The ctid represents the tuple's location (page number + offset inside it), while xmin and xmax can be considered as "birth date" and "death date" for tuples.īy understanding this behavior early on, you'll be better equipped to handle challenges related to disk space, bloat, and autovacuum processes that aim to clean up these dead tuples.Īnd here is my favorite basic example, a trivial but very important one: To help grasp these concepts, every table in Postgres has hidden columns that you can select: ctid, xmin, and xmax. It means that if you attempt to insert a record and then rollback that action, the tuple that was to be inserted is marked dead. Even a canceled INSERT creates a dead tuple, which can be surprising for many.Instead, it creates a new version of that row (a new tuple) and marks the old one as dead. Similarly, when you UPDATE a row, Postgres doesn't modify the existing tuple.If these dead tuples can accumulate and got deleted by vacuuming in large volumes, this leads to table and index bloat. Instead, the old tuple gets marked as dead but stays around untill VACUUM deletes it. When you execute a DELETE command, it doesn't immediately reclaim disk space.Here's what happens during various write operations: This versioning system is called MVCC, Multiversion concurrency control, and it is important to understand it to design well-performing systems. What this means is that when data in a row changes, rather than altering the existing data, Postgres adds a new version of that row, a tuple. In simple terms, a tuple in Postgres is the physical version of a row of data. One of the foundational aspects of PostgreSQL that surprises many newcomers is the concept of tuples. Understand early: tuples are physical versions of rows Here are those 10 tips (+bonus) Michael and I have discussed. After seeing the huge response to my tweet, which got over 200k views, 1200+ likes, and 200+ retweets, I wanted to dig deeper and share more about these essential tips. Our latest episode was all about helping newcomers to PostgreSQL. We've been at it for 55 weeks straight since July 2022, and we're not stopping anytime soon. Every Friday, Michael Christofides ( pgMustard) and I discuss these features on our podcast, Postgres.FM (there is also a video version on YouTube). It's more than just another database-it's a system packed with features that can change how you handle data. Getting started with PostgreSQL can be both exciting and challenging.
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