Berkeley DB Reference Guide: Access Methods
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Berkeley DB Reference Guide: Access Methods

Partial Record Storing and Retrieval

It is possible to both store and retrieve parts of data items in all Berkeley DB access methods. This is done by specifying the DB_DBT_PARTIAL flag to the DBT structure passed to the Berkeley DB interface.

The DB_DBT_PARTIAL flag is based on the values of two elements of the DBT structure, dlen and doff. The value of dlen is the number of bytes of the record in which the application is interested. The value of doff is the offset from the beginning of the data item where those bytes start.

For example, if the data item were ABCDEFGHIJKL, a doff value of 3 would indicate that the bytes of interest started at D, and a dlen value of 4 would indicate that the bytes of interest were DEFG.

When retrieving a data item from a database, the dlen bytes starting doff bytes from the beginning of the record are returned, as if they comprised the entire record. If any or all of the specified bytes do not exist in the record, the retrieval is still successful and the existing bytes or nul bytes are returned.

When storing a data item into the database, the dlen bytes starting doff bytes from the beginning of the specified key's data record are replaced by the data specified by the data and size structure elements. If dlen is smaller than size, the record will grow, and if dlen is larger than size, the record will shrink. If the specified bytes do not exist, the record will be extended using nul bytes as necessary, and the store call will still succeed.

The following are various examples of the put case for the DB_DBT_PARTIAL flag. In all examples, the initial data item is 20 bytes in length:

ABCDEFGHIJ0123456789