Tech
What Actually Happens When You UPDATE a Row in PostgreSQL?
Most developers think of a PostgreSQL table like this:
users
id | age
1 | 25
2 | 31
3 | 42
Then we run:
UPDATE users
SET age = 30
WHERE id = 1;
The obvious mental model is:
25 → 30
But PostgreSQL doesn't simply overwrite the old value.
To understand what actually happens, we need to look at how PostgreSQL stores rows and how MVCC (Multi-Version Concurrency Control) works.
Tables are stored as pages
PostgreSQL stores table data in fi...
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