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)