Db::set_bt_prefix()
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## Db::set_bt_prefix()

    #include <db_cxx.h>

    extern "C" {
        typedef size_t (*bt_prefix_fcn_type)(DB *, const *dbt1, const *dbt2);
    };
    int
    Db::set_bt_prefix(bt_prefix_fcn_type bt_prefix_fcn);

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 a DbException exception, 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

Database and Related Methods

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