Reg: Question about concurrency/locking

Hi,

I have a doubt referring to following scenario:

create table t1 (id integer primary key,desc varchar(10));

I insert 4 values as below:

id val
---------------------------------------
1 one
2 two
3 three
4 four

Note that the auto commit transaction for the session is off. I did
these inserts from user1 (session1). Now I opened another session
(session 2) of same user user1. I issued a update statement from
session 1 & from session 2 respectively as below:

Session 1: update t1 set val='ONE' where id=1;
Session 2: update t1 set val='TWO' where id=2;

Note that I haven's issued commit on either sessions. Both sessions
have auto commit off.

Now considering code internals for the above:

There will be one block/page/buffer allocated for table t1 and the
values are inserted to that block. When a commit is issued after
insert, the data is guranteed to be written to the datafile on the
disk. Now when the couple of updates are done from 2 sessions opened by
same user (user1), the page/buffer is found in the buffer/page frames
in the memory & when the update from session 1 is done - an exclusive
lock (update lock?) is held on the page - is this correct? Then when a
second update from session 2 is issued how does the update goes without
blocking, since all the four records are in the same physical block &
hence in the page/buffer frame in the memory. The second update also
need an exclusive lock before changing the record. Since the page is
already locked by session 1 in exclusive (update?) mode how does
session 2 succeed with getting it's own exclusive lock while session 1
is already holding one and haven't issued a commit/rollback yet?

Kindly clarify this & please provide me pointers into the wonderful
postgresql code, where it has been handled.

Thanks in advance.

Regards
Kalyankumar Ramaseshan

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