ERROR: Gin doesn't support full scan due to it's awful inefficiency

[Charlie Savage <cfis@xxxxxxxxxxxx>]

I've run across another GIN index issue - using postgresql 8.1.4 on 
Window/Linux with the GIN/tsearch2 patch.

I have two tables like this:

CREATE TABLE maps
(
  id serial,
  query tsearch2.tsquery
)


CREATE TABLE features
(
  id serial,
  vector tsearch2.tsvector
)

CREATE INDEX idx_features_tags_vector ON features USING gin (tags_vector);


Where maps.query contains cached tsquery (they are cached for 
performance reasons).

When I run this query:

select *
from maps, features
where to_tsquery('test') @@ features.tags_vector

I get this error:

ERROR:  Gin doesn't support full scan due to it's awful inefficiency

Here is explain (from a very small test database):

Nested Loop  (cost=0.00..1878.71 rows=370 width=208)
  ->  Seq Scan on maps  (cost=0.00..14.80 rows=480 width=136)
  ->  Index Scan using idx_features_tags_vector on features 
(cost=0.00..3.87 rows=1 width=72)
        Index Cond: ("outer".query @@ features.tags_vector)

I thought that this would solve my problem:

set enable_indexscan to off;

But it does not.

Interestingly, this does work:

select *
from features
where to_tsquery('test') @@ features.tags_vector;

Explain:

Index Scan using idx_features_tags_vector on features  (cost=0.00..3.87 
rows=1 width=72)
  Index Cond: ('''test'''::tsquery @@ tags_vector)

At first I thought the issue was that you couldn't use an Index Scan on 
gin index, but that now seems like an incorrect conclusion.

So, two things:

1.  How do I work around this issue?
2. Seems like postgresql should be smart enough to pick a query that 
will run.

Thanks,

Charlie