Background:
I've been using Postgres full-text search and it has met my needs quite well. Though there is some unexpected behavior that I cannot seem to wrap my head around. It has to do with the full-text search results returning the highlighted matches using the ts_headline function. It returns the correct matches most of the time, but often it will not return a match as I expect it. I think an example is the best way to demonstrate this.
Relevant Postgres full-text highlighting docs: https://www.postgresql.org/docs/current/textsearch-controls.html#TEXTSEARCH-HEADLINE
Versions: I have tried Postgres 15 and 12 and experienced this bug (feature?) in both.
Examples: In the following 3 full-text search highlighting queries, why do the first one's results not match the second and third's? Im trying to figure out what I can do to get matches on the first query. I first noticed that some of my highlight queries were coming back with no 'hits'. I created the following 3 queries to show the issue I'm having.
According to the docs, when there are no matches (identified by <b></b> tags) it simply returns the first MinWords. That's what is happening in the first query when I think that we should actually get the 2 results back as we do in the following two queries.
postgres=# SELECT ts_headline('english', 'beginning word word word word CHILD word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word SERVICE ending',
to_tsquery('english', 'CHILD & SERVICE'),
'MaxFragments=2,MinWords=5,MaxWords=10');
ts_headline
-------------------------------
beginning word word word word
(1 row)
Now let's increase MaxWords, now we get 2 matches
postgres=# SELECT ts_headline('english', 'beginning word word word word CHILD word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word SERVICE ending',
to_tsquery('english', 'CHILD & SERVICE'),
'MaxFragments=2,MinWords=5,MaxWords=11');
ts_headline
------------------------------------------------------------------------------------------------------------------------
beginning word word word word <b>CHILD</b> word word word word word ... word word word word word <b>SERVICE</b> ending
(1 row)
Now let's increase MaxFragments, now we get 2 matches
postgres=# SELECT ts_headline('english', 'beginning word word word word CHILD word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word word SERVICE ending',
to_tsquery('english', 'CHILD & SERVICE'),
'MaxFragments=3,MinWords=5,MaxWords=10');
ts_headline
---------------------------------------------------------------------------------------------------------
word word word word <b>CHILD</b> word word word word word ... word word word word <b>SERVICE</b> ending
(1 row)
I feel like something subtle is going on between all the MaxFragments, MinWords, MaxWords settings, or maybe this is undefined behavior or a bug. Im hoping to find a way to get the first query to match as I do believe it should. Please correct me if I'm wrong.
