Caveats of OUTER JOIN on nested JSON value

Question

I'm writing a query which is supposed to find the elements from a list which DO NOT exist in the DB. My first attempt at this was to use a nested query where the first query fetches the ids, then I right join on that query to get what I need, and this works well:

select v.id from (
    select distinct json_data ->> 'elementId' as elementId
    from content
    and json_data->> 'elementId' in ('id1', 'id2', 'id3')
) as a
right join (values('id1'), ('id2'), ('id3')) as v(id)
on v.id = a.elementId
where a.elementId is null

The above query works perfect except for the fact that I want to I should be able to reduce the nested query to a regular select if I do the comparison on json_data ->> 'elementId' directly.

My attempt:

select v.id
from content a
right join (values('id1'), ('id2'), ('id3')) as v(id)
on json_data ->> 'elementId' = v.id

After some debugging I realized that this will never work because the content table will always contain a row even if json_data ->>'elementId' is null.

Edit: I had an extra WHERE statement which wasn't stated in the question, once I moved this after the ON my query was fixed

My question is; Is there a way to avoid using a nested query when wanted to do a left join or right join on JSON data?

Answer 1

Use NOT EXISTS:

SELECT *
FROM  (VALUES ('id1'), ('id2'), ('id3')) AS v(id)
WHERE  NOT EXISTS (SELECT FROM content WHERE json_data ->> 'elementId' = v.id);

Or if you prefer a join:

SELECT v.id
FROM  (VALUES ('id1'), ('id2'), ('id3')) AS v(id)
LEFT   JOIN content c ON c.json_data ->> 'elementId' = v.id
WHERE  c.json_data IS NULL -- or use the PK column

Either is an "anti-join", technically; and both will probably result in the same query plan.

See:

• Select rows which are not present in other table

Consider upgrading to a current version of Postgres. 9.4 has reached EOL in Feb 2020.

Index

But even Postgres 9.4 already supports jsonb which (unlike json) allows a GIN index to support your query. See:

• How to get particular object from jsonb array in PostgreSQL?
• What's the proper index for querying structures in arrays in Postgres jsonb?

Or if you are focused on this query exclusively, a plain btree on an expression should be the optimum:

CREATE INDEX ON content ((json_data ->> 'elementId'));

Related:

• PostgreSQL operator uses index but underlying function does not