Who can explain the essence of PostgreSQL locks on the fingers?

Question

PostgreSQL

I read several articles on Habré, but for myself I did not understand the problems with locks. If the situation when someone writes, a situation may arise when someone tries to read, I still understand, then some other cases do not come to my mind. Can you explain them?

Answer 1

A typical case is a transaction.
You start doing a big update that will take, say, 3 minutes, while someone else is trying to change it. The table is locked for modification until the long transaction ends. And if you do select ... for update, then the table will be locked even for selects.
Rare case - alteration.
You modify the table by adding or deleting a field, and at that moment an analytical report occurs. At this point, the table is locked.
https://postgrespro.ru/docs/postgrespro/10/explici...