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Craig Kerstiens

Category: SQL

A look at Foreign Data Wrappers

There are two particular sets of features that continue to keep me very excited about the momentum of Postgres. And while PostgreSQL has had some great momentum in the past few years these features may give it an entirely new pace all together. One is extensions, which is really its own category. Dimitri Fontaine was talking about doing a full series just on extensions, so here’s hoping he does so I dont have to :)

One subset of extensions which I consider entirely separate is the other thing, which is foreign data wrappers or FDWs. FDWs allow you to connect to other data sources from within Postgres. From there you can query them with SQL, join across disparate data sets, or join across different systems. Recently I had a good excuse to give the postgres_fdw a try. And while I’ve blogged about the Redis FDW previously, the Postgres one is particularly exciting because with PostgreSQL 9.3 it will ship as a contrib module, which means all Postgres installers should have it… you just have to turn it on.

Postgres Dollar Quoting

After my most recent post on documenting your database I had a colleague and friend chime in:

{% blockquote @danfarina %} @craigkerstiens You may want to mention for another post the generality of dollar quoting: it’s not just for CREATE FUNCTION. {% endblockquote %}

Luckily I was able to convince him to create the post. You can read a bit more on him below, but without further adieu here’s a bit on dollar quoting within Postgres:

Documenting your PostgreSQL database

Just a few days ago I was surprised by what someone was doing with their database, and not in the typical horrifying travesty against mankind. Rather, it was a feature that while familiar with I’d never seen anyone fully take proper advantage of - COMMENT or describing tables. Postgres has a nice facility for you to provide a description for just about anything:

  • Table
  • Column
  • Function
  • Schema
  • View
  • Index
  • Etc.

hstore vs. JSON - Which to use in Postgres

If you’re deciding what to put in Postgres and what not to, consider that Postgres can be a perfectly good schema-less database. Of course as soon as people realized this then the common comes a question, is hstore or JSON better. Which do I use and in what cases. Well first, if you’re not familiar check out some previous material on them:

If you’re already up to date with both of them, but still wondering which to use lets dig in.

Pivoting in Postgres

Earlier today on an internal Heroku group alias there was a dataclip shared. The dataclip listed off some data grouped by a category, there was a reply a few minutes later with a modification to the query that used the crosstab function to pivot directly in SQL. There were immediately several reactions on the list that went something like this:


While a mostly simple function in Postgres (there are a few rough edges), it really is all too handy. So here it is in action. Taking some data that looks like

  • row identifier, in this case date
  • category grouping, in this case OS
  • value

Javascript Functions for PostgreSQL

Javascript in Postgres has gotten a good bit of love lately, part of that is from Heroku Postgres recently adding support for Javascript and part from a variety of people championing the power of it such as @leinweber (Embracing the web with JSON and PLV8) and @selenamarie (schema liberation with JSON and PLV8). In a recent conversation it was pointed out that it seems a bit of headache to have to create your own functions, or at least having an initial collection would make it that much more powerful. While many can look forward to PostgreSQL 9.3 which will have a bit more built in support for JSON a few functions can really help make it more useful today.

Explaining your PostgreSQL data

I’ve written a bit before about understanding the output from EXPLAIN and EXPLAIN ANALYZE in PostgreSQL. Though understandably getting a grasp on execution plans could probably use some more guidance. Yet, this time around I’m taking a bit of a cop out and highlighting a few tools instead of documenting myself, which I’ve done in a talk I’ve frequently given Postgres Demystified.

Explain explained

Postgres Indexes – Expression/Functional Indexing

Postgres is rich with options for indexing. First you’ve got a variety of types, and beyond that you can do a variety of things with each of these such as create unique indexes, use conditions to index only a portion of your data, or create indexes based on complex expressions or functions. In cases where you commonly use various PostgreSQL functions in your application or reporting you can get some great gains from this.

My SQL Bad Habits

I’m reasonably proficient at SQL – a coworker when pseudocoding some logic for him pointed out that my pseudocode is what he thought was executable SQL. I’m fully capable of writing clear and readable SQL – which most SQL is not. Despite that I still have several bad habits when it comes to SQL. Without further adieu heres some of my dirty laundry so hopefully others can not make the same mistakes.