Thread: Solution to UPDATE...INSERT problem

Solution to UPDATE...INSERT problem

From
"Christopher Kings-Lynne"
Date:
Hi Guys,

I just thought I'd share with you guys a very clever solution to the old
'update row.  if no rows affected, then insert the row' race condition
problem.  A guy at my work came up with it.

We were discussing this earlier on -hackers, but no-one could find a
solution that didn't involve locking the entire table around the
update...insert commands.

The problem is that sometimes the row will be inserted by another process
between your update and insert, causing your insert to fail with a unique
constraint violation.

So, say this is the insert:

INSERT INTO table VALUES (1, 'foo');  // 1 is in the primary key column

Rewrite it like this:

INSERT INTO table SELECT 1, 'foo' EXCEPT SELECT 1, 'foo' FROM table WHERE
pkcol=1;

See? So now that INSERT statement will insert the row if it doesn't exist,
or insert zero rows if it does.  You are then guaranteed that your
transaction will not fail and rollback, so you can repeat your update, or do
the insert first and then the update, etc.

Hope that's handy for people,

Chris


Re: [HACKERS] Solution to UPDATE...INSERT problem

From
Tom Lane
Date:
"Christopher Kings-Lynne" <chriskl@familyhealth.com.au> writes:
> INSERT INTO table SELECT 1, 'foo' EXCEPT SELECT 1, 'foo' FROM table WHERE
> pkcol=1;

> See? So now that INSERT statement will insert the row if it doesn't exist,
> or insert zero rows if it does.  You are then guaranteed that your
> transaction will not fail and rollback, so you can repeat your update, or do
> the insert first and then the update, etc.

Uh, why exactly do you think this is race-free?

It looks fancy, but AFAICS the SELECT will return info that is correct
as of its starting timestamp; which is not enough to guarantee that the
INSERT won't conflict with another transaction doing the same thing
concurrently.

            regards, tom lane


Re: Solution to UPDATE...INSERT problem

From
Lincoln Yeoh
Date:
AFAIK the "except" select won't see other inserts in uncommitted
transactions. If those transactions are committed you will end up with the
same problem. You can try it yourself, by manually doing two separate
transactions in psql.

You either have to lock the whole table, or lock at the application layer.
Some time back I suggested a "lock on arbitrary string" feature for
postgresql for this and various other purposes, but that feature probably
wouldn't scale in terms of management (it requires 100% cooperation amongst
all apps/clients involved).

There's no "select * from table where pkey=x for insert;" which would block
on uncommitted inserts/updates of pkey=x and other selects for insert/update.

In contrast "select ... for update" blocks on committed stuff.

Regards,
Link.

At 09:55 AM 3/27/03 +0800, Christopher Kings-Lynne wrote:

>Hi Guys,
>
>I just thought I'd share with you guys a very clever solution to the old
>'update row.  if no rows affected, then insert the row' race condition
>problem.  A guy at my work came up with it.
>
>We were discussing this earlier on -hackers, but no-one could find a
>solution that didn't involve locking the entire table around the
>update...insert commands.
>
>The problem is that sometimes the row will be inserted by another process
>between your update and insert, causing your insert to fail with a unique
>constraint violation.
>
>So, say this is the insert:
>
>INSERT INTO table VALUES (1, 'foo');  // 1 is in the primary key column
>
>Rewrite it like this:
>
>INSERT INTO table SELECT 1, 'foo' EXCEPT SELECT 1, 'foo' FROM table WHERE
>pkcol=1;
>
>See? So now that INSERT statement will insert the row if it doesn't exist,
>or insert zero rows if it does.  You are then guaranteed that your
>transaction will not fail and rollback, so you can repeat your update, or do
>the insert first and then the update, etc.
>
>Hope that's handy for people,
>
>Chris
>
>
>---------------------------(end of broadcast)---------------------------
>TIP 1: subscribe and unsubscribe commands go to majordomo@postgresql.org


Re: Solution to UPDATE...INSERT problem

From
Dennis Gearon
Date:
so the only real solution to this now is in application code outside of a
transatction, i.e. PHP,Perl,VB,C,Python, etc, right?

Lincoln Yeoh wrote:
> AFAIK the "except" select won't see other inserts in uncommitted
> transactions. If those transactions are committed you will end up with
> the same problem. You can try it yourself, by manually doing two
> separate transactions in psql.
>
> You either have to lock the whole table, or lock at the application
> layer. Some time back I suggested a "lock on arbitrary string" feature
> for postgresql for this and various other purposes, but that feature
> probably wouldn't scale in terms of management (it requires 100%
> cooperation amongst all apps/clients involved).
>
> There's no "select * from table where pkey=x for insert;" which would
> block on uncommitted inserts/updates of pkey=x and other selects for
> insert/update.
>
> In contrast "select ... for update" blocks on committed stuff.
>
> Regards,
> Link.
>
> At 09:55 AM 3/27/03 +0800, Christopher Kings-Lynne wrote:
>
>> Hi Guys,
>>
>> I just thought I'd share with you guys a very clever solution to the old
>> 'update row.  if no rows affected, then insert the row' race condition
>> problem.  A guy at my work came up with it.
>>
>> We were discussing this earlier on -hackers, but no-one could find a
>> solution that didn't involve locking the entire table around the
>> update...insert commands.
>>
>> The problem is that sometimes the row will be inserted by another process
>> between your update and insert, causing your insert to fail with a unique
>> constraint violation.
>>
>> So, say this is the insert:
>>
>> INSERT INTO table VALUES (1, 'foo');  // 1 is in the primary key column
>>
>> Rewrite it like this:
>>
>> INSERT INTO table SELECT 1, 'foo' EXCEPT SELECT 1, 'foo' FROM table WHERE
>> pkcol=1;
>>
>> See? So now that INSERT statement will insert the row if it doesn't
>> exist,
>> or insert zero rows if it does.  You are then guaranteed that your
>> transaction will not fail and rollback, so you can repeat your update,
>> or do
>> the insert first and then the update, etc.
>>
>> Hope that's handy for people,
>>
>> Chris
>>
>>
>> ---------------------------(end of broadcast)---------------------------
>> TIP 1: subscribe and unsubscribe commands go to majordomo@postgresql.org
>
>
>
> ---------------------------(end of broadcast)---------------------------
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>
> http://archives.postgresql.org
>


Re: [HACKERS] Solution to UPDATE...INSERT problem

From
Haroldo Stenger
Date:
El jue, 27-03-2003 a las 03:41, Tom Lane escribió:
> "Christopher Kings-Lynne" <chriskl@familyhealth.com.au> writes:
> > INSERT INTO table SELECT 1, 'foo' EXCEPT SELECT 1, 'foo' FROM table WHERE
> > pkcol=1;
>
> > See? So now that INSERT statement will insert the row if it doesn't exist,
> > or insert zero rows if it does.  You are then guaranteed that your
> > transaction will not fail and rollback, so you can repeat your update, or do
> > the insert first and then the update, etc.
>
> Uh, why exactly do you think this is race-free?
>
> It looks fancy, but AFAICS the SELECT will return info that is correct
> as of its starting timestamp; which is not enough to guarantee that the
> INSERT won't conflict with another transaction doing the same thing
> concurrently.

This approach certainly reduces significantly the time span within which
a race could occur, compared to, say, using two separate statements, or
worse, two statements in two consecutive transactions. But race
conditions either exist or they don't, so you're right.

Now, up to my knowledge this problem was only a an intractable one in
PostgreSQL because of transactions going into abort state once a unique
restriction violation happened. If savepoints/nested transactions were
there, one would simply put the insert within a protected area, and
retry as much as needed.

That's my bet why other databases don't seem to have a problem with this
one. Am I right? Or do they have some magic solution other than locking
the whole table?

Regards
Haroldo


Re: Solution to UPDATE...INSERT problem

From
Lincoln Yeoh
Date:
At 05:28 PM 3/27/03 +0800, Christopher Kings-Lynne wrote:
> > There's no "select * from table where pkey=x for insert;" which would
>block
> > on uncommitted inserts/updates of pkey=x and other selects for
>insert/update.
>
>How about user locks?  Isn't there something in contrib/ for that???  I
>could do a userlock on the primary key, whether it existed or not?

Depends on your case, whether you can correctly convert your potential
primary keys into integers to be locked on.

It still requires full cooperation by all relevant apps/clients.

Actually select ... for updates also require cooperation, but it's a
standard way of doing things,  so apps that don't cooperate can be said to
be broken :).

Is there a standard for "select ... for insert"? Or lock table for insert
where pkey=x?

Regards,
Link.