Like vs '='

I have a function to transform text into a
pseudo-metaphone variable, take this
example:

cddbsql=# select song, metatext(song) from cdsongs
where metatext(song) like
metatext('born to run') limit 3 ;   song     | metatext
-------------+----------Born To Run | brntornBorn To Run | brntornBorn To Run | brntorn
(3 rows)

Here is the problem: Depending on whether there is an
index or not, 'like'
behaves differently. Here is a transcript:

cddbsql=# select song, metatext(song) from cdsongs
where metatext(song) like
metatext('born to run') limit 1 ;   song     | metatext
-------------+----------Born To Run | brntorn
(1 row)
cddbsql=# create index cdsongs_meta_song on cdsongs
(metatext(song)) ;
CREATE
cddbsql=# select song, metatext(song) from cdsongs
where metatext(song) like
metatext('born to run') limit 1 ;song | metatext
------+----------
(0
rows)     

This happens in both 7.0 and
7.1.

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