Re: Regarding Postgresql Transaction isolation

Ajay P S <ajayps547(at)gmail(dot)com> writes:
> However, I see that in the heap_update(heapam.c) function there is a
> brief interval(Lock and unlock the buffer) where a writer may block
> readers if the writer is updating the same row which readers are
> reading.
> Could anyone please help me with the below query?

> 1) Is my understanding correct? In so, Is it not against the
> statements "readers does not block writers and writers does not block
> readers"

You should probably read that as "there are no macroscopic block
conditions between readers and writers". If you want to quibble
about whether a transient buffer lock violates the statement,
there are doubtless hundreds of other places where there's some
sort of short-term blockage. A trivial example is that just
finding the buffer in the first place can be transiently blocked
by spinlock or LWLock exclusion on the buffer lookup data structures.

regards, tom lane