# Db::set_bt_prefix

`

###

    #include <db_cxx.h>

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

# Description

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 function.

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_q_extentsize, 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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