Re: [LMDB] Append To Value

Yucheng Low wrote:

I see, correct me if I misunderstood: you are suggesting to use duplicate keys to store the vector? In other words, if I have an neighbor list for vertex 5, comprising of values [1,2,3], I will store it as 3 duplicate values for key 5.

Also, does the LMDB store the key just once for each duplicate value? Or multiple times once for each value.

The key is only stored once. Duplicating the key would be stupid. Also DUPFIXED values are stored contiguously, with no headers or other overhead. It will be effectively the same amount of storage as storing your single record.

Thanks, Yucheng

On Tue, Mar 26, 2013 at 11:35 AM, Howard Chu <hyc@symas.com mailto:hyc@symas.com> wrote:

Yucheng Low wrote:

    Hi,

    I am considering using LMDB to store a dynamic graph structure, and it
    will be
    very helpful to support "append to value" having the same semantics as
    "append" in Kyoto Cabinet.
    http://fallabs.com/__kyotocabinet/api/__classkyotocabinet_1_1BasicDB.__html#__a23e776e5bd1e3c5caa0f62edffb87__a54
    <http://fallabs.com/kyotocabinet/api/classkyotocabinet_1_1BasicDB.html#a23e776e5bd1e3c5caa0f62edffb87a54>

    For instance, if I were to store an adjacency list representation of a
    graph
    in LMDB, I would use vertex ID as the key, and the value is a vector
    of 64-bit
    IDs. To insert an edge into the graph will simply require appending 8
    bytes to
    the end of the vector. Re-reading and re-writing the entire value will be
    somewhat excessive (and will actually change the asymptotic runtime of the
    insertion).

    This is also one of those situations where a batch append might be
    useful. For
    instance, if I have an edge list on file which is too large to retain in
    memory. To convert it to an adjacency list in LMDB will require
    batch-append.
    Batch-writes are insufficient since that will require me to cache at
    least,
    large parts of the adjacency list in memory, and the original edge
    list could
    be ordered arbitrarily.

    The actual graph representation I am thinking of is somewhat more
    intelligent
    than a straight adjacency list, but similar concepts hold.


Based solely on the description here, I would use MDB_DUPFIXED and
MDB_APPENDDUP or MDB_MULTIPLE. You could later use MDB_GET_MULTIPLE to
read the values and reconstruct the vector.

--
   -- Howard Chu
   CTO, Symas Corp. http://www.symas.com
   Director, Highland Sun http://highlandsun.com/hyc/
   Chief Architect, OpenLDAP http://www.openldap.org/__project/
<http://www.openldap.org/project/>