# Db::set_bt_prefix

`

###

    #include <db_cxx.h>

    extern "C" {
    	typedef size_t (*bt_prefix_fcn_type)(const DBT *, const DBT *);
    };
    int
    Db::set_bt_prefix(bt_prefix_fcn_type bt_prefix_fcn);

# Description

Set the Btree prefix comparison function. The bt_prefix_fcn method must return the number of bytes of the second key argument that are necessary to determine that it is greater than the first key argument. If the keys are equal, the key length should be returned. The prefix comparison 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 comparison.

This 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 function 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 bt_prefix is not explicitly set, and no key comparison method is specified, a default lexical comparison method is used. If bt_prefix is not explicitly set and a key comparison method is specified, no prefix comparison is done. It is an error to set bt_prefix without also specifying a key comparison method.

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 method either returns a non-zero error value or throws an exception that encapsulates a non-zero error value on failure, and returns 0 on success.

# Errors

EINVAL
  An invalid flag value or parameter was specified.

  Called after Db::open was called.

### Class

Db

# See Also

Db::close, Db::cursor, Db::del, Db::err, Db::fd, Db::get, Db::get_byteswapped, Db::get_type, Db::join, Db::key_range, Db::open, Db::put, Db::remove, Db::set_bt_compare, Db::set_bt_minkey, Db::set_bt_prefix, Db::set_cachesize, Db::set_dup_compare, Db::set_errcall, Db::set_errfile, Db::set_errpfx, Db::set_flags, Db::set_h_ffactor, Db::set_h_hash, Db::set_h_nelem, Db::set_lorder, Db::set_malloc, Db::set_pagesize, Db::set_paniccall, Db::set_realloc, Db::set_re_delim, Db::set_re_len, Db::set_re_pad, Db::set_re_source, Db::stat, Db::sync, Db::upgrade and Db::verify.`

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