Trivial function query optimized badly

Well, once again I'm hosed because there's no way to tell the optimizer the cost for a user-defined function. I know this issue has already been raised (by me!) several times, but I have to remind everyone about this. I frequently must rewrite my SQL to work around this problem.

Here is the function definition:

CREATE OR REPLACE FUNCTION cansmiles(text) RETURNS text
AS '/usr/local/pgsql/lib/libchem.so', 'cansmiles'
LANGUAGE 'C' STRICT IMMUTABLE;

Here is the bad optimization:

db=> explain analyze select version_id, 'Brc1ccc2nc(cn2c1)C(=O)O' from version where version.isosmiles = cansmiles('Brc1ccc2nc(cn2c1)C(=O)O', 1);
QUERY PLAN
--------------------------------------------------------------------------------------------------------
Seq Scan on version (cost=0.00..23.41 rows=1 width=4) (actual time=1434.281..1540.253 rows=1 loops=1)
Filter: (isosmiles = cansmiles('Brc1ccc2nc(cn2c1)C(=O)O'::text, 1))
Total runtime: 1540.347 ms
(3 rows)

I've had to break it up into two separate queries. Ironically, for large databases, Postgres does the right thing -- it computes the function, then uses the index on the "isosmiles" column. It's blazingly fast and very satisfactory. But for small databases, it apparently decides to recompute the function once per row, making the query N times slower (N = number of rows) than it should be!

In this instance, there are 1000 rows, and factor of 10^4 is a pretty dramatic slowdown... To make it work, I had to call the function separately then use its result to do the select.

db=> explain analyze select cansmiles('Brc1ccc2nc(cn2c1)C(=O)O', 1);
QUERY PLAN
------------------------------------------------------------------------------------
Result (cost=0.00..0.01 rows=1 width=0) (actual time=1.692..1.694 rows=1 loops=1)
Total runtime: 1.720 ms
(2 rows)

db=> explain analyze select version_id, 'Brc1ccc2nc(cn2c1)C(=O)O' from version where version.isosmiles = 'Brc1ccc2nc(cn2c1)C(=O)O';
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------
Index Scan using i_version_isosmiles on version (cost=0.00..5.80 rows=1 width=4) (actual time=0.114..0.117 rows=1 loops=1)
Index Cond: (isosmiles = 'Brc1ccc2nc(cn2c1)C(=O)O'::text)
Total runtime: 0.158 ms
(3 rows)

Craig