GiST index slower than seqscan

I'm still trying to wrap my brain around this one. Please forgive me if this is
the proverbial "dead horse" that I'm beating.

In case you're unfamiliar with this particular horse, I'm using ltree to create
a full text index on some <= 50 char long fields for a lookup table. The idea
was to be able to tear through tons of data quickly finding case insensitive
substring matches.

Here's my index...

CREATE INDEX letter_search_vector_idx
ON letter_search
USING gist
(search_vector);

I know that the index is bigger than the table, but shouldn't it be able to
quickly scan the few branches that matter?

I've tried to do a varchar-based substring lookup table, and the size for that
table+index is enormous compared to the ltree table + index (which is huge
anyway)

I'm thinking that I've created something or am using something incorrectly.

I need to be set straight. Please advise!

[snip]

Welcome to psql 8.1.3, the PostgreSQL interactive terminal.

Type: \copyright for distribution terms
\h for help with SQL commands
\? for help with psql commands
\g or terminate with semicolon to execute query
\q to quit
data=# analyze letter_search;
ANALYZE
data=# explain select * from letter_search where search_vector ~
charslquery('669344');
QUERY PLAN

--------------------------------------------------------------------------------
-----------
Bitmap Heap Scan on letter_search (cost=59.14..4978.98 rows=1467 width=162)
Recheck Cond: (search_vector ~ '*.6.6.9.3.4.4.*'::lquery)
-> Bitmap Index Scan on letter_search_vector_idx (cost=0.00..59.14
rows=146
7 width=0)
Index Cond: (search_vector ~ '*.6.6.9.3.4.4.*'::lquery)
(4 rows)

data=# explain analyze select * from letter_search where search_vector ~
charslquery('669344');
QUERY PLAN

--------------------------------------------------------------------------------
-------------------------------------------------------------
Bitmap Heap Scan on letter_search (cost=59.14..4978.98 rows=1467 width=162)
(a
ctual time=63061.402..63072.362 rows=2 loops=1)
Recheck Cond: (search_vector ~ '*.6.6.9.3.4.4.*'::lquery)
-> Bitmap Index Scan on letter_search_vector_idx (cost=0.00..59.14
rows=146
7 width=0) (actual time=63058.094..63058.094 rows=2 loops=1)
Index Cond: (search_vector ~ '*.6.6.9.3.4.4.*'::lquery)
Total runtime: 63072.411 ms
(5 rows)

data=# set enable_bitmapscan=off;
SET
data=# explain select * from letter_search where search_vector ~ charslqu
ery('669344');
QUERY PLAN

--------------------------------------------------------------------------------
----------------------
Index Scan using letter_search_vector_idx on letter_search
(cost=0.00..5837.70
rows=1467 width=162)
Index Cond: (search_vector ~ '*.6.6.9.3.4.4.*'::lquery)
(2 rows)

data=# explain analyze select * from letter_search where search_vector ~
charslquery('669344');
QUERY
PLA
N
--------------------------------------------------------------------------------
-------------------------------------------------------------------------
Index Scan using letter_search_vector_idx on letter_search
(cost=0.00..5837.70
rows=1467 width=162) (actual time=14582.619..162637.887 rows=2 loops=1)
Index Cond: (search_vector ~ '*.6.6.9.3.4.4.*'::lquery)
Total runtime: 162637.977 ms
(3 rows)

data=# set enable_indexscan=off;
SET
data=# explain select * from letter_search where search_vector ~ charslqu
ery('669344');
QUERY PLAN
----------------------------------------------------------------------
Seq Scan on letter_search (cost=0.00..55232.18 rows=1467 width=162)
Filter: (search_vector ~ '*.6.6.9.3.4.4.*'::lquery)
(2 rows)

data=# explain analyze select * from letter_search where search_vector ~
charslquery('669344');
QUERY PLAN

--------------------------------------------------------------------------------
--------------------------------------
Seq Scan on letter_search (cost=0.00..55232.18 rows=1467 width=162) (actual
ti
me=4725.525..9428.087 rows=2 loops=1)
Filter: (search_vector ~ '*.6.6.9.3.4.4.*'::lquery)
Total runtime: 9428.118 ms
(3 rows)

[/snip]

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