cassandra - How wide-columns databases achieve dynamic columns on the storage layout? -


as known, in relational databases, when adding new column, data must reallocated (alter table without locking table?) maintain single row contiguous on disk.

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i understand how achieved on wide-columns storages such cassandra, sparses , can handle lots of dynamic columns insertions (http://www.datastax.com/dev/blog/thrift-to-cql3 (dynamic column family))

thanks!

in cassandra adding column adding bit adding row in relational database. can delete column specific row:

delete first_name user user_id='abcd';

in cql, alter table doesn't modify rows, in short modifies schema dictionary describe tables (look @ tables prefixed schema_ in system keyspace). changes cql parsing (the new column recognized) , interpretation (select * user meaning changed).

when drop column, data doesn't appear anymore in query results yet still present in sstables. data removed (and space freed) during future compaction (like tombstones).


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