PostgreSQL Parallel Query v2 | Hacker News Reader

PostgreSQL Parallel Query v2

rhaas.blogspot.com

265 pointsTostino42 comments

Besides parallel queries (specifically parallel index scans), the Postgres 10 feature I'm most looking forward to is the work Andres Freund has done on using LLVM to JIT compile expressions[0]. Based on the the layout of the database I work on[1], an insane amount of CPU goes to processing of partial index predicates. If the improvements are anywhere near what is being described in the mailing list, we're going to start saving a ton of money on CPU.

Parallel index scans will be an absolutely massive win for us. Currently Postgres does not perform any sort of prefetching. When performing an index scan, this results in serialized random I/O. In our case, queries wind up using ~1/10th the I/O they could be using if there were to do more I/O in parallel. We are expecting a dramatic speedup just from parallel index scans. This is in addition to the bugfix in 9.6 (we're currently on 9.5) which added proper support for index only scans over partial indexes. That bugfix should also result in a pretty large speedup for a lot of our queries.

As soon as Postgres 10 comes out, we are immediately going to upgrade. There are just so many huge wins for us.

[0] https://www.postgresql.org/message-id/20161206034955.bh33pae...

[1] https://blog.heapanalytics.com/running-10-million-postgresql...

Please share profiles on the list. Without known problematic production workloads, it's very hard for us to optimize appropriately. You mention predicate evaluation - it's possible this won't be affected by these changes (the index internals use lower level accesses than generic expression evaluation).

Re prefetching: PG does that for bitmap index scans, if you enable effective_io_concurrency > 1.

It should be interesting, but LLVM jit brings in a substantial up front cost. Maybe Andres found a way to mitigate it.

The plan is to just use interpretation of expressions for short-running queries. I've spent a lot of work in this release cycle making that faster, and allowing for an easier integration w/ less duplication between JIT and interpreted evaluation. There's two approaches for deciding when to JIT: a) based on query cost estimation. b) just do it after a threshold number of executions. I've not yet decided / experimented enough to make a call.

Additionally, at a later stage, caching of compiled programs will play a role, to reduce the frequency of llvm invocation.

Is it significant when you are talking about analytical queries which are running for minutes or hours? Seems like it wouldn't be a huge amount of code to JIT compared to the payoff?

Presto, which is designed for analytics, does JIT to bytecode on every query and still works fine on sub-second queries. One potential difference with LLVM is that the JVM will profile and only generate machine code for hot code, so short queries might use JVM interpreted bytecode.

But you are absolutely correct that for long running analytic queries, the up front cost of generating optimized code pays off.

Don't forget native logical replication.

Logical replication doesn't matter as much to us. We are using CitusDB (in other words sharded Postgres), and we have our own code for handling replication.

Citus should already be saturating i/o during a scan unless you're only hitting one shard. What am I missing about your use case?

In many cases all your data is actually in memory, especially when you can distribute your workload across many machines. If all your data is in memory, you'll very often hit CPU bottlenecks before memory bandwidth issues. Partially because bandwidth, disregarding latency, is just quite high, partially because expression evaluation in postgres isn't that well optimized (yet).

We shard our data by customer and individual queries touch only data for one customer. We do split a customer's data into multiple shards, but the number of shards we would need to split each customer into is not worth the overhead of maintaining that many shards.

Why? Is this a security issue? If you're sharing the same distributed table then sharding by customer is limiting the number of nodes/scans in parallel that can run.

Having millions of shards is a bit of a pain ops wise. There is also some other costs to having lots of shards. For example, the current Citus distribution planner takes time linear in the number of total shards. This isn't an issue 99% of the time, but since we use a composite type as our distribution key, we have an extremely high constant overhead for planning time. I believe the current median Citus Master planning time is ~350ms. We've talked to Citus about this and have discussed possible solutions. Currently we are trying to purge the composite type from our database.

Doesn't this inherently depend on the query? Shard-local i/o saturation affects the latency of the entire query if you're not expecting the i/o to the client to saturate.

This was exactly my point. But usually you're doing a full scan across all shards, so disk i/o is being saturated by virtue of the MPP sharding.

The relationship between EnterpriseDB and the at-large Postgres community has always intrigued me. Why do they turn over to the community what they could make part of their proprietary fork? Maybe they make a lot of money consulting and not just licenses and general at-large adoption is good for that side of business.

Part of it is that they build the basis for features, and work on getting them into core, so it makes their value added features in their proprietary fork easier to maintain. Sometimes giving back and having an easier time maintaining your own fork is worth the effort and costs associated I guess. I for one appreciate it.

I have never used EnterpriseDB and I am only aware of it from a distance but it has always struck me as the ideal open source arrangement. Sure, EnterpriseDB is never going to have the revenue numbers of Oracle but their management and business teams should have a much easier time sleeping at night. I think it is great that EnterpriseDB has identified that it is better for their company to exist as part of a healthy ecosystem instead of trying to pave paradise so to speak.

A healthy relationship with open source projects is a great way to attract talent and gain goodwill.

> but their management and business teams should have a much easier time sleeping at night

Why? What is Oracle doing so wrong that they cant sleep as well at night?

I think their secret sauce is Oracle compatibility, so if Postgres runs better, they run better.

And the Oracle compatibility piece breaks so much that it would never get merged into community Postgres.

Even if it didn't, for all the work they put into the community version one can't begrudge them trying to make a living on the enterprise side.

There are also plenty of devs in the community, outside the EnterpriseDB employees, who do do not want to pollute core PostgreSQL with alternative ways to do things it already can do just because of Oracle compatibility. Aliases have been added for ISO SQL compatibility, but adding features purely (i.e. features with zero value unless you are migrating from Oracle) for Oracle compatibility is not something core PostgreSQL does.

Well, Red Hat have demonstrated you can make a lot of money by open sourcing things (including buying proprietary software like ManageIQ and oVirt and open sourcing it). Given that EDB has ex-RHAT folks involved in setting it up, I imagine they're trying to replay the model.

I don't see how Red Hat has open sourced anything, the direction they are taking things is proprietary, not open.

RedHat Q4 FY2016 profit: $68M

Oracle Q4 FY2016 profit: $2B

You do realize you're comparing an amalgamation of software companies to a Linux distribution vendor, right? Oracle hasn't just been a database company for ... probably decades.

Apples : Citrus

will oracle be around in 10-20 years? will red hat? which will experience growth?

All things being equal it would take red hat ~30 quarters to make 2bn dollars, or roughly 7 years.

7 years to make what Oracle makes in a quarter.

I don't really have a dog in this fight but just FYI I think you missed ops point.

RedHat contributes to the common good, Oracle for a few millionaires. Therefore they create more value.

Does developing MySQL and Java somehow not contribute to the common good?

FWIW my feeling is that Oracle has made a net negative contribution to the Java ecosystem.

http://www.theregister.co.uk/2016/12/16/oracle_targets_java_...

I thought we didn't care about cash flows when valuing tech companies, just growth prospects ;)

This is a very common model now - several companies have open-source software (either their own or major contributor to) that is then covered with extra functionality, support and consulting services.

Some tradeoffs since the products are usually not as good and move slower than proprietary commercial offerings but the free access and large communities make up for it.

They do, they for example provide fork of Postgres that is compatible with Oracle which is not free.

Can't be for certain but one possible large factor is it reduces diffs from the upstream if you commit as much as you can back upstream. It's a major engineering drag to deal with that.

That is really an amazing amount of work that has gone into this next release.

This is really getting Postgres up to par with the commercial offerings, and hopefully it keeps progressing for v11.

It's especially important now since servers with more and more cores are becoming the norm.

Robert and his team are really doing an impressive amount of progress and work on this.

Definitely agreed.

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