BUG #10429: the same update return different result

El 23/05/2014 04:41 a.m., chuisiniu@hotmail.com escribió:

The following bug has been logged on the website:

Bug reference: 10429
Logged by: zhanglong
Email address: chuisiniu@hotmail.com
PostgreSQL version: 9.3.4
Operating system: window7 32bit
Description:

E:\postgresql-9.4beta1\db\bin>psql postgres
psql (9.4beta1)
Type "help" for help.

postgres=# drop table t1;
DROP TABLE
postgres=# create table t1(c1 int primary key);
CREATE TABLE
postgres=# insert into t1 values (1);insert into t1 values (2);update t1 set
c1 = c1 +1;
INSERT 0 1
INSERT 0 1
ERROR: duplicate key value violates unique constraint "t1_pkey"
DETAIL: Key (c1)=(2) already exists.
postgres=# select * from t1;
c1
----
1
2
(2 rows)

postgres=#
postgres=# drop table t1;
DROP TABLE
postgres=# create table t1(c1 int primary key);
CREATE TABLE
postgres=# insert into t1 values (2);insert into t1 values (1);update t1 set
c1 = c1 +1;
INSERT 0 1
INSERT 0 1
UPDATE 2
postgres=# select * from t1;
c1
----
3
2
(2 rows)

The data is all the same except the order I insert it,but the update get
different result.From the source code, it seems that db update the tuple
first found out.

the first case you insert
row number 1 value 1
row number 2 value 2
when update
row number 1 value 1 + 1=2 produces duplicate

the second case you insert
row number 1 value 2
row number 2 value 1
when update
row number 1 value 2 + 1=3
row number 1 value 1 + 1=2
OK!!!

perhaps you need define your constraint deferrable

http://www.postgresql.org/docs/9.3/static/sql-createtable.html

DEFERRABLE
NOT DEFERRABLE

This controls whether the constraint can be deferred. A constraint
that is not deferrable will be checked immediately after every
command. Checking of constraints that are deferrable can be
postponed until the end of the transaction (using the SET
CONSTRAINTS
<http://www.postgresql.org/docs/9.3/static/sql-set-constraints.html&gt;
command). NOT DEFERRABLE is the default. Currently, only UNIQUE,
PRIMARY KEY, EXCLUDE, and REFERENCES (foreign key) constraints
accept this clause. NOT NULL and CHECK constraints are not deferrable.

INITIALLY IMMEDIATE
INITIALLY DEFERRED

If a constraint is deferrable, this clause specifies the default
time to check the constraint. If the constraint is INITIALLY
IMMEDIATE, it is checked after each statement. This is the default.
If the constraint is INITIALLY DEFERRED, it is checked only at the
end of the transaction. The constraint check time can be altered
with the SET CONSTRAINTS
<http://www.postgresql.org/docs/9.3/static/sql-set-constraints.html&gt;
command.

Hi,

postgres=# update t1 set c1 = c1 +1;

This controls whether the constraint can be deferred. A constraint that is not deferrable will be checked immediately after every command.

My understanding would have been, that "update t1 set c1=c1+1;" is ONE command, so even in the NOT DEFERRABLE case the check would occur only after both values were changed and as such not find any violation of the constraint.

claudio
--
Claudio Nieder . mailto:claudio.nieder@inodes.ch . http://inodes.ch/
iNodes AG . Loogartenstrasse 14 . CH-8048 Zürich . +41 43 960 0066

Claudio Nieder <claudio.nieder.lists@inodes.ch> writes:

postgres=# update t1 set c1 = c1 +1;

This controls whether the constraint can be deferred. A constraint that is not deferrable will be checked immediately after every command.

My understanding would have been, that "update t1 set c1=c1+1;" is ONE command, so even in the NOT DEFERRABLE case the check would occur only after both values were changed and as such not find any violation of the constraint.

The important detail is a bit further down the page, in the
"Compatibility" section:

Non-deferred Uniqueness Constraints

When a UNIQUE or PRIMARY KEY constraint is not deferrable, PostgreSQL
checks for uniqueness immediately whenever a row is inserted or
modified. The SQL standard says that uniqueness should be enforced only at
the end of the statement; this makes a difference when, for example, a
single command updates multiple key values. To obtain standard-compliant
behavior, declare the constraint as DEFERRABLE but not deferred (i.e.,
INITIALLY IMMEDIATE). Be aware that this can be significantly slower than
immediate uniqueness checking.

regards, tom lane

--
Sent via pgsql-bugs mailing list (pgsql-bugs@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-bugs