How can I manually alter the statistics for a column?

I'd like to manually alter the statistics for a column, as for the column in
question the statistics are causing Postgres to do the wrong thing for my
purposes. (I.e., a Seq Scan, rather than an Index Scan.) If someone can
tell me how to achieve this, I would quite grateful.

Thanks!
|>ouglas

P.S. Actually, for this particular problem, just deleting the statistics
would be fine. I've tried doing:

alter table maindb_astobject alter column survey_id set statistics 0;

And then analyzing the column, but when "statistics" for a column are set
to 0, Postgres seems to leave the current statistics in place, which is not
the right thing for me at all. I can successfully set "statistics" to 1,
but that turns out to be one statistic too many.

I've tried settings the statistics via the table "pg_stats", but that turns
out to be a view, and Postgres won't allow to me to alter it.

Perhaps I can achieve the end by altering the "pg_statistic" table instead,
but that table is more than a bit opaque to me.

P.P.S The Seq Scan is 2-4 orders of magnitude slower than the Index Scan.