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Db::set_bt_prefix
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#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, and no particular alignment
of the memory to which the data field refers may be assumed.
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 key comparison
function.
The Db::set_bt_prefix interface may be used only 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
The Db::set_bt_prefix method may fail and throw an exception or return a non-zero error for the following conditions:
- EINVAL
- An invalid flag value or parameter was specified.
Called after Db::open was called.
The Db::set_bt_prefix method may fail and throw an exception or return a non-zero error for errors specified for other Berkeley DB and C library or system methods.
If a catastrophic error has occurred, the Db::set_bt_prefix method may fail and either
return DB_RUNRECOVERY or throw an exception encapsulating
DB_RUNRECOVERY, in which case all subsequent Berkeley DB calls will fail
in the same way.
Class
Db
See Also
Db::associate,
Db::close,
Db::cursor,
Db::del,
Db::err, Db::errx
Db::fd,
Db::get,
Db::pget,
Db::get_byteswapped,
Db::get_type,
Db::join,
Db::key_range,
Db::open,
Db::put,
Db::remove,
Db::rename,
Db::set_alloc,
Db::set_append_recno,
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_feedback,
Db::set_flags,
Db::set_h_ffactor,
Db::set_h_hash,
Db::set_h_nelem,
Db::set_lorder,
Db::set_pagesize,
Db::set_paniccall,
Db::set_q_extentsize,
Db::set_re_delim,
Db::set_re_len,
Db::set_re_pad,
Db::set_re_source,
Db::stat,
Db::sync,
Db::truncate,
Db::upgrade,
and
Db::verify.
Copyright Sleepycat Software
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