6 ms·

Creating a Document-Store Hybrid in Postgres 9.5

65 points by craigkerstiens 11y ago

jarjoura 11y ago
Only thing I would caution here is that Postgres HA is notoriously difficult to set up and get right. Something like CouchDB is designed from the ground up for high availability and scalability. So if your design calls for only a document store, probably best to avoid Postgres.
- empthought 11y ago
  This is flat-out misleading. PostgreSQL is also designed "from the ground up for high availability and scalability," it's just optimized for a different -- and far more common and useful -- set of system assumptions and constraints. Unfortunately those assumptions and constraints don't make good blog or product marketing material copy. "We have a typical LOB app," "our data needs are nothing extraordinary," etc.
  - coderzach 11y ago
    Postgres isn't designed from the ground up for high availability or scalability. Most of the in-built functionality for setting up HA wasn't introduced until 9.0, and the process of setting up HA postgres with automatic failover requires third-party tools. As for scalability, postgres can only scale vertically. If you need a solution that can scale writes to more than than one node, or a solution that has first party support for HA with automatic failover, you shouldn't be using postgres. As an aside, I find the dogmatic "just use postgres for everything" choir just as bad as the marketing BS associated with noSQL databases.
    - snuxoll 11y ago
      Unless you are using synchronous replication (locks up transactions if there is an issue with replication) there's no safe way to do automatic failover with an RDBMS without using shared storage (expensive, pain in the butt). Corosync and pacemaker make setting up automatic failover with shared storage relatively painless. I don't do it this way personally because shared storage is a giant pain.
    - empthought 11y ago
      "High availability" and "high availability with automatic failover" are not the same thing. (The tricky bit is actually the un-failing-over part.) I assure you that people were setting up high availability systems with PostgreSQL 7.x in 2004 or so. Very few organizations need to scale their database in any other direction than vertically. If it works for Stack Overflow, it's certainly likely to work for your application.
      - takeda 11y ago
        It's also lack of understanding the problem you're trying to solve. For example at my previous job we had a dedicated data store for performing lookups such as ip -> zip code and latitude/longitude -> zip code. The company decided to use Oracle Coherence and store all data in memory, because it'll be fast. To store all of that information they needed 16 m3.medium machines. Last year they had a great success optimizing it, because they managed to replace 16 x m3.medium machines to just 3 x c3.2xlarge machines running MongoDB (the data was ~12GB). I did a POC and put the data in PostgreSQL with proper columns and indices (I just needed to install ip4range and PostGIS), the whole data fit in 600MB! the queries took under at most 2ms on cold cache but generally were under miliseconds, because all the data fit in RAM.
        threeseed 11y ago
        It seems more of a problem with the people than technologies. Why would you need 16 m3.medium (60GB RAM total) to store 600MB of data ? I've used Oracle Coherence and other grid technologies and something doesn't sound right here. It is just a couple of distributed Java HashMaps we are taking about here. Likewise if 600MB of data is expanding to 12GB in MongoDB then something is very, very wrong with the design of your schema.
        empthought 11y ago
        > It seems more of a problem with the people than technologies. Isn't that what "lack of understanding of the problem you're trying to solve" means? Though in this case, if the problem had "IP4 ranges" and "geographic data and computation functions" in its scope, then MongoDB is quite inadequate compared to PostgreSQL.
  - brightball 11y ago
    My best attempt at PostgreSQL marketing material copy. :-) http://www.brightball.com/postgresql/why-should-you-learn-postgresql http://www.brightball.com/postgresql/why-should-you-learn-po...
    - merb 11y ago
      Good stuff expect Listen Notify. it's too dependent on the driver. I mean some drivers needs some kind of polling which is, aweful. Mostly you could go around this issue by opening another connection without going through your database pool and building a simple driver just for listen/notify. (jdbc I'm looking at you)
      - brightball 11y ago
        Most of the uses of it that I've seen have taken this approach with a listener on a thread using a different connection.
- snuxoll 11y ago
  PostgreSQL HA is not that difficult to setup, repmgr makes the task pretty easy. If you are talking about automatic failover then that's another story entirely, because it is simply not appropriate for relational databases unless you are using shared storage (expensive, painful) - but if you want to go that route corosync and pacemaker have your back.
- __jal 11y ago
  PG HA really isn't difficult to set up. You have to know about your environment and how you use the DB, but if that's limiting factor then I'm really not sure why you'd try to set up HA.
- bsaul 11y ago
  I've read many answers to your comment saying "but it's not that hard". So to all those people : please, write a blog or send a link to one that explains how to setup a HA cluster with automatic failover. I've spent some time trying to setup a simple pg cluster once ( not even HA, just duplicate writes) and it was nothing but straightforward.
orf 11y ago
Interesting post, but I don't understand his way of deleting rows that aren't in data.json. Creating a separate table seems overly complex: why not just store a list of ID's in memory and delete any that aren't in that set? Creating a list of 100,000,000 integers in Python takes 900mb (4.2gb for a set!), which isn't that much. Stick it in one big 'DELETE WHERE id NOT IN (...)' query. Or better yet just TRUNCATE beforehand.
- fritzy 11y ago
  You're right there. This was a naive stab at it that I hadn't properly thought through. I'll try to update the post this weekend.
- empthought 11y ago
  > Stick it in one big 'DELETE WHERE id NOT IN (...)' query. I think that's a terrible idea, because it will scan the entire NOT IN (...) array for each row. Since it's NOT IN, it will even have to scan the whole thing every time. A temporary table is generally the way to go for unbounded lists of things; there's even support for it in PostgreSQL and the SQL standard.
  - orf 11y ago
    I always assumed pg would be smart and use something with a constant lookup time. This[1] is a query plan 9.5 made with a table containing 10,000,000 sequential rows, executing "DELETE WHERE id IN (select * from generate_series(1, 1000000));". Maybe your right, it spends 5.8 seconds on the nested loop. I've only used this for smaller volumes where an extra table might be overkill, but I guess with loads more rows a temporary table is better. 1. http://tatiyants.com/pev/#/plans/plan_1454709615696 http://tatiyants.com/pev/#/plans/plan_1454709615696
    - empthought 11y ago
      PG is doing the smart thing; it's optimizing for the common case of short IN/NOT IN literals in SQL queries. Simple arrays have much better constant factors. If the clause is a subselect (like yours) and is also an actual table (unlike yours), it will be smart and rewrite it as a JOIN, which will generally have better complexity if the indexes are correct.
Sanddancer 11y ago
Neat little tutorial. One of the things I'd suggest to the author is to perhaps add in something on creating standard and materialized views for some of the more complicated queries, to show more of the interesting data manipulation options that postgresql gives you.
SwellJoe 11y ago
I'm often surprised at how many of the new web frameworks only support something like MongoDB. There's nothing wrong with MongoDB (as far as I know), but when I'm building something new, I'm planning for a long haul, and I expect to want/need real database features in the future, even if I don't need them today. Given that PostgreSQL has JSON data types, and performance when using those data types is competitive with the dedicated JSON or document databases, I honestly can't think of any reason to use something else. Especially given the long reliability history, the large pool of expertise, the tooling, etc. of PostgreSQL vs. the other options. Am I missing something in this assessment of the situation? I see so many smart people deploying web applications (in particular) on MongoDB and others...why are they choosing it over PostgreSQL?
- plasticxme 11y ago
  Mongodb easy; PostreSQL hard. The latter requires extensive planning, which slows you down when features need to go out now.
  - mbreese 11y ago
    Oh no! Someone needs to model their data before writing code??? The horror! You're going to pay for getting the data model right eventually. The only question is - do you want to pay for it now or later? If you push it off until later, it's generally more painful to make changes. Or, you have to maintain a sub-optimal model for as long as the application exists.
    - threeseed 11y ago
      I take it you must be new to software development. Because I have never seen or heard of a situation where the data model has been designed upfront and never changed along the way. And you seem to be turning this into a black/white scenario. In almost all cases you do your best effort upfront and then iterate. The advantages of these NoSQL/Schemaless style databases is that you can do this migration in your app instead of in your app+database. And unfortunately schema/data migration tools for databases really aren't that great - even after all these decades.
ahachete 11y ago
Most of this work, and much more, including compatibility with the MongoDB protocol (including replication) is already done in ToroDB (https://github.com/torodb/torodb https://github.com/torodb/torodb). Hope that this self-plug is hopeful :)

Only thing I would caution here is that Postgres HA is notoriously difficult to set up and get right. Something like CouchDB is designed from the ground up for high availability and scalability. So if your design calls for only a document store, probably best to avoid Postgres.

empthought 11y ago
This is flat-out misleading. PostgreSQL is also designed "from the ground up for high availability and scalability," it's just optimized for a different -- and far more common and useful -- set of system assumptions and constraints. Unfortunately those assumptions and constraints don't make good blog or product marketing material copy. "We have a typical LOB app," "our data needs are nothing extraordinary," etc.
- coderzach 11y ago
  Postgres isn't designed from the ground up for high availability or scalability. Most of the in-built functionality for setting up HA wasn't introduced until 9.0, and the process of setting up HA postgres with automatic failover requires third-party tools. As for scalability, postgres can only scale vertically. If you need a solution that can scale writes to more than than one node, or a solution that has first party support for HA with automatic failover, you shouldn't be using postgres. As an aside, I find the dogmatic "just use postgres for everything" choir just as bad as the marketing BS associated with noSQL databases.
  - snuxoll 11y ago
    Unless you are using synchronous replication (locks up transactions if there is an issue with replication) there's no safe way to do automatic failover with an RDBMS without using shared storage (expensive, pain in the butt). Corosync and pacemaker make setting up automatic failover with shared storage relatively painless. I don't do it this way personally because shared storage is a giant pain.
  - empthought 11y ago
    "High availability" and "high availability with automatic failover" are not the same thing. (The tricky bit is actually the un-failing-over part.) I assure you that people were setting up high availability systems with PostgreSQL 7.x in 2004 or so. Very few organizations need to scale their database in any other direction than vertically. If it works for Stack Overflow, it's certainly likely to work for your application.
    - takeda 11y ago
      It's also lack of understanding the problem you're trying to solve. For example at my previous job we had a dedicated data store for performing lookups such as ip -> zip code and latitude/longitude -> zip code. The company decided to use Oracle Coherence and store all data in memory, because it'll be fast. To store all of that information they needed 16 m3.medium machines. Last year they had a great success optimizing it, because they managed to replace 16 x m3.medium machines to just 3 x c3.2xlarge machines running MongoDB (the data was ~12GB). I did a POC and put the data in PostgreSQL with proper columns and indices (I just needed to install ip4range and PostGIS), the whole data fit in 600MB! the queries took under at most 2ms on cold cache but generally were under miliseconds, because all the data fit in RAM.
      - threeseed 11y ago
        It seems more of a problem with the people than technologies. Why would you need 16 m3.medium (60GB RAM total) to store 600MB of data ? I've used Oracle Coherence and other grid technologies and something doesn't sound right here. It is just a couple of distributed Java HashMaps we are taking about here. Likewise if 600MB of data is expanding to 12GB in MongoDB then something is very, very wrong with the design of your schema.
        empthought 11y ago
        > It seems more of a problem with the people than technologies. Isn't that what "lack of understanding of the problem you're trying to solve" means? Though in this case, if the problem had "IP4 ranges" and "geographic data and computation functions" in its scope, then MongoDB is quite inadequate compared to PostgreSQL.
- brightball 11y ago
  My best attempt at PostgreSQL marketing material copy. :-) http://www.brightball.com/postgresql/why-should-you-learn-postgresql http://www.brightball.com/postgresql/why-should-you-learn-po...
  - merb 11y ago
    Good stuff expect Listen Notify. it's too dependent on the driver. I mean some drivers needs some kind of polling which is, aweful. Mostly you could go around this issue by opening another connection without going through your database pool and building a simple driver just for listen/notify. (jdbc I'm looking at you)
    - brightball 11y ago
      Most of the uses of it that I've seen have taken this approach with a listener on a thread using a different connection.
snuxoll 11y ago
PostgreSQL HA is not that difficult to setup, repmgr makes the task pretty easy. If you are talking about automatic failover then that's another story entirely, because it is simply not appropriate for relational databases unless you are using shared storage (expensive, painful) - but if you want to go that route corosync and pacemaker have your back.
__jal 11y ago
PG HA really isn't difficult to set up. You have to know about your environment and how you use the DB, but if that's limiting factor then I'm really not sure why you'd try to set up HA.
bsaul 11y ago
I've read many answers to your comment saying "but it's not that hard". So to all those people : please, write a blog or send a link to one that explains how to setup a HA cluster with automatic failover. I've spent some time trying to setup a simple pg cluster once ( not even HA, just duplicate writes) and it was nothing but straightforward.

orf 11y ago

Interesting post, but I don't understand his way of deleting rows that aren't in data.json. Creating a separate table seems overly complex: why not just store a list of ID's in memory and delete any that aren't in that set? Creating a list of 100,000,000 integers in Python takes 900mb (4.2gb for a set!), which isn't that much. Stick it in one big 'DELETE WHERE id NOT IN (...)' query. Or better yet just TRUNCATE beforehand.

fritzy 11y ago
You're right there. This was a naive stab at it that I hadn't properly thought through. I'll try to update the post this weekend.
empthought 11y ago
> Stick it in one big 'DELETE WHERE id NOT IN (...)' query. I think that's a terrible idea, because it will scan the entire NOT IN (...) array for each row. Since it's NOT IN, it will even have to scan the whole thing every time. A temporary table is generally the way to go for unbounded lists of things; there's even support for it in PostgreSQL and the SQL standard.
- orf 11y ago
  I always assumed pg would be smart and use something with a constant lookup time. This[1] is a query plan 9.5 made with a table containing 10,000,000 sequential rows, executing "DELETE WHERE id IN (select * from generate_series(1, 1000000));". Maybe your right, it spends 5.8 seconds on the nested loop. I've only used this for smaller volumes where an extra table might be overkill, but I guess with loads more rows a temporary table is better. 1. http://tatiyants.com/pev/#/plans/plan_1454709615696 http://tatiyants.com/pev/#/plans/plan_1454709615696
  - empthought 11y ago
    PG is doing the smart thing; it's optimizing for the common case of short IN/NOT IN literals in SQL queries. Simple arrays have much better constant factors. If the clause is a subselect (like yours) and is also an actual table (unlike yours), it will be smart and rewrite it as a JOIN, which will generally have better complexity if the indexes are correct.

Sanddancer 11y ago

Neat little tutorial. One of the things I'd suggest to the author is to perhaps add in something on creating standard and materialized views for some of the more complicated queries, to show more of the interesting data manipulation options that postgresql gives you.

SwellJoe 11y ago

I'm often surprised at how many of the new web frameworks only support something like MongoDB. There's nothing wrong with MongoDB (as far as I know), but when I'm building something new, I'm planning for a long haul, and I expect to want/need real database features in the future, even if I don't need them today. Given that PostgreSQL has JSON data types, and performance when using those data types is competitive with the dedicated JSON or document databases, I honestly can't think of any reason to use something else. Especially given the long reliability history, the large pool of expertise, the tooling, etc. of PostgreSQL vs. the other options. Am I missing something in this assessment of the situation? I see so many smart people deploying web applications (in particular) on MongoDB and others...why are they choosing it over PostgreSQL?

plasticxme 11y ago
Mongodb easy; PostreSQL hard. The latter requires extensive planning, which slows you down when features need to go out now.
- mbreese 11y ago
  Oh no! Someone needs to model their data before writing code??? The horror! You're going to pay for getting the data model right eventually. The only question is - do you want to pay for it now or later? If you push it off until later, it's generally more painful to make changes. Or, you have to maintain a sub-optimal model for as long as the application exists.
  - threeseed 11y ago
    I take it you must be new to software development. Because I have never seen or heard of a situation where the data model has been designed upfront and never changed along the way. And you seem to be turning this into a black/white scenario. In almost all cases you do your best effort upfront and then iterate. The advantages of these NoSQL/Schemaless style databases is that you can do this migration in your app instead of in your app+database. And unfortunately schema/data migration tools for databases really aren't that great - even after all these decades.

ahachete 11y ago

Most of this work, and much more, including compatibility with the MongoDB protocol (including replication) is already done in ToroDB (https://github.com/torodb/torodb https://github.com/torodb/torodb). Hope that this self-plug is hopeful :)