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

Attached 42th version of the patches.

On 18. 1. 2020 v 18:46 Nikita Glukhov <n.gluhov@postgrespro.ru> wrote:

Attached 41th version of the patches.

Changes since previous version:* Enabled DEFAULT clause for ON ERROR/ON EMPTY behaviors in JSON_QUERY()* Added RETURNING clause to JSON_EXISTS() ("side effect" of implementation   EXISTS PATH columns in JSON_TABLE)* ARRAY in EMPTY ARRAY ON ERROR clause is optional now for better Oracle   compatibility

On 17.01.2020 9:54, Pavel Stehule wrote:

I tested cumulative patch - sent in json_table patch.

I almost satisfied by quality of this patch. There is very good conformance with standard and with Oracle. Unfortunately MySQL in this part of JSON support is not compatible.

I found one issue, when I tested some examples from Oracle.

SELECT JSON_VALUE('{a:100}', '$.a' RETURNING int) AS value;

then the result was null.

But it is wrong, because it should to raise a exception, because this json is broken on Postgres (Postgres requires quoted attribute names)

json_query has same problem

postgres=# SELECT JSON_QUERY('{a:100, b:200, c:300}', '$') AS value;
┌───────┐
│ value │
╞═══════╡
│ ∅     │
└───────┘
(1 row)

It should to check if input is correct json

By the standard, it is implementation-defined whether JSON parsing errors 
should be caught by ON ERROR clause.

SQL/JSON query functions use "JSON API common syntax" which is a combination 
of JSON context item and JSON path.  It passes context item to JSON path 
engine with ALREADY PARSED flag set to False.  ALREADY PARSED flag can enable 
special parsing rules.

Corresponding quotes from the standard:

10.14 <JSON API common syntax> <JSON API common syntax> (    Parameter: "JSON API COMMON SYNTAX" ) Returns: "STATUS" and "SQL/JSON SEQUENCE"

General Rules:
...
3) General Rules of Subclause 9.39, "SQL/JSON path language: syntax and   semantics", are applied with P as PATH SPECIFICATION, C as CONTEXT ITEM,  False as ALREADY PARSED, and PC as PASSING CLAUSE; let ST be the STATUS   and let SEQ be the SQL/JSON SEQUENCE returned from the application of   those General Rules.

9.39 SQL/JSON path language: syntax and semantics

"SQL/JSON path language: syntax and semantics" [General Rules] ( Parameter: "PATH SPECIFICATION", Parameter: "CONTEXT ITEM", Parameter: "ALREADY PARSED", Parameter: "PASSING CLAUSE"
) Returns: "STATUS" and "SQL/JSON SEQUENCE"

General Rules:
...

4) If ALREADY PARSED is False, then it is implementation-defined whether the   following rules are applied: a) The General Rules of Subclause 9.36, "Parsing JSON text", are applied with    JT as JSON TEXT, an implementation-defined <JSON key uniqueness constraint>     as UNIQUENESS CONSTRAINT, and FO as FORMAT OPTION; let ST be the STATUS and    let CISJI be the SQL/JSON ITEM returned from the application of those     General Rules. b) If ST is not successful completion, then ST is returned as the STATUS of    this application of these General Rules, and no further General Rules of     this Subclause are applied.


I decided to apply this rules, so the parsing errors are caught now by ON ERROR 
(NULL ON ERROR is by default).

postgres=# SELECT JSON_VALUE('error', '$' ERROR ON ERROR);
ERROR:  invalid input syntax for type json
DETAIL:  Token "error" is invalid.
CONTEXT:  JSON data, line 1: error

I'm not sure if it would be better to add an implicit cast to json type that
will be executed before so that parsing errors can no longer be caught.
But implicit casting can simplify a bit execution of SQL/JSON query functions.


I have checked error handling in JSON parsing in Oracle 18c/19c, and it behaves
like our current implementation.  But Oracle seems to do JSON parsing on demand:

Oracle19c> SELECT JSON_VALUE('{a:1 error, b:2}', '$.a' ERROR ON ERROR) FROM dual;
1

Oracle19c> SELECT JSON_VALUE('{a:1 error, b:2}', '$.b' ERROR ON ERROR) FROM dual;
ORA-40441: JSON syntax error

Oracle19c> SELECT JSON_VALUE('{a:1 error, b:2}', '$.b') FROM dual;
NULL

Everywhere I don't like default masking error. I think so can be very confusing to get NULL
(by default) instead a error of broken format.

I vote for check of input is correct JSON, and if it, then start processing. Else to raise a error.

More - our JSON Parser is different than Oracle's JSON parser. And if somebody will run Oracle's JSONs,
then he get some result on Oracle. But on Postgres, same JSON can be invalid, and he get NULL.

The raising some errors looks like only one safe variant.

I have removed handling of parsing errors in SQL/JSON functions and JSON_TABLE.
Now, FORMAT JSON expressions (implicit or explicit) are simply transformed into
ordinary casts to json type, and these casts are executed before execution of
SQL/JSON functions.  Previously, separate expression was created for such casts,
and it was executed in the separate subtransaction in ExecEvalJsonExpr().  So,
this change also simplifies the code a bit.