limit over attribute size if index over it exists

Hi everybody,

I have got an issue with PostgreSQL. There is a limitation on the
column length of a tuple, in case there is an index over it. In the
actual project I am working on, I meet such a situation. I have got an
attribute over which I am doing a search (that is, I need an index over
it), but this attribute can be in some cases very large (100KB+).

The log message I get from Postgres, if I try to insert a tuple with
such a big attribute (e.g. 10K) is the following:

ERROR: index row requires 15704 bytes, maximum size is 8191

(PostgreSQL 8.07 under Linux. The index is a btree index.)

I have thought of a possible workaround. I would like to know if it
seems reasonable. The idea would be to build a hash, on the client
side, over the problematic column (let's say column a). I then store in
the db the attribute a (without index) and the hash(a) (with an index).
Then when I am doing a select, I use firstly a sub-select to choose all
tuples with the right hash (quick, with index), and then an outer
select to choose the tuple with the right attribute a (slow, sequential
scan, but normally few tuples, because few collisions). Something like
that:

SELECT b
FROM (
SELECT a, b
FROM foo
WHERE hash='<hash(a)>'
) as bar
WHERE bar.a='<a>'

(Actually, in my case the situation is slightly more complicated
because I don't have just one attribute but 2+, so there are some index
types that I cannot use. Anyway the principle is the same).

Does this solution seem reasonable, or is there other (more elegant)
ways to do that?

Thank you in advance.
Cheers,

Pat