You can store the md5 (or any hash) in a new column and use it in the index instead of the string column. It will still be a string index but much shorter. You have to be aware of hash collision but in my case it was a multi column index so the risk was close to zero. MD5 was maybe not the best choice but it's builtin so available everywhere.

What I did to not maintain a second column is to use the function directly in the index:

``` CREATE UNIQUE INDEX CONCURRENTLY "groupid_md5_uniq" ON "crawls" ("group_id", md5("url")); ```

``` SELECT * FROM crawls WHERE group_id= $0 AND md5(url) = md5($1) ```

This simple trick, that did not required an extensive refactor, speed up the query time by a factor of thousand.