ctid and line pointers: why a row's physical address isn't a stable id
Simple terms
A row's address is two numbers: which page, and which slot on that page. Not a byte offset, a slot. That one layer of indirection is the whole trick. Indexes point at the slot, so Postgres can shuffle a row's bytes around inside its page to tidy up and every index entry still lands correctly. The catch is that slots get recycled as rows come and go. A ctid tells you where a row lives right now, not which row it is. Never store one as a permanent id.
You might be asked
What does a row's ctid actually point at, is the second number a byte offset into the page? Walk me through what happens to a row's ctid when you UPDATE it, how an index entry still finds the row afterward, and explain why ctid is a dangerous thing to store as a permanent row identifier.
Pro concept
Full answer and evidence depth sit behind Pro
You have the question and a plain-English lead. Pro unlocks the short answer, what the docs say, what the code does, the labeled lab evidence or reproduction protocol, how to use it under pressure, and the references.
Unlock the full breakdown for ctid and line pointers: why a row's physical address isn't a stable id
- How you'd answer it, the full senior-level short answer
- What the docs say, the manual's actual wording, with the citations
- What the code does, the mechanism decoded from PostgreSQL's own source at a pinned tag
- Proof from a real run, labeled as raw output, a captured run summary, or a run-it-yourself protocol
- Using it under pressure, the situation, the call you'd make, what goes wrong, and what people get wrong
- Version notes, what changed across PostgreSQL releases
Card required. Cancel before day 7 and you are not charged.
How this was verified
The open teaser is the question and a plain-English lead. The short answer, the manual walkthrough, the source decode, the lab evidence, and how to use it under pressure unlock with Pro. Evidence is labeled as raw output, a captured run summary, or a run-it-yourself protocol.
Connected
Where this concept connects
How this concept links across the library, the interview questions that test it, its plain-English glossary definition, and the guided pathways it belongs to. Open the full map to explore further.