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Why Uber Engineering Switched from Postgres to MySQL

• appleflaxen 10y ago
  This was a great overview and write-up. Anyone know why they are using MySQL over MariaDB[1]? 1. https://mariadb.org/ https://mariadb.org/
  • 0xmohit 10y ago
    It's hard to say, but one can often find people switching from Postgres to MySQL (and vice-versa). For example, see this [0] and the related HN discussion [1]. [0] http://insights.dice.com/2015/03/19/why-i-choose-postgresql-over-mysqlmariadb/ http://insights.dice.com/2015/03/19/why-i-choose-postgresql-... [1] https://news.ycombinator.com/item?id=9231751 https://news.ycombinator.com/item?id=9231751
  • mwpmaybe 10y ago
    I'm not saying this is the case at Uber, but it's quite common for MariaDB users to refer to it as MySQL, or to use the two names interchangeably. It's difficult to get out of the habit as all of the CLI tools are still mysql*, clients (i.e. the protocol) are all named "mysql", CM modules (like Ansible's) are all named "mysql", etc. And sometimes it's just simpler (if lazier) to say you're using MySQL rather than explain what MariaDB is and why it's preferable.
    • zokier 10y ago
      I still occasionally call LibreOffice OpenOffice. Besides mysql has less syllables than mariadb.
  • Illniyar 10y ago
    Well there new database is basically a key value store built on MySQL, so I think there isn't much to be gained by using MariaDB and it will add needless additional complexity.
• swasheck 10y ago
  this reads like a laundry list of buzzwords that were designed to justify not throwing any effort into postgresql and just going with a new shiny toy (not mysql. yes. i know it's been around for a while). it happens everywhere.
  • edgyswingset 10y ago
    From the post: > [...] This design difference means that the MySQL replication binary log is significantly more compact than the PostgreSQL WAL stream. Doesn't sound like what you described at all.
    • swasheck 10y ago
      so you're pulling one line from the article to tell me that i'm wrong? on-disk format/write amplification: > For tables with a large number of secondary indexes, these superfluous steps can cause enormous inefficiencies. For instance, if we have a table with a dozen indexes defined on it, an update to a field that is only covered by a single index must be propagated into all 12 indexes to reflect the ctid for the new row. How wide is their data? Depending on the answer to this, it could be that they've over-indexed, have a poor indexing strategy, or are reacting to the poor queries generated by an ORM (not sure if they use one, or if they hand-code their own SQL). data corruption: everyone has bugs. https://bugs.mysql.com/search.php?search_for=&status=Active&severity=1&limit=10&order_by=&cmd=display&direction=ASC&os=0&phpver=&bug_age=0 https://bugs.mysql.com/search.php?search_for=&status=Active&... add in mysql's tendency to loosely-adhere to the SQL standard and there are many ways that you can actually corrupt your own data. i'm not here to debate whether postgres is better than mysql. i'm just saying that it seems like a lot of research went into justifying a switch. who knows, maybe that research could have been spent optimizing their current environment.
      • coldtea 10y ago
        >so you're pulling one line from the article to tell me that i'm wrong? Better than telling them that they are wrong while not only not pulling even one line from their article, but misattributing it to be something very different from what it is. In what world does the response to the concerns and analysis in the article can ever be: "data corruption: everyone has bugs"...
• BadassFractal 10y ago
  I've heard from technical leaders at multiple now well established unicorns how they'd never use postgres or switched from postgres simply because MySQL has a lot more tooling built it and many more people are exposed to its shortcomings at "web scale" so that it's very well known where and when things will break. Disclaimer, I'm a hardcore Postgres user myself, but I also keep tabs on the other tools.
  • davidw 10y ago
    OTOH, it's important to consider that most places are not and will never be "unicorns", so analyzing things at that scale may not make sense. Just because Facebook managed to make something work doesn't mean that you should follow the same path.
  • segmondy 10y ago
    I've been a hardcore and exclusive Postgres user since 2004. Always rolled my eyes when folks say they use MySQL or moved to it from Postgres. Reading about that bug sent shiver's down my spine. I know no system is bug free, but Postgres is one of those systems that have held up very well for me, and now I'm super paranoid. :-( I guess I understand tho, I moved from BSD to Linux for more tooling.
• 0xmohit 10y ago
  Facebook maintains it's own fork [0] of MySQL. A couple of interesting talks are also available: MySQL at Facebook, Current and Future [1] and Massively Distributed Backup at Facebook Scale [2]. [0] https://github.com/facebook/mysql-5.6 https://github.com/facebook/mysql-5.6 [1] https://www.youtube.com/watch?v=jqwegP9xwVE https://www.youtube.com/watch?v=jqwegP9xwVE [2] https://www.youtube.com/watch?v=UBHcmP2TSvk https://www.youtube.com/watch?v=UBHcmP2TSvk
  • raverbashing 10y ago
    "its own fork" And yes, MySQL has a lot of problems, but it seems to be better for "seat of the pants" usage (that is, when you don't need too many SQL capabilities and wants to use it mostly as a data store)
  • lazyant 10y ago
    from what I understand FB uses Mysql as permanent storage, not as relational database
    • yeukhon 10y ago
      I believe they use it for almost everything. They built all the graphs on top of MySQL data. I don't know what permanent storage is, but if you are referring to storing images and videos, I doubt. Highly doubt that. They may be storing pointers, but as a file system, I doubt. But I wouldn't know for sure, I don't work there. Oh, they do have Cassandra (well they built Cassandra)...
      • jhartmann 10y ago
        They use a system called haystack for binary large object storage. They very much don't use mysql for those.
        • yeukhon 10y ago
          That's my feeling they wouldn't use MySQL as a blob store.
• NhanH 10y ago
  Well, this is heresy. Does that mean we are now officially boycotting Uber? Joke asides, one thing I've been trying to figure out for awhile is the limitation at which certain components/ systems broke down. Basically, something along the line of "given X records, this operations would take Y time, or would cause Z A B C problems". I've actually got developers friends asking me how fast a simple "SELECT * FROM X WHERE index=?" would take on a million row table, since they were surprised that some NoSQL DB could do a query on hundred million rows in a few seconds. I guess that's part of why you only learned how to scale after having done it once.
  • Illniyar 10y ago
    "surprised that some NoSQL DB could do a query on hundred million rows in a few seconds" The tone seems to suggest this is fast, a simple index query in an RDBMS of even a hundred million rows will take milliseconds on even a weak computer.
    • xaprb 10y ago
      Justify that with actual math, please? I don't think you know how computers (processors, bus, memory access, etc) work. How exactly do you think an RDBMS can query an index at a hundred billion rows per second?
      • gkop 10y ago
        Illniyar is right, a b-tree with a branching factor of 100 can index 100M rows in 4 levels. Even with the index on a spinning disk, should have no trouble coming in well under 100ms.
        • ad-hominem 10y ago
          And that right there's the math
        • falcolas 10y ago
          Indexes like this are also typically in memory to begin with, making it even faster, since you only have to find the actual record on disk. Even better, if you only need one field from the record, and it's part of a compound index, you can frequently return the data from just the indexes; no disk seeks required. Small tip with InnoDB on MySQL - any non-primary key index is automatically a compound index with the primary key.
• deleted 10y ago
  [deleted]
• thinkingkong 10y ago
  Posts like this are important. We too often rely on a buzz-word heuristic and that's how you end up with dozens of random technologies that are harder to maintain and don't necessarily solve any of your problems. This method is good, because it shows that when you understand the problem the right way, you can find the right solution, even if by popularity it looks like a "step backwards" Massive Kudos.
  • millamox 10y ago
    I strongly disagree. it would have been useful f they'd stuck to problems without well-known solutions. Sadly, they also mixed in issues which are easily solved, or in a particularly egregious case, where they just complain about a bug. As though MySQL never had a bug. That was silly. My read of it was: Postgres annoyed us a few times, and we got fed up with its, so now something different will annoy us. Please look forward to our blog post in 4 years about how we're using X instead of MySQL/Schemaless because those were also imperfect.
    • stephengillie 10y ago
      Almost all problems are already solved. "Solving" a problem today is mostly ego-stroking.
      • GuiA 10y ago
        Sweet, I have a few PhD thesis and NSF grant proposals I'd like your help on.
        • stephengillie 10y ago
          I'd love to help stroke your ego, but I see you're busy doing it yourself.
    • thinkingkong 10y ago
      The issue is all solutions are imperfect. You just make tradeoffs and shuffle the problems around. Doing so deliberately is whats commendable.
• zeeshanm 10y ago
  I like the sample data they have used: id first last birth_year 1 Blaise Pascal 1623 2 Gottfried Leibniz 1646 3 Emmy Noether 1882 4 Muhammad al-Khwārizmī 780 5 Alan Turing 1912 6 Srinivasa Ramanujan 1887 7 Ada Lovelace 1815 8 Henri Poincaré 1854
  • antonius 10y ago
    Mind explaining the significance? I didn't pick up on it.
    • digitalinfinity 10y ago
      They're all famous mathematicians
    • houli 10y ago
      They're all famous scientists/mathematicians.
    • praccu 10y ago
      old mathematicians
      • klancaster 10y ago
        No, dead mathematicians
    • muddyrivers 10y ago
      It caught my eyes as well. Like it very much.
      • deleted 10y ago
        [deleted]
    • muddyrivers 10y ago
• frik 10y ago
  Also check out highscalability.com for more stories that value MySQL and its great InnoDB engine: http://highscalability.com/blog/category/mysql http://highscalability.com/blog/category/mysql
• slantedview 10y ago
  The connection handling section was surprising to me, reading that Postgres uses a process per connection! This is pretty shocking to me, in a bad way.
  • iampims 10y ago
    pgbouncer works pretty well for that. I kinda wish it was part of the default Postgres install.
    • scrollaway 10y ago
      How does pgbouncer work with Amazon RDS (where you can't install separate software)?
      • rpedela 10y ago
        pgbouncer does not need to run on the same machine as the DB. It can run on an EC2 instance in front of your RDS instance.
      • dijit 10y ago
        you install it on the clients of the servers themselves and "connect locally" to the bouncer or have a bouncer step (server pair) in front of the database connections themselves.
  • 0xmohit 10y ago
    One could use connection pooling. Quoting from the tuning guide [0]: max_connections sets exactly that: the maximum number of client connections allowed. This is very important to some of the below parameters (particularly work_mem) because there are some memory resources that are or can be allocated on a per-client basis, so the maximum number of clients suggests the maximum possible memory use. Generally, PostgreSQL on good hardware can support a few hundred connections. If you want to have thousands instead, you should consider using connection pooling software to reduce the connection overhead. Replication, Clustering, and Connection Pooling [1] [0] https://wiki.postgresql.org/wiki/Tuning_Your_PostgreSQL_Server https://wiki.postgresql.org/wiki/Tuning_Your_PostgreSQL_Serv... [1] https://wiki.postgresql.org/wiki/Replication,_Clustering,_and_Connection_Pooling https://wiki.postgresql.org/wiki/Replication,_Clustering,_an...
• trequartista 10y ago
  Wow, this is such a detailed analysis. Having used Postgres and suffered issues with data replication as well as database crashes, this post was really helpful.
• raisyer 10y ago
  while Postgress might be better if you use it 'as-is'.... community of MySQL is much better and the tools available are more mature... just goes on to prove that even if something is not-that-good.. it still might be successful,scalable and popular if there is a strong community behind it..
  • madhusudhan000 10y ago
    >community of MySQL is much better and the tools available are more mature Can you backup your statements with some facts. I have seen the postgres community to be much better particularly now with Oracle taking ownership.
• jasode 10y ago
  Fyi... a related (not duplicate) discussion of a previous Uber story: https://news.ycombinator.com/item?id=10923848 https://news.ycombinator.com/item?id=10923848
• polskibus 10y ago
  Does anyone know if citusdb or enterprisedb improve on the postgresql issues mentioned in the post vs last postgresql version?
  • brandur 10y ago
    CitusDB is essentially vanilla Postgres now that they've moved their implementation to an extension. It won't specifically address the problems posted here, but will solve other ones like trying to scale a system beyond a single node.
• pritambaral 10y ago
  I wonder what the design decisions are behind (or what it would take) to make Postgres store secondary indexes on disk like InnoDB does. Sure, the extra index lookup through the primary index is a cost, but it seems like write-amplification can sure be a greater concern too. Ultimately, it would be nice if Postgres gave the DBA a choice of — if not move outright to — secondary-index indirection through the primary index like InnoDB does.
  • saurik 10y ago
    I would definitely not want PostgreSQL to "move to" the mechanism used by InnoDB as that mechanism is slower for reads (which this article even admits). This disk layout is one of the reasons I continue to use PostgreSQL. If you care more about write performance than reads you might even want to look into something entirely different than a traditional database. FWIW, PostgreSQL's mitigation to the write amplification problem mentioned here are "heap only tuples". It is highly likely that Uber could have prevented a lot of this pain by learning more about this feature--which is notably not mentioned even once in this entire article, which to me completely undermines the feel they are trying to achieve of "we really really really know what we are doing"--and tuning their table density parameters to take maximal advantage. Instead of "why we moved off of X" it would be much better to see "we are considering moving off of X: anyone know what we are doing wrong?". Sure, maybe Uber knows about HOT, and did extensive analysis to determine it wasn't a solution to their problem; but it frankly does not seem at all to be the case. Anyone who knows a lot about PostgreSQL would have explained HOT to them, so they probably didn't even consult with PostgreSQL experts behind the scenes. Sadly, "we are looking for help with a complex technical challenge" is something the market punishes under the premise that everyone has to be entirely self-sufficient gods of their technology stack :(. The only time I remember ever having seen a company reach out to the community for help was reddit (with respect to something involving PostgreSQL or Cassandra... I don't remembee the specific issue).
    • pritambaral 10y ago
      Thank you for introducing me to "heap only tuples", I did not know about it. From what I read about it from Postgres's documentation[0], it would have helped in the updates where no indexed column was updated. 0: https://wiki.postgresql.org/wiki/Index-only_scans#Interaction_with_HOT https://wiki.postgresql.org/wiki/Index-only_scans#Interactio...
• hyperion2010 10y ago
  This is a fantastic read. I hope the pg folks can turn as many of the issues brought up here into bug reports as possible (I think many of the issues, especially re: replication, are known), this kind of feedback is invaluable.
  • davidw 10y ago
    Some of these may be tradeoffs rather than bugs. I'd love to read a reply from someone knowledgeable about Postgres internals - it'd be very interesting.
    • hyperion2010 10y ago
      Yep! saurik's reply above sheds some light on the issue (read more important than write). Also, to the downvoter, sometimes the solution to a bug report is improved and more visible documentation of existing features.
• 0xmohit 10y ago
  Worth quoting from the article: Accordingly, using pgbouncer to do connection pooling with Postgres has been generally successful for us. However, we have had occasional application bugs in our backend services that caused them to open more active connections (usually “idle in transaction” connections) than the services ought to be using, and these bugs have caused extended downtimes for us.
  • fdr 10y ago
    What I don't understand: no system gets to maintain open transactions for free. MySQL keeps UNDO logs, so the effect there, much like Oracle, is possibly running out of UNDO space, as well as slowing down reads that have to apply UNDO to get the last-committed row version. So what gives? Did Uber Engineering fix the dangling transaction issues while migrating off, or are they relying on some other property of MySQL vs. Postgres?
    • 0xmohit 10y ago
      They seem to be relying on the database take care of those :)
• sam_pointer 10y ago
  We did something very similar at EA Playfish, at least one alumni of which is part of the Uber engineering team. We used a 2 column InnoDB-backed table for all of our data storage, massively sharded, and run in a 3-host master-slave-slave configuration. At that time EC2 would routinely kill hosts without the courtesy of a poke via ACPI and as such we became very good at quickly recovering shards. In a nutshell this mechanism was to have the new host contact a backup slave, perform an lvm snap, pipe the compressed snap over a TCP connection, unroll it and carry on, letting replication take up the delta. That enabled us to not only manage the 10 million or so daily active users of that title, but was also the platform under the 12 or so additional titles that studio had. We had lots and lots of very simple things and failures were contained. I think at the time we were the 3rd-largest consumer of EC2 after Netflix and "another" outfit I never learned the name of. EA being what it was, however, we were never permitted to open source a lot of the cool stuff Netflix and ourselves seemed to develop in parallel.
  • sandGorgon 10y ago
    this is frikking awesome! do you have any of the lvm and pipe scripts publicly available ? i'm kinda struggling with building and setting up lvm on ec2 automatically and was wondering if there is any tidbits you can pass along.
    • sam_pointer 10y ago
      Unfortunately not. That company and codebase is very much dead, and I don't own any of it. Even if I did it would be a bunch of context-less shell (in the first iteration) and then a bunch of context-less Chef (in the second platform iteration). Things I do remember: - We used ephemeral volumes for all data stores. This was pre-provisioned IOPs and EBS was flaky as heck back then. I can't remember the disk layout, although we did experiment a great deal. - We took great pains to ensure there was enough space to make the snapshot (IIRC is was a telemetry/monitoring item) - The pipe scripts were essentially "netcat". The best I can offer is this talk: http://vimeo.com/57861199 http://vimeo.com/57861199
• vbezhenar 10y ago
  Why would anyone run hundreds of connections? A server can only process number_of_processor_cores connections at once. Sure, few connections might wait for I/O, but not hundreds, unless database is very untypical.
  • brianwawok 10y ago
    For example: You are using Python and you have 10 web servers and 20 background servers connected to a common DB. Each server has 10 threads, and each thread holds 1 connection open. That is 300 open connections. Opening and closing connections is very slow and expensive, so almost always better to keep these 300 connections open than to try to be fancy. You COULD try to say give each server only 3 connections and make them share, which cuts you to 90 connections.. but then you have to try to share state between different python processes (not easy), and will often end up with deadlocks and sync overhead.
    • tracker1 10y ago
      Agreed, which is why many clients create a pool of connections that gets reused. Connection cost is expensive, and the rdbms already handles concurrency and even a couple thousand connections shouldn't be a significant overhead.
      • brianwawok 10y ago
        I'm trying to find evidence on what memory usage is for MySQL for 1k connections vs postgres with 1k connections. I am finding a lot of people saying postgres has heavier connections but for 1k connections what's the difference. 1MB of memory? 1GB?
        • kasey_junk 10y ago
          https://wiki.postgresql.org/wiki/Number_Of_Database_Connections https://wiki.postgresql.org/wiki/Number_Of_Database_Connecti... Its a little outdated but I've found it largely holds. That is, you'd want a mighty box for 1000 concurrent connections. That said, connection queuing works really well with postgres such that throughput is frequently better at lower connection counts than at higher ones. I have no experience with mysql that is less than 15 years out of date.
• ledjon 10y ago
  I would argue that most of these Postgres "flaws" are actually advantages over MySQL when you look at them holistically rather than the very specific Uber use-case. Postgres's MVCC is superior (can rollback DDL, can add indexes online, can have open read transactions for a VERY long time without impacting other parts of the system) Postgres supports many types of indexes, not just b-tree. One thing it doesn't have is clustered b-tree indexes... which is really what MySQL does that makes it somewhat "better." I wonder how Uber adds an index to a table that already has 1B+ rows in it with mysql? Postgres have WAL level replication is a better guarantee of actually replicating the data correctly. I cannot tell you how many times I've had to tell my boss that the "mysql replicas might be slightly out of sync with the master" because of various replication issues. The way it handles triggers and scheduled events alone is garbage and can very easily break replication and/or silently cause inconsistency. As for data corruption, if there is a bug that causes corruption, then there is a bug. I don't think that is a fundamental design flaw as implied in this article. You shouldn't rely on 1/2 assed replication design to accidentally save you from the data corruption bug. There are many downsides to the design MySQL has that are simply not listed here. I have been both a professional MySQL administrator as well as Postgresql (as well as SQL Server and many NoSQL engines). Many of these Postgres issues are only issues at crazy huge scale, and I would say at that point you probably want to move away from relational anyway. MySQL has its own very large set of problems at scale as well. It sounds like Uber is using MySQL as just a data bucket with primary keys ("Schemaless") which is good -- because you can't alter tables to save your life with MySQL. At the end of the data each developer/business needs to use what works for them, but I would really shy away from pointing to this article as a linchpin in the "MySQL vs. Postgres" war (if there even is such a thing.)
  • fjordan 10y ago
    Tools such as the percona toolkit (https://www.percona.com/software/mysql-tools/percona-toolkit https://www.percona.com/software/mysql-tools/percona-toolkit) provide the ability to perform safe and performant online alters for MySQL. Behind the scenes it actually creates a new table with the new schema and utilizes triggers to apply writes to the new table as the original data is being copied. Once it completes the table is renamed and the triggers are removed. Percona also recommends using this tool to safely perform alters for any Galera-based replication product (Percona XtraDB Cluster, MariaDB Galera Cluster, etc.)
• brandur 10y ago
  Interesting post! While I suspect that a MySQL installation is just as likely to have its own problems in the long run, I'm not smart enough to provide any kind of compelling point-by-point refutation. However, a number of the points made strike me as having possible trade-offs that were not really addressed in-depth. My summary of the arguments against Postgres and some basic thoughts on each: 1. Writes are more expensive because all secondary indexes must be updated with a new physical location. This may be true, but the MySQL model of using primary keys from secondary indexes will mean that reads are inherently expensive. They even mention this: > This design means that InnoDB is at a slight disadvantage to Postgres when doing a secondary key lookup, since two indexes must be searched with InnoDB compared to just one for Postgres. So it seems like a classic read vs. write trade-off. I'm also a little skeptical of any performance claims that don't include any numbers. It's possible that efficient coding in Postgres makes this much more of a wash in terms of performance than claimed here. 2. Replication is less efficient because it's sending a lot of physical information out along the stream. This is quite true, but IMO unlikely to be a major issues for most users unless they're dealing with a huge amount of data and streaming it over a slow connection (i.e. across the continent like Uber's disaster recovery center). 3. Data corruption from a bug found in 9.2. Certainly a bad situation, but IMO not really a valid claim for situation. 9.2 is way behind at this point, and there's not much to say that they wouldn't have encountered a similar bug or something worse in MySQL in all that time, especially operating at scale. To give a counter-anecdote, I operated Postgres at scale for a long time across many versions starting at 9.1 and was lucky enough to have never once encountered a bug with data corruption. 4. Postgres' MVCC model makes it easy for replicas to accidentally fall behind their master. This one is valid (and annoying), but there are very good reasons for it, and you have some switches to control the behavior based on value transactions finishing on followers or prompt replication more highly. 5. Upgrades are difficult because the WAL stream works at a physical level and is not compatible between database versions. Again, this is valid, but the statement-based replication is a scary idea. Row-level replication is more interesting and probably something that Postgres should have though. Some good news is that Postgres is getting closer to logical WAL streaming, which should make in-place upgrades possible.
  • codedokode 10y ago
• pella 10y ago
  Roadmaps ( PostgreSQL ) 2016-2017-... * Postgres Professional roadmap ( Pluggable storages, Multimaster cluster with sharding, Effective partitioning, Adaptive query planning, Page-level data compression, Connection pooling, Native querying for jsonb with indexing support, ....) https://wiki.postgresql.org/wiki/Postgres_Professional_roadmap https://wiki.postgresql.org/wiki/Postgres_Professional_roadm... * EnterpriseDB database server roadmap ( Parallelism, Replication, Vertical Scalability, Performance ) https://wiki.postgresql.org/wiki/EnterpriseDB_database_server_roadmap https://wiki.postgresql.org/wiki/EnterpriseDB_database_serve... ==== And "Scalable PostgreSQL for real-time workloads https://www.citusdata.com https://www.citusdata.com " --> https://github.com/citusdata/citus https://github.com/citusdata/citus
  • leblancfg 10y ago
    My big question here is why they decided to move over to MySQL instead of using the Citus (also open source) Postgres extension. They don't mention it, so we don't know whether it was considered, and if so, why it was not selected. Postgres' rapid feature growth in the past years is interesting in itself.
    • pella 10y ago
      Citus is Open Sourced since "Mar 2016", so it is not so old. https://github.com/citusdata/citus/blob/release-5.1/CHANGELOG.md https://github.com/citusdata/citus/blob/release-5.1/CHANGELO... "citus v5.0.0 (March 24, 2016) - Public release under AGPLv3 - PostgreSQL extension compatible with PostgreSQL 9.5 and 9.4"
    • bonesmoses 10y ago
      Citus currently has major problems joining non-distributed tables with distributed tables, and it doesn't fully support schemas yet. It's almost there, but not quite.
      • leblancfg 10y ago
        Thank you!
• xaprb 10y ago
  Why not PostgreSQL? (Sorry, someone had to say it.)
• forgotpwtomain 10y ago
  Great write up. A couple points - I'm not sure this post is illustrative of any generally applicable considerations (re: the title) in the choice of Postgresql vs MySQL, since Uber seems to no longer be using a relational model for most of their data and is using MySQL effectively as a key-value store. > say a developer has some code that has to email a receipt to a user. Depending on how it’s written, the code may implicitly have a database transaction that’s held open until after the email finishes sending. While it’s always bad form to let your code hold open database transactions while performing unrelated blocking I/O, the reality is that most engineers are not database experts and may not always understand this problem, especially when using an ORM that obscures low-level details like open transactions. I have to very seriously disagree here, ORMs make a lot of things easy - and you can get away with building stuff for a while without understanding the underlying databases or SQL but only to a certain scale (I'd say more like medium-scale, definitely not large or Uber level). If you have engineers writing code that interacts with a database without understanding transactional semantics, the engineer in question not the database is the problem. > We started out with Postgres 9.1 and successfully completed the upgrade process to move to Postgres 9.2. However, the process took so many hours that we couldn’t afford to do the process again. There seem to be ways [0][1] to do online upgrades with Postgres (before logical decoding in 9.4), although I haven't personally used them. Not sure if they explored these options at Uber or not? [0] https://github.com/markokr/skytools https://github.com/markokr/skytools [1] http://slony.info/ http://slony.info/
  • spotman 10y ago
    > I have to very seriously disagree here, ORMs In spirit I agree with you, its the engineer's fault for not reading the documentation of their ORM or equivalent. But in these big ships with hundreds of programmers, leaving transactions open in hibernate is a daily occurrence somewhere. Usually caught before production, but happens oh-so-frequently, that anything that exacerbates the pain from this would be seen in a negative light for sure.
• cdelsolar 10y ago
  So basically, if you don't intend to use it as a relational database, and you have enough scale to run cross-data-center (and across-the-world) master-master replication, then you should maybe switch from PostgreSQL to MySQL?
• fusiongyro 10y ago
  The article could be summed up as "Postgres is not a distributed database." MySQL isn't either, although it certainly has more friendly replication technology. I think it's a lot more likely that what's really happening here is that they've designed their "schemaless" schema or its supporting software to handle the kind of soft errors that MySQL is permitting and Postgres was not. We have MySQL replication across the country where I work and I certainly wouldn't characterize it as robust; it fails every 3-6 months. MySQL replication is certainly a lot older and easier to use than Postgres's, but SQL databases are fundamentally CP systems. When you say "This design means that replicas can routinely lag seconds behind master, and therefore it is easy to write code that results in killed transactions" it sounds like you're blaming the way replication was implemented for a physical problem. There is no way to design a replication system such that two highly-consistent databases can achieve perfect availability in the face of real-world networks. A worse protocol can exacerbate the problem, but a better one can't make it go away. I have never seen corruption with Postgres (unlike MySQL), but I have never tried cross-datacenter replication with it. Apart from that, Postgres generally seems to do much better with consistency than MySQL does, where DDL statements are not transactional, etc. So I am not surprised to hear that their system trips harder on Postgres's more aggressive consistency. In short, I suspect a more robust solution to their problem is a NoSQL database. On the other hand, it sounds like they want a combination of availability and consistency that will be difficult to get off-the-shelf. I'm glad they found a way to make it work. I wouldn't generally choose Postgres for a scalable system with an aggressive availability constraint--but then again, I wouldn't choose MySQL either, and I generally avoid problems that demand highly scalable, highly available solutions.
• Illniyar 10y ago
  So the major issue detailed here is that postgres basically uses immutables rows which creates performance issues with writes. Just read about their new schemaless db in their blog an the first paragraph contains this: "The basic entity of data is called a cell. It is immutable, and once written, it cannot be overwritten. (In special cases, we can delete old records.) A cell is referenced by a row key, column name, and ref key. A cell’s contents are updated by writing a new version with a higher ref key but same row key and column name." So, mmm..., not saying that postgres didn't pose a problem for them but I think postgres' db model fits better to their new db then mysql. They probably had to work really hard to get mysql to work like postgres. Without this issue, it looks like two things needed.to be done with postgres that would have solved their problems have indexes that point to primary id and do logical replication (which they say a plugin solved in 9.4). Is this a case of "I got burned by something so I won't use it again"
  • Illniyar 10y ago
    So more from here: https://eng.uber.com/schemaless-part-two/ https://eng.uber.com/schemaless-part-two/ "Each Schemaless shard is a separate MySQL database, and each MySQL database server contains a set of MySQL databases. Each database contains a MySQL table for the cells (called the entity table) and a MySQL table for each secondary index, along with a set of auxiliary tables." So... 1 table, with 1 index and manually created and updated secondary index tables. With this scheme I can only assume postgres will work just as well or better.
• postila 10y ago
  Well done :-) https://twitter.com/LeviNotik/status/757991465649778689 https://twitter.com/LeviNotik/status/757991465649778689
• mspradley 10y ago
  Why did they not consider Oracle or MS SQL Server? They can afford the licensing and both have numerous replication technologies to choose from.
  • je42 10y ago
    Oracle ??? Let's start: - No transactions for DDL changes. - Oldschool commandline client. auto commit disabled by default. no history. - Weird sql syntax + semantics. f.e. null == empty string.
    • mschuster91 10y ago
      I'd add - Oracle InstantClient SDK needs an Oracle account to download and is closed source, and is not available as a package for anything except rpm-based distros - getting it to run with PHP is a major PITA, once again due to the above-mentioned issues - Holy f..ing cow, why does it translate everything down to ASCII by default instead of returning raw bytes? - It's expensive as f..k - Try to run a query using InstantClient with a ; at the end, it will barf - DID I MENTION IT CANNOT DO A SIMPLE LIMIT AFTER AN ORDER BY?! (at least not until 12.1, which was released in 2013; due to various issues, one including the pricing, I have seen multiple orgs running way older versions. But come on, over 30 years with only ROWNUM?!)
    • quicksilver03 10y ago
      I'm with you on all of your points except auto-commit: having it off by default is much better, it forces you to explicitly commit when you change data and thus think if you really want to persist the changes.
      • je42 10y ago
        yes. except that all other clients have auto commit on. thus if you transition from that to auto-commit off. You face two issues: 1. you don't know. and you thing you commited something but actually you didn't 2. you forgot and you committed something. but transaction keeps an row lock open and you bring down the server... :)
• distantsounds 10y ago
  Perhaps their engineers can design a web page that allows the scroll wheel to work.
• snarfy 10y ago
  I wonder how much of this could have been solved by using a different file system. There is all of this talk about the physical layer but no mention of the file system used. > Typically, write amplification refers to a problem with writing data to SSD disks: a small logical update (say, writing a few bytes) becomes a much larger, costlier update when translated to the physical layer. This is exactly the type of problem solved by the file system layer.
  • pritambaral 10y ago
    How would any filesystem help with that? SSDs typically write entire blocks, even if the OS asks them to only write a few bytes. That's just how SSDs work.
    • Terretta 10y ago
      Rethink the file system. Think log-structured.
      • pritambaral 10y ago
        Doesn't matter. Even a log-structured fs, when told to write 4 bytes of new data to disk will have to write those 4 bytes immediately and return. If an application asks the OS (via fsync), and the OS asks the fs and the fs doesn't write to disk but tells the OS it did, then the fs just lied and risked data loss. If power is lost between the fs lying to the OS and its subsequently actually writing to disk, the data that the fs lied about is lost. You don't want that with a DB on top of it.
• jedberg 10y ago
  > MySQL supports multiple different replication modes: > Statement-based replication replicates logical SQL statements (e.g., it would literally replicate literal statements such as: UPDATE users SET birth_year=770 WHERE id = 4) Postgres has that too (using a 3rd party tool, but it's an officially supported tool). We were using it on reddit 10 years ago. It caused a lot of problems. I wouldn't call that an advantage for Mysql. Honestly, reading this it seems like the summary is: "We don't follow great engineering practices so we need a database more forgiving". Which is fine if that's how you want to run your business, but isn't really the death knell for Postgres. A specific example: > This problem might not be apparent to application developers writing code that obscures where transactions start and end. For instance, say a developer has some code that has to email a receipt to a user. Depending on how it’s written, the code may implicitly have a database transaction that’s held open until after the email finishes sending. While it’s always bad form to let your code hold open database transactions while performing unrelated blocking I/O, the reality is that most engineers are not database experts and may not always understand this problem, especially when using an ORM that obscures low-level details like open transactions. Your developer should understand database transactions. But you should make it easier for them by abstracting it so that they don't have to. And in this particular case, I'd say they shouldn't be using the database to do locking around sending a receipt. It should be put into a queue and that queue should be processed separately, which avoids the transaction problem altogether.
  • spotman 10y ago
    Try enforcing this on teams that use ORMs like hibernate with 500 developers. Super, duper, common issue, you will find this at every large shop at some point in its life time, usually around the time of hiring people and expanding extremely fast, and taking on some tech debt. All functions of extreme scale, hyper growth, and yeah, not following the absolute best practices all the time, but tech debt is like any debt, you get something now, and pay later. If they continue going up and to the right they will be able to afford it.
    • forgotpwtomain 10y ago
      Yes, but there is no point in blaming the database for that. It's seriously not a database issue. Particularly Postgres has well exposed internals for monitoring and killing of long running transactions (if you are not setting statement timeout).
• manigandham 10y ago
  They should use SQL Server (which has great replication abilities, although horizontal scale out is still difficult) or MemSQL (which is distributed, scalable, and can do everything they need). Or use Cassandra which is a perfect fit (or ScyllaDB which is a better version of it). This all sounds like an aversion to just paying for or using better products when the problem is easily solved.
  • darklajid 10y ago
    SQL Server needs a couple hits with a cluebat to even satisfy the basic three requirements for a database system they listed (I'm talking about the MVCC line of course). Honestly, if a commercial database provides what you require and you have the budget? Sure, why not. But I'd always try to avoid that myself, because the serious players (MS, Oracle, is anyone still using DB2?) are really, really expensive.
    • manigandham 10y ago
      > basic three requirements SQL Server works just fine and has lots of concurrency control to do whatever they need. And the way they're using the database doesn't seem to really make this an issue outside of their replication. > are really, really expensive This whole janky setup they have sounds even worse. None of the commercial relational databases are really that expensive considering what they offer, and we're talking about Uber here. We're a small startup that pays for both. Money for (a better) working product with support is the right call, not build it yourself. This is just poor tech decision (outside of using a RDBMS in the first place).
      • brianwawok 10y ago
        Maybe you should call up Google, Facebook,linked in, Twitter, and tell them they all made mistakes and should use MSSQL.
        • manigandham 10y ago
          Those are all massive scale tech companies that actually needed to invent many of the datastore technologies used for big data today. Uber is not among them. But I'm sure you know all this as it seems you work for an enterprise database company that actually makes the exact product Uber should use.
• ismdubey 10y ago
  The fact that Uber scaled to so many users with Postgress gives me such a relief. For now, I am good !!
• drob 10y ago
  We've hit a lot of the same fundamental limits scaling PostgreSQL at Heap. Ultimately, I think a lot of the cases cited here in which PostgreSQL is "slower" are actually cases in which it does the Right Thing to protect your data and MySQL takes a shortcut. Our solution has been to build a distribution layer that makes our product performant at scale, rather than sacrificing data quality. We use CitusDB for the reads and an in-house system for the writes and distributed systems operations. We have never had a problem with data corruption in PostgreSQL, aside from one or two cases early on in which we made operational mistakes. With proper tuning and some amount of durability-via-replication, we've been able to get great results, and that's supporting ad hoc analytical reads. (For example, you can blunt a lot of the WAL headaches listed here with asynchronous commit.)
• deleted 10y ago
  [deleted]
• markpapadakis 10y ago
  Great write-up. A few observations: 1. The encoding and translation schemes of Postgres and mySQL/InnoDB are well described in the blog post, and I would also agree that InnoDB’s design is, all things considered, better for all the reasons outlined in the post. 2. I don’t understand why anyone still uses lseek() followed by read()/write() and not pread()/pwrite() syscalls. It’s trivial to replace the pair of calls with one. Aerospike is another datastore that resorts to pairs of seek/red-write instead of pread/pwrite calls. 3. Process/connection model makes no real sense nowadays - although to be fair, there is, today, practically almost no difference in terms of footprint between OS threads and OS processes (other than memory and FDs sharing semantics, they are practically the same). It’s still more appropriate to use threads (although I ‘d argue maintaining a pool of threads for processing requests and one/few threads for multiplexing network I/O is the better choice). 4. ALTER TABLE is obviously a pain point with mySQL, although I am not really sure many users with large datasets care; they probably figured out long ago it’s going to be an issue and they designed and expanded accordingly. It’s also a relatively rare operation. That said, other than using mySQL (or any other RDBMS) to build the data plane for an elaborate, distributed KV store, one should consider Salesforce’s approach too. Their tables have some 50 or so columns, and the column names are generic (e.g column_0, column_1, … ). They have a registry where they assign column indices (e.g column_0) to a specific high-level entity type (e.g customer title, or price), and whenever they need to query, they just translate from the high level entity to the actual column names and it works. They also, IIRC, use other tables to index those columns (e.g such an index table can have just 3 columns, table id, column index, value) and they consult that index when needed (FriendFeed did something similar). 5. Cassandra should have no problem supporting the operations and semantics of Shemaless ass described in their blog posts. However, given they already operate it in production, they probably considered it and decided against it.
  • fdr 10y ago
    w.r.t. 2: https://www.postgresql.org/message-id/6248.1046130083%40sss.pgh.pa.us https://www.postgresql.org/message-id/6248.1046130083%40sss.... Manfred Spraul <manfred(at)colorfullife(dot)com> writes: > Tom Lane wrote: >> It seems unlikely to me that eliminating lseek on some platforms would >> be worth the hassle of maintaining two code paths. lseek is mighty >> cheap as system calls go. >> > It was considered expensive enough to write a syscall avoidance layer > that caches the file pointer and skips lseek if fpos==offset. You're missing the point: that layer is mostly there to ensure that we don't foul up the kernel's readahead recognition for sequential fetches. It's nice that Linux doesn't care, but Linux is not the only platform we worry about. regards, tom lane
• cnfjdnx 10y ago
  Uber """"""""""engineering""""""""""
• scotty79 10y ago
  Funny how the article is just: - we used X in a fashion that suited us best - it caused us problems Y because of some technicalities of X - so we switched to Z and we could avoid Y thanks to how Z handles the technicalities differently than Y and the top rated HN comments are: - you used the X wrong - all the technicalities of X that caused you problems Y are actually superior features of X
  • zihotki 10y ago
    But think how funny that'd be if we assume that "all the technicalities of X that caused you problems Y are actually superior features of X" is actually true?
• pella 10y ago
  anno 2013 "MIGRATING UBER FROM MYSQL TO POSTGRESQL" https://www.yumpu.com/en/document/view/53683323/migrating-uber-from-mysql-to-postgresql https://www.yumpu.com/en/document/view/53683323/migrating-ub...
• kev009 10y ago
  I like this transparency, I know I never want to work at uber.
• vanviegen 10y ago
  One major advantage of MySQL's clustered indexes the article doesn't mention is that, although secondary key reads may be a little slower, primary key reads will be faster. The row data lives in the primary key index, so there is no need for referencing an additional database page (possibly causing random I/O). This is especially relevant when doing range queries over the primary key. Imagine a table containing billions of chat messages, from which you want to retrieve a single conversation history. With a clustered primary key on (conversation id, message id), MySQL would need to process just a couple of database pages. Postgres, on the other hand, would need to reference a semi-random page for each of the messages. Now imagine a 10k message chat conversation, a table too large to fit into RAM, and storage by means of spinning rust (yeah, yeah, I know what year it is :-)). The difference would be somewhere between 2 and 3 orders of magnitude.
  • pritambaral 10y ago
    Uhh ... Postgres supports Index-only scans; as long as the data you're asking for is in the index, that is. So if you have an index on (conversation_id, message_id), and you try to retrieve message ids of a specific conversation, only the index will be touched.
    • vanviegen 10y ago
      Doh, you're right of course! We were having this problem in the Postgres 9.1 era, before index-only scans were a thing. Still, it's quite inefficient maintaining an extra copy of the data that is never actually used. Though no longer multiple orders of magnitude less efficient. However, I'd guess that programmers don't often think to add these seemingly useless fields to an index, as it feels inefficient and just wrong. But at least this offers an out in pathetic cases.
• viraptor 10y ago
  I'd like to see their migration strategy as well. I mean, they say moving from pgsql 9.2 to higher version (which then allows online upgrades) is too much work. Yet they'll have to migrate to mysql, which will take much more engineering effort. For anything close to realtime, they'll need to copy the old data, while at the same time forking the new writes into both pgsql slaves and new mysql servers. And they cannot use WAL for that without some advanced processing. I hope this follows in the next blog post.
• nierman 10y ago
  with respect to "Difficulty upgrading to newer releases": pg_upgade has a --link option which uses hard links in the new cluster to reference files from the old cluster. This can be a very fast way to do upgrades even for large databases (most of the data between major versions will look the same; perhaps only some mucking with system catalogs is required in the new cluster). Furthermore, you can use rsync with --hard-links to very quickly upgrade your standby instances (creating hard links on the remote server rather than transferring the full data). that is all referenced in the current documentation: https://www.postgresql.org/docs/current/static/pgupgrade.html https://www.postgresql.org/docs/current/static/pgupgrade.htm...
• jswny 10y ago
  It really is sad that we all just can't appreciate a company like Uber giving us insight into their internal engineering choices. So many people on here think Postgres is so perfect that if you don't like it you must be using it wrong. Postgres is a tool like anything else. It has good use cases, and bad use cases.
• sriharis 10y ago
  A common solution to conserve bandwidth is to use compression. This can be done easily in PostgreSQL by using ssh tunnels and turning on compression. I wonder why they didn't try that.
  • Terretta 10y ago
    Reading this, I wondered if they deeply understood the difference between bandwidth and latency. I doubt compression would be the thing, as I'd guess they had a latency problem, not a bandwidth problem. You see this kind of misunderstanding commonly pooled with other "a computer has physically moving parts" misunderstandings like the ORM or connection pooling concerns outlined. After tyranny of abstractions, nobody knows how the moving parts really work. // As alternatives given you'd like to keep the immutable data approach which brings a lot of goodness, consider a log-structured file system for the disk concerns, and geo-sensible replication for the latency concerns. At this scale, for near real-time app, bi-coastal DB is a bad model. You shouldn't have all users in SF querying a database in DC. Given the nature of the business model, they can share geographically at one time scale, and roll up and replicate geo diverse data at a high latency leisure.
• dschiptsov 10y ago
  Nice to see how Postgres (a relative of Informix) follows an old-school maxim to focus on Consistency and Durability by being "append-only" and never over-writing the data. Sticking to the right principles is better than over-optimization. The Uber engineers should, perhaps, take a look at Changelogs of last 5 or so releases of MySQL to see how many bugs in InnoDB has been found in each release and read stories about data loses due to inability to repair storages. According to old-school DBA tradition, it is much better to have an straightforward storage engine based on right principles written in C than fancy storage written in C++. At least if one values ones data. Well, in the age of in-memory "databases", "durability through replication" and "eventual consistency" old school focus on disk commits might sound a bit outdated, until one gets that moment when shards got messed up and there is no way to know how many writes are missing and where. Database is a durable storage which guarantees data consistency and ability to roll-back to a clean state through direct-access (by passing all caches) writes. At lest this is what we had in glorious times of IDS 7.3
• exabrial 10y ago
  My big think with MySQL is that the last time I needed to make this decision (about 3yr ago), the support tooling, community knowledge, and documentation around MySQL was light years ahead of Postgres. There were literally hundreds of MySQL clients and utilities and Postgres was "just a database". Competition breeds excellence.
• quotemstr 10y ago
  > Each of these system calls incurs a context switch System calls are not context switches. I wish people would distinguish between them. A system call is just a change of privilege level and is efficient --- there's no cache invalidation required on almost any system. A context switch, on the other hand, involves a call to the scheduler, saving and restoring of much more CPU register state, and various kinds of cache invalidation. (It's even more expensive if you're switching between different processes instead of different threads in the same process.) The kernel may perform a context switch while executing a system call --- this context switch is what makes blocking calls blocking. But even IO system calls do not necessarily cause context switches, especially in the case where an operation can be satisfied by accessing only the page cache. tl;dr A system call is not necessarily a context switch
• morekozhambu 10y ago
  There is this interesting talk on MySQL vs Postgres. https://www.youtube.com/watch?v=emgJtr9tIME https://www.youtube.com/watch?v=emgJtr9tIME
• madhusudhan000 10y ago
  So let me try to summarise this. Poor replica MVCC support They are actually pointing to a blog article written in 2010 -> http://blog.2ndquadrant.com/tradeoffs_in_hot_standby_deplo/ http://blog.2ndquadrant.com/tradeoffs_in_hot_standby_deplo/ Do they realise that it is 2016 ? Guess they did't bother to understand the hot standby feedback system. > Postgres’s design resulted in inefficiencies and difficulties for our data at Uber. What kind of inefficiency ? The explain what is purpose of WAL and replication which every database person knows about but didn't care to explain the actual problem at hand ? Data corruption > During a routine master database promotion to increase database capacity, we ran into a Postgres 9.2 bug Why the heck didn't they upgrade to a newer version ? Did you report this bug to pg dev , did they take so much time to fix this, or were you just assuming that the bug could fix itself ? > The bug we ran into only affected certain releases of Postgres 9.2 and has been fixed for a long time now. However, we still find it worrisome that this class of bug can happen at all. Postgres 9.2 is pretty old and there has been 3 major releases after that. WTF ? I can say countless instances where MySQL data corruption was a constant nuisance with version 5.5 and they have fixed it with newer releases. Replication > During peak traffic early on, our bandwidth to the storage web service simply wasn’t fast enough to keep up with the rate at which WALs were being written to it So you have run into a hardware limitation and then blame postgres. What was limit that you hit ? I don't understand this point at all. Concept of context switching I am surprised that this is actually an issue, in a database the slowest part is always the disk and not the CPU. Confused on how did they hit this limitation first without actually touching others. Time taken by a context switch : http://stackoverflow.com/questions/21887797/what-is-the-overhead-of-a-context-switch http://stackoverflow.com/questions/21887797/what-is-the-over... Which is in microseconds. InnoDB buffer pool > By comparison, the InnoDB storage engine implements its own LRU in something it calls the InnoDB buffer pool Postgres has something similar called shared_buffer. They are speaking as if postgres relies entirely on the operating system which is false. > It makes it possible to implement a custom LRU design. For instance, it’s possible to detect pathological access patterns that would blow out the LRU and prevent them from doing too much damage Not sure what kind of damage they are speaking. In a postgres sequential scan (full table scan), a ring buffer is used instead and does not result in the shared buffers being blown away. If you need a custom LRU design, there is definitely something wrong in the way that you are using an OLTP database. Connection Handling This is complete BS. Nobody uses databases without connection pools. Agree that a thread is more lightweight than a process, but you would never hit this limit at all in real time which is in the order of microseconds again. In a production system, one would open connections immediately and then hold them in the connection pool. This overhead is almost not visible at all. If you are constantly opening and closing connections then there is something seriously wrong with your design. > However, we have had occasional application bugs in our backend services that caused them to open more active connections (usually “idle in transaction” connections) than the services ought to be using, and these bugs have caused extended downtimes for us So they are blaming the database for a bug in their design/system. Computers are no match for human stupidity. > Accordingly, using pgbouncer to do connection pooling with Postgres has been generally successful for us. Again what is the problem, the whole article smells more and more like a useless rant, just because you dont know how to use them ? Conclusion Another thing is that they have not given any kind of query/access pattern in which they use postgres/mysql. They put in a couple of low level things and then say that postgres is badly designed. I can think of only two logical explanations 1) The article writer was already familiar with MySQL and they didn't bother to even dig into postgres deeper 2) They have been paid by oracle :P
• denishpatel 10y ago
  They migrated from MySQL to Postgres (https://www.yumpu.com/en/document/view/53683323/migrating-uber-from-mysql-to-postgresql https://www.yumpu.com/en/document/view/53683323/migrating-ub...) with almost same reasons and now they are counter argumenting for their inability to use software and lack of skills to upgrade!
• _navaneethan 10y ago
  From this article, Can i assume the below point? why postgres is designed such a way of "physical replication" rather than the design of mysql's ONLY "logical replication"? Because postgres empowering data integrity with help of forceful constraints.
• mace 10y ago
  I think a fair summary is "We had a few problems with PostgreSQL mostly due to our rapid growth. We rethought the problem and changed the way we use a relational database for large-scale storage and are now using MySQL as a dumb key-value store." The conclusion reinforces this: "Postgres served us well in the early days of Uber, but we ran into significant problems scaling Postgres with our growth." I read this a both endorsement of PostgreSQL as well as highlighting some of the problems that any large-scale use of it would run into.
• simon2Q 10y ago
  I've tried to produce a full reply to most of the technical points raised there. http://blog.2ndquadrant.com/thoughts-on-ubers-list-of-postgres-limitations/ http://blog.2ndquadrant.com/thoughts-on-ubers-list-of-postgr...