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Hi,
I'm having some trouble with transaction isolation levels, and would
appreciate some input on this.
According to the documentation of PostgreSQL 8.0.3, section 12.2:
"Read Committed is the default isolation level in PostgreSQL. When a
transaction runs on this isolation level, a SELECT query sees only data
committed before the query began; it never sees either uncommitted data
or changes committed during query execution by concurrent transactions."
Therefore, in this isolation level, I should not see data committed by
another concurrent transaction.
Now I open two terminals with a 'pgsql' command, and do the following:
terminal 1:
geertj=> \set AUTOCOMMIT off
geertj=> set transaction isolation level read committed;
SET
geertj=> create table products (id integer not null, code char(20)
not null);
CREATE TABLE
geertj=> commit; begin;
COMMIT
BEGIN
terminal 2:
geertj=> \set AUTOCOMMIT off
geertj=> set transaction isolation level read committed;
SET
geertj=> select * from products;
id | code
----+------
(0 rows)
terminal 1:
geertj=> insert into products values (10, 'pr10');
INSERT 34071 1
geertj=> commit;
COMMIT
terminal 2:
geertj=> select * from products;
id | code
----+----------------------
10 | pr10
As you see, the row inserted in the transaction from terminal 1 appears
into terminal 2, but terminal 2 had a transaction open that did not commit.
The top of section 12.2 of the manual tells that nonrepeatable reads may
happen in the 'read committed' isolation level. I can understand the
above behaviour in terms of this. However, the start of section 12.2.1
tells that in read committed mode, you get a snapshot of the database
when the transaction starts, and that snapshot will not inlude committed
changes from other transactions. Who is right here?
Regards,
Geert
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