jsonb nested values and indexes

Hello

I don't know a doc about jsonb indexes

"But in jsonb_path_ops, each index item is a hash of both the value and the
key(s) leading to it; for example to index {"foo": {"bar": "baz"}}, a
single index item would be created incorporating all three of foo, bar, and
baz into the hash value. Thus a containment query looking for this
structure would result in an extremely specific index search; but there is
no way at all to find out whether foo appears as a key. On the other hand,
a jsonb_ops index would create three index items representing foo, bar, and
baz separately;"

What keys I can search on this jsonb?

postgres=# create table x(a jsonb);
CREATE TABLE
postgres=# insert into x values('{"foo": {"bar": "baz"}}');
INSERT 0 1

postgres=# select * from x where a ? 'foo';
a
-------------------------
{"foo": {"bar": "baz"}}
(1 row)

postgres=# select * from x where a ? 'bar';
a
---
(0 rows)

postgres=# select * from x where a ? 'baz';
a
---
(0 rows)

???

Regards

Pavel