size_t (*bt_prefix_fcn)(DB *, const DBT *, const DBT *));
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
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 method configures operations performed using the specified
DB handle, not all operations performed on the underlying
The DB->set_bt_prefix interface may not be called after the DB->open
interface 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 DB->set_bt_prefix method may fail and return a non-zero error for the following conditions:
- An invalid flag value or parameter was specified.
Called after DB->open was called.
The DB->set_bt_prefix method may fail and return a non-zero error for errors specified for other Berkeley DB and C library or system functions.
If a catastrophic error has occurred, the DB->set_bt_prefix method may fail and
in which case all subsequent Berkeley DB calls will fail in the same way.
Databases and Related Methods
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