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Comment by arghwhat | Hacker News Reader

#1 seems a bit weird. You could theoretically have the DB automatically create new buckets for a hash partition, but hash partitions shouldn't have hot spots unless you hash on a bad column, and creating new buckets will require all existing rows to be moved.

For other types of partitions, partition hotspots would be the fault of the DBA partitioning on a bad column. The DB could theoretically half partition ranges if a range partition is very large, but there are other things that are more important.

#2 is a nice performance feature which has been talked about before, but do any DB's actually have this yet? I suspect that it is quite tricky, and won't be universally beneficial.

Is #3 really a problem? I certainly wouldn't expect schema changes to perform well, and increasing complexity just to help in this area would seem silly.

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> Is #3 really a problem? I certainly wouldn't expect schema changes to perform well, and increasing complexity just to help in this area would seem silly.

Not the parent, but obviously they refer to the online schema migration problem. Without that online schema migrations are basically impossible. Or you get handed the burden of implementing or finding a tool that does it for you.

And yes, pt-online-schema-change[1] is such a tool, but for MySQL. MariaDB has this feature built-in with their `ALTER ONLINE TABLE` [2].

So something similar for Postgres would be godsent.

[1] https://www.percona.com/doc/percona-toolkit/LATEST/pt-online...

[2] https://mariadb.com/kb/en/library/alter-table/

mysql 5.7 has pretty good online DDL support[1]

I have been using it exclusively now instead of pt-online-schema-change.

[1] https://dev.mysql.com/doc/refman/5.7/en/innodb-online-ddl.ht...

Oracle has had it for awhile (though I forget what its called.)

SQL Server is getting it in 2017 https://docs.microsoft.com/en-us/sql/relational-databases/pe...

Oracle is also your first born expensive and Postgres is free.

Really? Someone was offended and/ or defending the obscene pricing of Oracle?

Oracle has the right to charge money for their products if they want to. Most people here make a living from exchanging things for money that are better in some way than things you can get for free.

(I didn't downvote you, btw)

Of course. And likewise, people have the right to disagree with or be upset about their pricing model, billing approach and business practices.

They could charge a trillion dollars for their software. I am not poo-pooing that. I am just upset with their practices in extracting those payments.

> hash partitions shouldn't have hot spots unless you hash on a bad column

In the nice world of all data having an even load, sure...

But in the real world, you can easily get a handful of users in your "Users" table sending millions of requests per second, and in that case, you would really like to re-partition so that they don't happen to all end up on the same partition.

Implementation can be as simple as allowing splittable partitions, so that whenever load/size gets too high, a partition can be split in half, and half the records moved to a new host. The partition map is only a few kilobytes, so is globally shared/updated. There are no concurrency issues, because during the splitting process, either the old or new partition is responsible for each record, and both the old and new partition hosts can respond to a read or write query, either themselves or by forwarding it to the other host.

Whenever two neighbouring partitions both see low load/space usage, join them with the same method in reverse. By joining only neighbouring partitions, you can't suffer terrible fragmentation and blowing up the size of the global partition map.

I'm not sure what you mean by "move to a new host" - all the built-in partitioning features are single-host for now, so this does not make much sense. There are plans to support remote (foreign) partitions, but that is in the future.

A more immediate use case is partition-aware algorithms. For example joining equi-partitioned tables can be done at the partition level, and the smaller the partitions the faster the join (likely). Or partition-aware aggregation, when the partition key is included in the GROUP BY keys. And so on. In that case it makes sense to split the largest partitions to make them smaller.

Partitions provide separation, not load balancing, and does thus not concern themselves with load or size.

While a can imagine a way to make hashed partitioning sub-divide buckets, it won't really be useful, as you can still have a single user blowing their bucket out of proportions with others being almost empty, which partitioning can never solve.

What you want is round robin distribution and parallel query execution, not partitioning. This of course only makes sense if the tables are on different disk systems/hosts or if all tables fit in memory.

No, that's not true. When combined with parallelism, ability to place partitions to other hosts and partition-aware algorithms (partition-wise joins/aggregation etc.) it can be a powerful scalability feature.

It is true. However, while partitioning is not load balancing, partitioning can be used to make some form of load distribution.

Partitioning primarily deals with isolating related data into small pools so that queries do not need to touch full tables. The purpose of the buckets is to group related data so that a single partition may serve a query (instead of having to touch the entire table), thereby lowering the load from each individual query, not distributing all queries.

Load balancing deals with distributing all queries to ensure even node load, not increasing efficiency of queries (which it does not do at all). Data distribution leading to round-robin accesses across nodes/"partitions" is best. This is the exact opposite of partitioning, which tries to group all that data into a single partition.

Hash partitioning on serial or uuid might give you round-robin like behavior, but it's not really what partitioning is meant for.

I'm not really sure what is your point. I was not suggesting partitioning alone is a load-balancing solution, but that partitioning in combination with other features is a powerful scalability feature. Assuming there is a (1) way to keep the partitions withing some size limit automatically, and (2) the partitioning scheme is such that the queries can leverage the partitioning scheme (eliminate most partitions, join tables partitioned in the same way).

We don't have (1) yet - at least not in core PostgreSQL, but this part should be doable in an extension until it gets into the core. We do have (2) to a large extent, although I'm sure there are gaps and room for improvement.

And when combined with ability to place partitions to other hots, that will be another step.

No one is claiming partitioning alone magically solves scalability, but IMHO it's an important building block in doing that (for a large class of applications).

My point is that partitioning is the polar opposite of load balancing, and the wrong tool to implement it.

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