Обсуждение: The mystery of concurrent access

Hi all

I thought enough experimentation would reveal this mystery, but its very
confusing, so I'll ask.

When 2 processes/people are simultaneously working on a table

* does Postgres automatically enforce some sort of locking?
* do the changes made by process A become visible to process B immediately
(of course, the result of a SELECT wouldn't change dynamically, but suppose
process B did another SELECT, would it access process A's records?)

Thanks

PVS

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P.V.,

> When 2 processes/people are simultaneously working on a table
>
> * does Postgres automatically enforce some sort of locking?
> * do the changes made by process A become visible to process B
> immediately
> (of course, the result of a SELECT wouldn't change dynamically, but
> suppose
> process B did another SELECT, would it access process A's records?)

Please read:

http://www.postgresql.org/idocs/index.php?mvcc.html

-Josh

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What you really need to do is read the following
chapter from the PostgreSQL User's guide:

http://www.postgresql.org/idocs/index.php?mvcc.html

It explains all you need to know about how PostgreSQL
handles concurrent access.  Pay special attention to
section 9.3 as Read Committed Isolation Level is the
default for PostgreSQL.

--- "P.V. Subramanian" <pvsmian@hotmail.com> wrote:
> Hi all
>
> I thought enough experimentation would reveal this
> mystery, but its very
> confusing, so I'll ask.

Yes, I imagine that experimenting would be very
confusing.  Fortunately the documentation is very
good.

> When 2 processes/people are simultaneously working
> on a table
>
> * does Postgres automatically enforce some sort of
> locking?

Yes.  And it does so at the row level.  It's very
cool.

> * do the changes made by process A become visible to
> process B immediately
> (of course, the result of a SELECT wouldn't change
> dynamically, but suppose
> process B did another SELECT, would it access
> process A's records?)

It depends on the isolation level, and on what you
mean by "immediately."  In the default isolation level
if transaction A starts and then transaction B starts
transaction A cannot see any of the changes from
transaction B until transaction B is committed.

Bruce Momjian explains it much better than I ever
could, you should also take a look at the relevant
chapter of his book:

http://www.ca.postgresql.org/docs/aw_pgsql_book/node98.html

> Thanks
>
> PVS


I hope this is helpful,
Jason


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