-- does it work

testdb=# select ts_headline('The purpose of FTS is to find documents, which satisfy
query and optionally return them in some order. Most common case: Find
documents containing all query terms and return them in order of their
similarity to the query.', to_tsquery('english', 'documents'),
'MaxFragments=1');
                                  ts_headline                                  
-------------------------------------------------------------------------------
 purpose of FTS is to find <b>documents</b>, which satisfy
 query and optionally return them in some order. Most common case: Find
 <b>documents</b> containing all query terms and return them in order of their
 similarity
(1 row)

-- does it respect MinWords

testdb=# select ts_headline('The purpose of FTS is to find documents, which satisfy query and optionally return them in some order. Most common case: Find documents containing all query terms and return them in order of their similarity to the query.', to_tsquery('english', 'nosuchterm'), 'MaxFragments=1,MinWords=5');
      ts_headline      
-----------------------
 The purpose of FTS is

-- does it respect MaxWords

testdb=# select ts_headline('The purpose of FTS is to find documents, which satisfy query and optionally return them in some order. Most common case: Find documents containing all query terms and return them in order of their similarity to the query.', to_tsquery('english', 'document'), 'MaxFragments=1,MinWords=5,MaxWords=8');
                           ts_headline                            
------------------------------------------------------------------
 find <b>documents</b>, which satisfy query and optionally return

-- does it exclude ShortWord in the end ( "in" is excluded)

testdb=# select ts_headline('The purpose of FTS is to find documents, which satisfy query and optionally return them in some order. Most common case: Find documents containing all query terms and return them in order of their similarity to the query.', to_tsquery('english', 'document'), 'MaxFragments=1,MinWords=5,MaxWords=10');
                              ts_headline                              
-----------------------------------------------------------------------
 find <b>documents</b>, which satisfy query and optionally return them
(1 row)

-- does it exclude ShortWord in the front ( "The" is excluded)

testdb=# select ts_headline('The purpose of FTS is to find documents, which satisfy query and optionally return them in some order. Most common case: Find documents containing all query terms and return them in order of their similarity to the query.', to_tsquery('english', 'document'), 'MaxFragments=1,MinWords=5,MaxWords=13');
                                      ts_headline                                      
---------------------------------------------------------------------------------------
 purpose of FTS is to find <b>documents</b>, which satisfy query and optionally return

-- when multiple words are used, the cover is shown in middle of the fragment (cover size <= MaxWords)

testdb=# select ts_headline('The purpose of FTS is to find documents, which satisfy query and optionally return them in some order. Most common case: Find documents containing all query terms and return them in order of their similarity to the query.', to_tsquery('english', 'optional & order'), 'MaxFragments=1,MinWords=5,MaxWords=10');
                                ts_headline                                
---------------------------------------------------------------------------
 query and <b>optionally</b> return them in some <b>order</b>. Most common

-- does it choose the smallest cover (there are three covers between positions (7,17), (17, 22), and (22, 31). The chosen one is (17, 22))

testdb=# select ts_headline('The purpose of FTS is to find documents, which satisfy query and optionally return them in some order. Most common case: Find documents containing all query terms and return them in order of their similarity to the query.', to_tsquery('english', 'order & documents'), 'MaxFragments=1,MinWords=5,MaxWords=10');
                                   ts_headline                                   
---------------------------------------------------------------------------------
 some <b>order</b>. Most common case: Find <b>documents</b> containing all query


-- does it show multiple fragments

testdb=# select ts_headline('The purpose of FTS is to find documents, which satisfy query and optionally return them in some order. Most common case: Find documents containing all query terms and return them in order of their similarity to the query.', to_tsquery('english', 'query & documents'), 'MaxFragments=2,MinWords=5,MaxWords=10');
                                                                           ts_headline                                                                            
------------------------------------------------------------------------------------------------------------------------------------------------------------------
 find <b>documents</b>, which satisfy <b>query</b> and optionally return them ... common case: Find <b>documents</b> containing all <b>query</b> terms and return
(1 row)

-- does it exclude overlapping covers (even when MaxFragments = 2, the overlapping covers are excluded)

testdb=# select ts_headline('The purpose of FTS is to find documents, which satisfy query and optionally return them in some order. Most common case: Find documents containing all query terms and return them in order of their similarity to the query.', to_tsquery('english', 'query & order & documents'), 'MaxFragments=2,MinWords=5,MaxWords=15');
                                                   ts_headline                                                   
-----------------------------------------------------------------------------------------------------------------
 them in some <b>order</b>. Most common case: Find <b>documents</b> containing all <b>query</b> terms and return
(1 row)


-- when cover size is greater than MaxWords, does it break covers into fragments (first with MaxFragments = 1 and then with maxFragments = 2)

testdb=# select ts_headline('The purpose of FTS is to find documents, which satisfy query and optionally return them in some order. Most common case: Find documents containing all query terms and return them in order of their similarity to the query.', to_tsquery('english', 'purpose & similarity'), 'MaxFragments=1,MinWords=5,MaxWords=10');
                           ts_headline                           
-----------------------------------------------------------------
 <b>purpose</b> of FTS is to find documents, which satisfy query
(1 row)

testdb=# select ts_headline('The purpose of FTS is to find documents, which satisfy query and optionally return them in some order. Most common case: Find documents containing all query terms and return them in order of their similarity to the query.', to_tsquery('english', 'purpose & similarity'), 'MaxFragments=2,MinWords=5,MaxWords=10');
                                                    ts_headline                                                    
-------------------------------------------------------------------------------------------------------------------
 <b>purpose</b> of FTS is to find documents, which satisfy query ... order of their <b>similarity</b> to the query

-- using Oleg suggestions for testing some boundry cases

testdb=# select ts_headline('1 2 3 4 5 1 2 3 1','1&3'::tsquery, 'MaxFragments=1');
    ts_headline    
-------------------
 <b>3</b> <b>1</b>

testdb=# select ts_headline('1 2 3 4 5 1 2 3 1','1&3'::tsquery, 'MaxFragments=2');
                ts_headline                
-------------------------------------------
 <b>1</b> 2 <b>3</b> ... <b>3</b> <b>1</b>

testdb=# select ts_headline('1 2 3 4 5 1 2 3 1','1&2'::tsquery, 'MaxFragments=2');
               ts_headline               
-----------------------------------------
 <b>1</b> <b>2</b> ... <b>1</b> <b>2</b>

testdb=# select ts_headline('1 2 3 4 5 1 2 3 1','2'::tsquery, 'MaxFragments=2');
      ts_headline      
-----------------------
 <b>2</b> ... <b>2</b>

testdb=# select ts_headline('1 2 3 4 5 1 2 3 1','1&4'::tsquery, 'MaxFragments=2');
     ts_headline     
---------------------
 <b>4</b> 5 <b>1</b>

-- Checking FragmentDelimiter

testdb=# select ts_headline('1 2 3 4 5 1 2 3 1','2'::tsquery, 'MaxFragments=2,FragmentDelimiter=***');
     ts_headline     
---------------------
 <b>2</b>***<b>2</b>
(1 row)