Fulltext - multiple single column indexes

Hi,
I have table with several columns and need to perform fulltext search over
volatile number of columns. 
I can't use multicolumn gist index or gin index over concatenated columns,
so I've created several single column indexes (one for each column I want to
search) and now I need to query them like this:

to_tsvector('cs', coalesce(annotation, '')) || to_tsvector('cs',
coalesce(resume, '')) || ...
@@ to_tsquery('cs', 'Query text');

This query works, but EXPLAIN has shown me, that postgres doesn't use the
indexes, so the query over a table with several thousands of records last
very long time. I've figured out, that indexes probably cannot be used this
way. What is a recommendation for this scenario?
Indexes over static number of columns work fine, but I can't use them,
because in my application logic I want to let user choose which columns to
search.

Thank you for your reply.
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