On Sun, Mar 17, 2019 at 8:06 PM Jonathan S. Katz <jkatz@postgresql.org> wrote:
> On 3/17/19 1:02 PM, Alexander Korotkov wrote:
> >
> > Thank you for the explanation. Is it jsonb_ops or jsonb_path_ops?
>
> I just used "USING gin(col)" so jsonb_ops.
I see. So, jsonb_ops extracts from this query only existence of
.length key. And I can bet it exists in all (or almost all) the
documents. Thus, optimizer thinks index might be useful, while it's
useless. There is not much can be done while we don't have statistics
for jsonb (and access to it from GIN extract_query). So, for now we
can just refuse from extracting only keys from jsonpath in jsonb_ops.
But I think it would be better to just document this issue. In future
we should improve that with statistics.
------
Alexander Korotkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company
