perform update operations on columns of type JSONB

2015/07/29 15:38
阅读数 39

Looking through the documentation for the Postgres 9.4 datatype JSONB, it is not immediately obvious to me how to do updates on JSONB columns.

Documentation for JSONB types and functions:

http://www.postgresql.org/docs/9.4/static/functions-json.htmlhttp://www.postgresql.org/docs/9.4/static/datatype-json.html

As an examples, I have this basic table structure:

CREATE TABLE test(id serial, data jsonb);

Inserting is easy, as in:

INSERT INTO test(data) values ('{"name": "my-name", "tags": ["tag1", "tag2"]}');

Now, how would I update the 'data' column? This is invalid syntax:

UPDATE test SET data->'name' = 'my-other-name' WHERE id = 1;

Is this documented somewhere obvious that I missed? Thanks.

答案======================

Ideally, you don't use JSON documents for data that you want to manage inside a relational database. Use a normalized relational design instead.

JSON is primarily intended to store whole documents that do not need to be manipulated inside the RDBMS.

Updating a row in Postgres always writes a new version of the whole row. That's the basic principle ofPostgres' MVCC model. From a performance perspective, it hardly matters whether you change a single piece of data inside a JSON object or all of it: a new version of the row has to be written.

Thus the advice in the manual:

JSON data is subject to the same concurrency-control considerations as any other data type when stored in a table. Although storing large documents is practicable, keep in mind that any update acquires a row-level lock on the whole row. Consider limiting JSON documents to a manageable size in order to decrease lock contention among updating transactions. Ideally, JSON documents should each represent an atomic datum that business rules dictate cannot reasonably be further subdivided into smaller datums that could be modified independently.

The gist of it: to modify anything inside a JSON object, you have to assign a modified object to the column. Postgres supplies limited means to build and manipulate json data in addition to its storage capabilities. The arsenal of tools has grown substantially with every new release since version 9.2. The upcoming version 9.4 is adding quite a bit.

Instructions how to work with the tools of Postgres 9.3:

答案2=========================

This is coming in 9.5 in the form of jsonb_set by Andrew Dunstan based on an existing extensionjsonbx that does work with 9.4

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