#include <db.h>
int
DB->set_bt_prefix(DB *db,
size_t (*bt_prefix_fcn)(DB *, const DBT *, const DBT *));
Set the Btree prefix function. 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 Berkeley DB Reference Guide for more details about how this works. The usefulness of this is data-dependent, but can produce significantly reduced tree sizes and search times in some data sets.
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 Btree key comparison function.
The DB->set_bt_prefix() method configures operations performed using
the specified DB handle,
not all operations performed on the underlying database.
The DB->set_bt_prefix() method may not be called after the
DB->open() method is called. If
the database already exists when DB->open()
is called, the information specified to DB->set_bt_prefix() must be the same as that
historically used to create the database or corruption can occur.
The DB->set_bt_prefix()
method returns a non-zero error value on failure and 0 on success.
The bt_prefix_fcn function is the application-specific Btree prefix function. The prefix function takes three parameters:
The bt_prefix_fcn function must return the number of bytes of the second key parameter that would be required by the Btree key comparison function to determine the second key parameter's ordering relationship with respect to the first key parameter. 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, and no particular alignment of the memory to which the data field refers may be assumed.
The DB->set_bt_prefix()
method may fail and return one of the following non-zero errors:
If the method was called after DB->open() was called; or if an invalid flag value or parameter was specified.