[HACKERS] SQL/JSON in PostgreSQL - Mailing list pgsql-hackers

Hi there,

Attached patch is an implementation of SQL/JSON data model from SQL-2016 standard (ISO/IEC 9075-2:2016(E)), which was published 2016-12-15 and is available only for purchase from ISO web site (https://www.iso.org/standard/63556.html). Unfortunately I didn't find any public sources of the standard or any preview documents, but Oracle implementation of json support in 12c release 2 is very close (http://docs.oracle.com/database/122/ADJSN/json-in-oracle-database.htm), also we used https://livesql.oracle.com/  to understand some details.

Postgres has already two json data types - json and jsonb and implementing another json data type, which strictly conforms the standard, would be not a good idea. Moreover, SQL standard doesn’t describe data type, but only data model, which “comprises SQL/JSON items and SQL/JSON sequences. The components of the SQL/JSON data model are:

1) An SQL/JSON item is defined recursively as any of the following:

a) An SQL/JSON scalar, defined as a non-null value of any of the following predefined (SQL) types:

character string with character set Unicode, numeric, Boolean, or datetime.

b) An SQL/JSON null, defined as a value that is distinct from any value of any SQL type.

NOTE 122 — An SQL/JSON null is distinct from the SQL null value.

c) An SQL/JSON array, defined as an ordered list of zero or more SQL/JSON items, called the SQL/JSON

elements of the SQL/JSON array.

d) An SQL/JSON object, defined as an unordered collection of zero or more SQL/JSON members….

“

Our jsonb corresponds to SQL/JSON with UNIQUE KEYS and implicit ordering of keys and our main intention was to provide support of jsonb as a most important and usable data type.

We created repository for reviewing (ask for write access) - https://github.com/postgrespro/sqljson/tree/sqljson

Examples of usage can be found in src/test/regress/sql/sql_json.sql

The whole documentation about json support should be reorganized and added, and we plan to do this before release. We need help of community here.

Our goal is to provide support of main features of SQL/JSON to release 10, as we discussed at developers meeting in Brussels (Andrew Dunstan has kindly agreed to review the patch).

We had not much time to develop the complete support, because of standard availability), but hope all major features are here, namely, all nine functions as described in the standard (but see implementation notes below):

“All manipulation (e.g., retrieval, creation, testing) of SQL/JSON items is performed through a number of SQL/JSON functions. There are nine such functions, categorized as SQL/JSON retrieval functions and SQL/JSON construction functions. The SQL/JSON retrieval functions are characterized by operating on JSON data and returning an SQL value (possibly a Boolean value) or a JSON value. The SQL/JSON construction functions return JSON data created from operations on SQL data or other JSON data.

The SQL/JSON retrieval functions are:

— <JSON value function>: extracts an SQL value of a predefined type from a JSON text.

— <JSON query>: extracts a JSON text from a JSON text.

— <JSON table>: converts a JSON text to an SQL table.

— <JSON predicate>: tests whether a string value is or is not properly formed JSON text.

— <JSON exists predicate>: tests whether an SQL/JSON path expression returns any SQL/JSON items.

The SQL/JSON construction functions are:

— <JSON object constructor>: generates a string that is a serialization of an SQL/JSON object.

— <JSON array constructor>: generates a string that is a serialization of an SQL/JSON array.

— <JSON object aggregate constructor>: generates, from an aggregation of SQL data, a string that is a serialization

of an SQL/JSON object.

— <JSON array aggregate constructor>: generates, from an aggregation of SQL data, a string that is a serialization

of an SQL/JSON array.

A JSON-returning function is an SQL/JSON construction function or JSON_QUERY.”

The standard describes SQL/JSON path language, which used by SQL/JSON query operators to query JSON. It defines path language as string literal. We implemented the path language as  JSONPATH data type, since other approaches are not friendly to planner and executor.

The functions and JSONPATH provide a new functionality for json support, namely, ability to operate (in standard specified way) with json structure at SQL-language level - the often requested feature by the users.

The patch is consists of about 15000 insertions (about 5000 lines are from tests), passes all regression tests and doesn’t touches critical parts, so we hope with community help to bring it to committable state.

Authors: Nikita Glukhov, Teodor Sigaev, Oleg Bartunov and Alexander Korotkov

Implementation notes:

1. We didn’t implemented ‘datetime’ support, since it’s not clear from standard.

2. JSON_OBJECT/JSON_OBJECTAGG (KEY <key> VALUE <value>, ...) doesn’t implemented, only (<key>:<value>, …) and (<key> VALUE <value>, …) are supported, because of  grammar conflicts with leading KEY keyword.

3. FORMAT (JSON|JSONB))  in JSON_ARRAYAGG with subquery  doesn’t supported, because of grammar conflicts with non-reserved word FORMAT.

4. JSONPATH implemented only for  jsonb data type , so JSON_EXISTS(), JSON_VALUE(), JSON_QUERY() and JSON_TABLE() doesn’t works if context item is of json data type.

5. Some methods and predicates for JSONPATH not yet implemented, for example  .type(), .size(), .keyvalue(),  predicates  like_regex, starts with, etc. They are not key features and we plan to make them in next release.

6. JSONPATH doesn’t support expression for index array, like [2+3 to $upperbound], only simple constants like [5, 7 to 12] are supported.

7. JSONPATH extensions to standard: .** (wildcard path accessor), .key (member accessor without leading @).

8. FORMAT JSONB extension to standard for returning jsonb - standard specifies possibility of returning custom type.

9. JSON_EXISTS(), JSON_VALUE(), JSON_QUERY() are implemented using new executor node JsonExpr.

10. JSON_TABLE() is transformed into joined subselects with JSON_VALUE() and JSON_QUERY() in target list.

11. JSON_OBJECT(), JSON_ARRAY() constructors and IS JSON predicate are transformed into raw function calls.

12. Added explicit casts bytea=>jsonb and jsonb=>bytea (for jsonb=>bytea output using RETURNING bytea FORMAT JSONB and corresponding bytea=>jsonb input using <jsonb_bytea_expr> FORMAT JSONB).

Best regards,

Oleg