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incoherent view of serializable transactions

From: "Kevin Grittner" <Kevin(dot)Grittner(at)wicourts(dot)gov>
To: <pgsql-hackers(at)postgresql(dot)org>
Subject: incoherent view of serializable transactions
Date: 2008-12-22 17:00:53
Message-ID: 494F7365.EE98.0025.0@wicourts.gov (view raw or flat)
Thread:
Lists: pgsql-hackers
As I've understood limitations of the PostgreSQL implementation of
SERIALIZABLE transactions, at least the only example given in the
documentation, revolve around a rather unlikely situation:
 
Given concurrent transactions T1 and T2 and non-overlapping sets of
data A and B, T1 reads data including A and uses the data to modify B
while T2 reads data including B and uses that data to modify A, where
the modifications performed by either would affect the modifications
made by the other, if they were visible.
 
For reasons I'll omit here, that scenario didn't worry me for my
current uses of PostgreSQL.
 
I've found another form of deviation from the standard SERIALIZABLE
behavior, though, which does worry me.  Although the above appears to be
the only situation where the end result after everything commits is
inconsistent with standard SERIALIZABLE behavior, the PostgreSQL
implementation allows transactions to view the data in states which
would never be possible during the application of the transactions in
series in the order they will appear to have been applied after the
commit.
 
Imagine, as an example, a system which involves recording receipts,
each of which must go into a daily deposit.  There is a control table
with one row containing the current deposit date for receipts. 
Somewhere mid-afternoon that date is updated, all subsequent receipts
fall into the new day, and a report is run listing the receipts for the
day and giving the deposit total.
 
Under a standard-compliant implementation of SERIALIZABLE, this is
straightforward: a transaction which is inserting a receipt selects the
deposit date to use in its transaction, and any SELECT of receipts for a
date prior to the current deposit date will see the accurate, final
data.  Under the PostgreSQL implementation, although data eventually
gets to a coherent state, there can be a window of time where a SELECT
can return an incomplete list of receipts for a date which appears to be
closed, even if all transactions for modifying and viewing data are
SERIALIZABLE.
 
-- setup
create table ctl (k text not null primary key, deposit_date date not
null);
insert into ctl values ('receipt', date '2008-12-22');
create table receipt (receipt_no int not null primary key, deposit_date
date not null, amount numeric(13,2));
insert into receipt values (1, (select deposit_date from ctl where k =
'receipt'), 1.00);
insert into receipt values (2, (select deposit_date from ctl where k =
'receipt'), 2.00);

-- connection 1
start transaction isolation level serializable ;
insert into receipt values (3, (select deposit_date from ctl where k =
'receipt'), 4.00);

-- connection 2
start transaction isolation level serializable ;
update ctl set deposit_date = date '2008-12-23' where k = 'receipt';
commit transaction;
start transaction isolation level serializable ;
select * from ctl;
-- (deposit_date shows as 2008-12-23)
select * from receipt;
-- (Only receipts 1 and 2 show for 2008-12-22.)
commit;

-- connection 1
commit transaction;

-- connection 2
start transaction isolation level serializable ;
select * from receipt;
-- (All receipts for the 2008-12-22 deposit date now show.)
commit transaction;
 
At this point, SERIALIZABLE transactions appear to have worked, with
receipt 3 happening before the update of deposit_date; however, there
was a window of time when the update to deposit_date was visible and
receipt 3 was not.
 
This absolutely can't happen in a standard-compliant implementation.
At a minimum, this window where visible data lacks coherency should be
noted in the documentation.  I don't know if there's any way to fix
this without killing performance.
 
-Kevin

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Next:From: Martijn van OosterhoutDate: 2008-12-22 17:54:59
Subject: Re: incoherent view of serializable transactions
Previous:From: Tom LaneDate: 2008-12-22 16:52:05
Subject: Some semantic details of the window-function spec

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