Last week, I covered how MVCC, Postgres’s storage system, works on a very high level. Near the end, I also mentioned that it doesn’t quite lend itself well to certain use cases, such as rapidly mutating session storage. Well, there is one caveat to that statement that I’d forgotten about because of its relatively limited utility: unlogged tables.
As a DBA, I strive not to live in an isolated ivory tower, away from the developers that are destined to fill our databases with volumes of data from a myriad of applications. It’s important, I think, to occasionally come back to the basics. So I’d like to discuss one of the core elements that Postgres DBAs might be intimately familiar with, but comes up often enough that some clarification is warranted. How does PostgreSQL store data? The answer may surprise you, and is critical to understand before regularly interacting with a Postgres system.
Postgres has had anonymous blocks since the release of 9.0 in late 2010. But it must either be one of those features that got lost in the shuffle, or is otherwise considered too advanced, because I rarely see it used in the wild. If that’s the case, it’s a great shame considering the raw power it conveys. Without committing to a function, we can essentially execute any code in the database, with or without SQL input.
Fairly recently, a friend of mine presented a problem he wanted to solve with some JSON he had in a table. After he presented the end result he was trying to reach, I made the assumption that this would be pretty easy to do. But then I looked at the JSON Functions to try and find that quick fix. Though I read extensively and used rather liberal interpretations of the functions, there’s no way to directly manipulate JSON object contents with PostgreSQL.
High availability of PostgreSQL databases is incredibly important to me. You might even say it’s a special interest of mine. It’s one reason I’m both excited and saddened by a feature introduced in 9.4. I’m Excited because it’s a feature I plan to make extensive use of, and saddened because it has flown under the radar thus far. It’s not even listed in the What’s new in PostgreSQL 9.4 Wiki page. If they’ll let me, I may have to rectify that.