Berkeley DB Hash Tables: hash, berkeley, db, Berkeley DB

Berkeley DB Hash Tables

If you choose to use Hash tables with Berkeley DB, are keys actually stored in the hash table, or does the hash table store only hashes of keys, along with the actual record?

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One can not reconstruct the original value from a hash code, one-to-one,
and two entries can have the same hash code. So if one just would store the hash, the original key would be lost. So with all likliness they do store the original data also.

If one looks throught the sources, on can read in hash_page.c:
      /*
                   * Figure out how many bytes we need on the new
                   * page to store the key/data pair.
                   */

and in db_page.h
/*
* The first and second types are H_KEYDATA and H_DUPLICATE, represented
* by the HKEYDATA structure:
*
*      +-----------------------------------+
*      | type | key/data ... |
*      +-----------------------------------+
*
* For duplicates, the data field encodes duplicate elements in the data
* field:
*
*      +---------------------------------------------------------------+
*      | type | len1 | element1 | len1 | len2 | element2 | len2 |
*      +---------------------------------------------------------------+
*
* Thus, by keeping track of the offset in the element, we can do both
* backward and forward traversal.
*/

and if you look through the docs you can find an example:
http://www.oracle.com/technology/documentation/berkeley-db/db/ref/am/curget.html

and there they write:
/* Initialize the key/data return pair. */ memset(&key, 0, sizeof(key)); memset(&data, 0, sizeof(data));

/* Walk through the database and print out the key/data pairs. */ while ((ret = dbcp->c_get(dbcp, &key, &data, DB_NEXT)) == 0) printf("%.*s : %.*s\n", (int)key.size, (char *)key.data, (int)data.size, (char *)data.data); if (ret != DB_NOTFOUND) { dbp->err(dbp, ret, "DBcursor->get"); goto err;

I bets this is not a hash code but the original string. If I'm not totally mistaken. Then the key is storeds as-is in the DB also.

Regards
Friedrich

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Berkeley DB Hash Tables

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Databases Miscellaneous

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