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. Thanks, Yucheng On Tue, Mar 26, 2013 at 11:35 AM, Howard Chu <[email protected]> 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/> >
