On 22/06/11 10:00, Greg Smith wrote: > On 06/21/2011 10:00 AM, Vick Khera wrote: >> Postgres has nothing quite like the MySQL cluster mode with NDB. You >> will have to re-think your solution if you want to use postgres to >> distribute your queries and data across multiple servers. >> > > The closest thing to a NDB cluster in PostgreSQL is using PL/Proxy to > split data across multiple nodes. Both use similar hash-key methods to > distribute things across more than one system, and you can run queries > that return a combined set of results bigger than any single node could > have handled. Doesn't PL/Proxy, like NDB, offer no referential integrity enforcement between nodes? Enforcing referential integrity in a multi-master cluster is one of the things that adds synchronization overhead and makes multi-master clustered systems slower. In extremely rough terms, before a node can do certain operations it has to ask one or more other nodes "can I do this?" and wait for them to reply (possibly waiting for their disk I/O or for other tasks to finish). Each node then has to say "I've done this" when it's ready to commit the change and make it visible across the cluster, and other nodes have to handle that change. For example, if you want to delete a tuple that's in a table targeted by a referential integrity constraint you have to ask all the other nodes whether they have any tuples that reference yours before you can go ahead and delete it. As you can imagine, this gets slow. There are various caching and optimistic execution tricks used to help with these issues in some clustered RDBMSes, but they're all limited to various degrees. The more effective tricks come with application-visible downsides: for example, they can allow transactions to abort at commit time because a concurrent modification occurred on another cluster node. -- Craig Ringer -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
