Indexing on jsonb keys in postgresql - postgresql

Question

I'm using PostgreSQL.
Is there any way to create index just on dictionary keys, not values.
For example imagine a jsonb column like:
select data from tablename where id = 0;
answer: {1:'v1', 2:'v2'}
I want to index on the key set (or key list) which is [1, 2]. To speed up queries like:
select count(*) from tablename where data ? '2';
As you can see in docs, there is a way for indexing the column entirely (keys + values):
CREATE INDEX idxgin ON api USING GIN (jdoc);
This is not good for me, considering that I store a large amount of data in values.
I tried this before:
CREATE INDEX test ON tablename (jsonb_object_keys(data));
The error was:
ERROR: set-returning functions are not allowed in index expressions
Also, I don't want to store keys in the dictionary as a value.
Can you help me?

Answer 1

Your example doesn't make much sense, as your WHERE clause isn't specifying a JSON operation, and your example output is not valid JSON syntax.
You can hide the set-returning function (and the aggregate) into an IMMUTABLE function:
create function object_keys(jsonb) returns text[] language SQL immutable as $$
select array_agg(jsonb_object_keys) from jsonb_object_keys($1)
$$;
create index on tablename using gin ( object_keys(data));
If you did it this way, you could then query it formulated like this:
select * from tablename where object_keys(data) #> ARRAY['2'];
You could instead make the function return a JSONB containing an array rather than returning a PostgreSQL text array, if you would rather query it that way:
select * from tablename where object_keys_jsonb(data) #> '"2"';
You can't use a ? formulation, because in JSONB that is specifically for objects not arrays. If you really wanted to use ?, you could instead write a function which keeps the object as an object, but converts all the values to JSON null or to empty string, so they take up less space.