Weird ranking results with ts_rank

Hi everybody.

I'm implementing a solution that uses PostgreSQL's full text search
capabilities and I have come across a particular set of results for ts_rank
that don't seem to make sense according to the documentation. I have tried
the following queries in PostgreSQL 10, 11 and 12.
In both cases only the word "box" is matching, but adding a non-matching
word with OR to the query increases the ranking. If I keep adding more
non-matching words with OR the ranking starts to decrease again, but I
would imagine that the second option should have the highest score and it
would start decreasing from there the more non-matching words I add.
Is there something I'm not understanding?

Thanks.

postgres=# select ts_rank(to_tsvector('search for a text box'),
to_tsquery('circle | lot <-> box'));
ts_rank
-------------
0.020264236
(1 row)

postgres=# select ts_rank(to_tsvector('search for a text box'),
to_tsquery('lot <-> box'));
ts_rank
---------
1e-20
(1 row)

--
Javier Ayres
Data Engineer
+1 855 636 5811 <+18556365811> - sophilabs.co