Why do SQL FullText queries slow down when you OR?

Question

In SQL Server (2008), I have a FullText index on two columns, call them Table1.FirstNames and Table2.LastNames. After profiling some queries, I came up with the following results:

SELECT *
FROM (Table1 LEFT JOIN Table2 ON Table1.SomeKey=Table2.SomeKey)
WHERE CONTAINS(FirstNames, 'Bob') OR CONTAINS(LastNames, 'Bob')

=> 31 197ms

SELECT *
FROM (Table1 LEFT JOIN Table2 ON Table1.SomeKey=Table2.SomeKey)
WHERE (FirstNames LIKE '%Bob%') OR CONTAINS(LastNames, 'Bob')

=> 1941ms

SELECT *
FROM (Table1 LEFT JOIN Table2 ON Table1.SomeKey=Table2.SomeKey)
WHERE CONTAINS(FirstNames, 'Bob') OR LastNames LIKE '%Bob%'

=> 3201ms

SELECT *
FROM (Table1 LEFT JOIN Table2 ON Table1.SomeKey=Table2.SomeKey)
WHERE CONTAINS(FirstNames, 'Bob')

=> 565ms

SELECT *
FROM (Table1 LEFT JOIN Table2 ON Table1.SomeKey=Table2.SomeKey)
WHERE FirstNames LIKE '%Bob%'

=> 670ms

SELECT *
FROM (Table1 LEFT JOIN Table2 ON Table1.SomeKey=Table2.SomeKey)
WHERE CONTAINS(LastNames, 'Bob')

=> 17ms

SELECT *
FROM (Table1 LEFT JOIN Table2 ON Table1.SomeKey=Table2.SomeKey)
WHERE LastNames LIKE '%Bob%'

=> 3ms

This behaviour persists even if I rebuild the FullText index.

FullText is usually much faster than a LIKE query over large sets of data in a specific language, but why do query speeds slow down by an order of magnitude when I OR together two FullText clasues?

Answer 1

Does changing to using ContainsTable help?

It did here Adding more OR searches with CONTAINS Brings Query to Crawl

And the same answerer (Joe Stefanelli) managed to bring about a similar improvement by changing FREETEXT predicates combined with OR to a FREETEXTTABLE here SQL Server full text query across multiple tables - why so slow?