);
foo_exists | foo |  err 
------------+-----+------t          |   1 | true   
(1 row)

b) When searched value is not scalar, then it returns null. This behave can be 
suppressed by clause FORMAT Json. I found a different behave, and maybe I found
a bug.  On MySQL this clause is by default for JSON values (what has sense).

SELECT *FROM
      JSON_TABLE(
        '[{"a":[1,2]}]',
        '$[*]'
        COLUMNS(
         aj JSON PATH '$.a' DEFAULT '{"x": 333}' ON EMPTY 
        )
      ) AS tt;

It returns null, although it should to return [1,2]. 

Yes, regular (non-formatted) JSON_TABLE columns can accept only scalar values. 
Otherwise an error is thrown, which can be caught by ON ERROR clause. This 
behavior is specified by the standard.

FORMAT JSON is not implicitly added for json[b] columns now. The current SQL
standard does not have any json data types, so I think we can add implicit 
FORMAT JSON for json[b] typed-columns.  But I'm a bit afraid that different 
behavior can be standardized after introduction of json data types in SQL.

There is another bug maybe. Although there is DEFAULT clause. It returns NULL.

ON ERROR should be used if "not a scalar" error needs to be caught:

SELECT *
FROM   JSON_TABLE(       '[{"a":[1,2]}]',       '$[*]'       COLUMNS(           aj JSON PATH '$.a' DEFAULT '{"x": 333}' ON ERROR       )   ) AS tt;
    aj     
------------{"x": 333}
(1 row)


ON EMPTY catches only empty-result case (for example, non-existent path in 
lax mode):

SELECT *
FROM   JSON_TABLE(       '[{"a":[1,2]}]',       '$[*]'       COLUMNS(           aj JSON PATH '$.foo' DEFAULT '{"x": 333}' ON EMPTY        )   ) AS tt;    aj     
------------{"x": 333}
(1 row)

I got correct result when I used FORMAT JSON clause. 
I think it should be default behave for json and jsonb columns.

I agree that FORMAT JSON could be implicit for json[b] columns.  But I think
there could be one minor problem if we want to verify that returned value is 
scalar.

Without FORMAT JSON this is verified by the underlying JSON_VALUE expression:

SELECT *
FROM   JSON_TABLE(       '[{"a":[1,2]}]',       '$[*]'       COLUMNS (           aj JSON PATH 'lax $.a' ERROR ON ERROR        )   ) AS tt;
ERROR:  JSON path expression in JSON_VALUE should return singleton scalar item

(This error message with the reference to implicit JSON_VALUE needs to be fixed.)


But with FORMAT JSON we need to construct complex jsonpath with a filter and 
override ON EMPTY behavior:

SELECT *
FROM   JSON_TABLE(       '[{"a":[1,2]}]',       '$[*]'       COLUMNS (           aj JSON FORMAT JSON            -- strict mode is mandatory to prevent array unwrapping           PATH 'strict $.a ? (@.type() != "array" && @.type() != "object")'           ERROR ON EMPTY ERROR ON ERROR       )   ) AS tt;
ERROR:  no SQL/JSON item

please, check the behave of other databases. I think so good conformance with other RDBMS is important. More this method for checking if value is object or not looks little bit scary.

maybe we can implement some functions like JSON_IS_OBJECT(), JSON_IS_ARRAY(), JSON_IS_VALUE()?

 

More - we have this functionality already

ostgres=# select json_typeof('[10,20]');
┌─────────────┐
│ json_typeof │
╞═════════════╡
│ array       │
└─────────────┘
(1 row)

Implicit FORMAT JSON is used for columns of json[b], array and composite types now.
The behavior is similar to behavior of json_populate_record().

Example:

CREATE TYPE test_record AS (foo text[], bar int);

SELECT * 
FROM JSON_TABLE( '{"foo": ["bar", 123, null]}', '$'  COLUMNS (    js json PATH '$',     jsonb_arr jsonb[] PATH '$.foo',     text_arr text[] PATH '$.foo',     int_arr int[] PATH '$.foo' DEFAULT '{}' ON ERROR,     rec test_record PATH '$'  )
);            js              |      jsonb_arr       |    text_arr    | int_arr |         rec         
-----------------------------+----------------------+----------------+---------+---------------------{"foo": ["bar", 123, null]} | {"\"bar\"",123,NULL} | {bar,123,NULL} | {}      | ("{bar,123,NULL}",)
(1 row)

Another question - when I used FORMAT JSON clause, then I got syntax error
on DEFAULT keyword .. . Is it correct? 

Why I cannot to use together FORMAT JSON and DEFAULT clauses?

JSON_TABLE columns with FORMAT JSON, like JSON_QUERY, can have only 
ERROR, NULL, EMPTY ARRAY, EMPTY OBJECT behaviors.

This syntax is specified in the SQL standard:

<JSON table formatted column definition> ::= <column name> <data type> FORMAT <JSON representation> [ PATH <JSON table column path specification> ] [ <JSON table formatted column wrapper behavior> WRAPPER ] [ <JSON table formatted column quotes behavior> QUOTES [ ON SCALAR STRING ] ] [ <JSON table formatted column empty behavior> ON EMPTY ] [ <JSON table formatted column error behavior> ON ERROR ]

<JSON table formatted column empty behavior> ::= ERROR | NULL | EMPTY ARRAY | EMPTY OBJECT

<JSON table formatted column error behavior> ::= ERROR | NULL | EMPTY ARRAY | EMPTY OBJECT


But I also think that DEFAULT clause could be very useful in JSON_QUERY and 
formatted JSON_TABLE columns.

DEFAULT clause was enabled in JSON_QUERY() and formatted JSON_TABLE columns:

SELECT * 
FROM JSON_TABLE( '{"foo": "bar"}', '$'  COLUMNS (    baz json FORMAT JSON DEFAULT '"empty"' ON EMPTY  )
);  baz   
---------"empty"
(1 row)