Prefix compression adds a lot of complexity. If you insert a new record that makes an existing prefix non-unique, you then have to expand and rewrite every key

Prefix compression adds a lot of complexity. If you insert a new record that makes an existing prefix non-unique, you then have to expand and rewrite every key in the page, and that will ripple up to every parent page as well. And the expansion can then cause the records to no longer fit in their current page, so then you need to do a bunch of page splits. It's gross and ugly and easy to get wrong. (The btree.c that LMDB was based on has prefix compression. We ripped it out after finding bugs in it.) If your data values are small enough, you can use DUPSORT records instead, and manage your prefixes manually. There is a linear search involved to open a DBI handle, yes. But you only do that once, and keep it open for the life of the program. Not a big deal.