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Berkeley DB Reference Guide: Access Methods

Specifying a database hash

The database hash determines in which bucket a particular key will reside. The goal of hashing keys is to distribute keys equally across the database pages, therefore it is important that the hash function work well with the specified keys so that the resulting bucket usage is relatively uniform. A hash function that does not work well can effectively turn into a sequential list.

No hash performs equally well on all possible data sets. It is possible that applications may find that the default hash function performs poorly with a particular set of keys. The distribution resulting from the hash function can be checked using db_stat utility. By comparing the number of hash buckets and the number of keys, one can decide if the entries are hashing in a well-distributed manner.

The hash function for the hash table can be specified by calling the DB->set_h_hash method. If no hash function is specified, a default function will be used. Any application-specified hash function must take a reference to a DB object, a pointer to a byte string and its length, as arguments and return an unsigned, 32-bit hash value.

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