why index scan not working when using 'like'?

Hi all,

I want to use index on the gene_symbol column in my
query and gene_symbol is indexed. but when I use
lower (gene_symbol) like lower('%mif%'), the index
is not used. While when I change to
lower(gene_symbol) = lower('mif'), the index is used
and index scan works, but this is not what I like. I
want all the gene_symbols containing substring
'mif' are pulled out, and not necessarily exactly match.

could anybody give me some hints how to deal with
this. If I do not used index, it take too long for
the query.

PGA> explain select distinct probeset_id, chip,
gene_symbol, title, sequence_description, pfam from
affy_array_annotation where lower(gene_symbol) like
upper('%mif%');
QUERY PLAN
-----------------------------------------------------------------------------------------
Unique (cost=29576.44..29591.44 rows=86 width=265)
-> Sort (cost=29576.44..29578.59 rows=857
width=265)
Sort Key: probeset_id, chip, gene_symbol,
title, sequence_description, pfam
-> Seq Scan on affy_array_annotation
(cost=0.00..29534.70 rows=857 width=265)
Filter: (lower((gene_symbol)::text)
~~ 'MIF%'::text)
(5 rows)

PGA=> explain select distinct probeset_id, chip,
gene_symbol, title, sequence_description, pfam from
affy_array_annotation where lower(gene_symbol) =
upper('%mif%');

QUERY PLAN
---------------------------------------------------------------------------------------------------------------------
Unique (cost=3433.44..3448.44 rows=86 width=265)
-> Sort (cost=3433.44..3435.58 rows=857 width=265)
Sort Key: probeset_id, chip, gene_symbol,
title, sequence_description, pfam
-> Index Scan using gene_symbol_idx_fun1
on affy_array_annotation (cost=0.00..3391.70
rows=857 width=265)
Index Cond:
(lower((gene_symbol)::text) = '%MIF%'::text)
(5 rows)

Regards,
William