Set the Btree prefix function. The prefix function must return the
number of bytes of the second key argument that would be required by
the Btree key comparison function to determine the second key argument's
ordering relationship with respect to the first key argument. If the
two keys are equal, the key length should be returned. The prefix
function must correctly handle any key values used by the application
(possibly including zero-length keys). The data and
size fields of the DBT are the only fields that may be
used for the purposes of this determination.
The prefix function is used to determine the amount by which keys stored
on the Btree internal pages can be safely truncated without losing their
uniqueness. See the Btree
prefix comparison section of the Reference Guide for more details about
how this works. The usefulness of this is data dependent, but in some
data sets can produce significantly reduced tree sizes and search times.
If no prefix function or key comparison function is specified by the
application, a default lexical comparison function is used as the prefix
function. If no prefix function is specified and a key comparison
function is specified, no prefix function is used. It is an error to
specify a prefix function without also specifying a key comparison
The DB->set_bt_prefix interface may only be used to configure Berkeley DB before
the DB->open interface is called.
The DB->set_bt_prefix function returns a non-zero error value on failure and 0 on success.