### 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: Db::set_bt_prefix

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 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.

### Parameters

bt_prefix_fcn
  The bt_prefix_fcn function is the application-specific Btree prefix function. The prefix function takes three parameters:

  db
    The db parameter is the enclosing database handle.

  dbt1
    The dbt1 parameter is a Dbt representing a database key.

  dbt2
    The dbt2 parameter is a Dbt representing a database key.

  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.

### Errors

The Db::set_bt_prefix method may fail and throw DbException, encapsulating one of the following non-zero errors, or return one of the following non-zero errors:

EINVAL
  If the method was called after Db::open was called; or if an invalid flag value or parameter was specified.

### Class

Db

### See Also

Databases and Related Methods`

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