Обсуждение: On duplicate ignore
Hi,
Ours is a web-based application. We're trying to implement ON DUPLICATE
IGNORE for one of our application table, named EMAILLIST. After a quick
Google search, I'm finding the following "easy & convenient" single SQL
statement syntax to follow with:
INSERT INTO EMAILLIST (EMAIL)
SELECT 'john@example.net'
WHERE NOT EXISTS (SELECT 1 FROM EMAILLIST WHERE EMAIL =
'john@example.net');
My question is, in a single threaded INSERT, this will *definitely* work.
Since ours is a web-based application, will this work out in a concurrent
multi-threaded environment too? In other words, will it be truly unique
when INSERT calls are concurrent?
Regards,
Gnanam
Just create a unique index on EMAIL column and handle error if it comes
Thanks,
Atul Goel
-----Original Message-----
From: pgsql-general-owner@postgresql.org [mailto:pgsql-general-owner@postgresql.org] On Behalf Of Gnanakumar
Sent: 18 January 2012 11:04
To: pgsql-general@postgresql.org
Subject: [GENERAL] On duplicate ignore
Hi,
Ours is a web-based application. We're trying to implement ON DUPLICATE IGNORE for one of our application table, named
EMAILLIST. After a quick Google search, I'm finding the following "easy & convenient" single SQL statement syntax to
followwith:
INSERT INTO EMAILLIST (EMAIL)
SELECT 'john@example.net'
WHERE NOT EXISTS (SELECT 1 FROM EMAILLIST WHERE EMAIL = 'john@example.net');
My question is, in a single threaded INSERT, this will *definitely* work.
Since ours is a web-based application, will this work out in a concurrent multi-threaded environment too? In other
words,will it be truly unique when INSERT calls are concurrent?
Regards,
Gnanam
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> Just create a unique index on EMAIL column and handle error if it comes Thanks for your suggestion. Of course, I do understand that this could be enforced/imposed at the database-level at any time. But I'm trying to find out whether this could be solved at the application layer itself. Any thoughts/ideas?
Hey Gnanakumar,
2012/1/18 Gnanakumar <gnanam@zoniac.com>
// Dmitriy.> Just create a unique index on EMAIL column and handle error if it comesThanks for your suggestion. Of course, I do understand that this could be
enforced/imposed at the database-level at any time. But I'm trying to find
out whether this could be solved at the application layer itself. Any
thoughts/ideas?
Exactly at the application level you just need to ignore
an unique constraint violation error reported by the backend.
You may also wrap INSERT statement in the PL/pgSQL
function or in the DO statement and catch the exception
generated by the backend.
an unique constraint violation error reported by the backend.
You may also wrap INSERT statement in the PL/pgSQL
function or in the DO statement and catch the exception
generated by the backend.
No way you can make sure at application level. Think in sense of an uncommitted row and other session inserting at the samemoment in time. Thanks, Atul Goel -----Original Message----- From: Gnanakumar [mailto:gnanam@zoniac.com] Sent: 18 January 2012 12:59 To: Atul Goel; pgsql-general@postgresql.org Subject: RE: [GENERAL] On duplicate ignore > Just create a unique index on EMAIL column and handle error if it > comes Thanks for your suggestion. Of course, I do understand that this could be enforced/imposed at the database-level at anytime. But I'm trying to find out whether this could be solved at the application layer itself. Any thoughts/ideas? The information contained in this email is strictly confidential and for the use of the addressee only, unless otherwiseindicated. If you are not the intended recipient, please do not read, copy, use or disclose to others this messageor any attachment. Please also notify the sender by replying to this email or by telephone (+44 (0)20 7896 0011) andthen delete the email and any copies of it. Opinions, conclusions (etc) that do not relate to the official business ofthis company shall be understood as neither given nor endorsed by it. IG Group Holdings plc is a company registered inEngland and Wales under number 04677092. VAT registration number 761 2978 07. Registered Office: Cannon Bridge House, 25Dowgate Hill, London EC4R 2YA. Listed on the London Stock Exchange. Its subsidiaries IG Markets Limited and IG Index Limitedare authorised and regulated by the Financial Services Authority (IG Markets Limited FSA registration number 195355and IG Index Limited FSA registration number 114059).
* Gnanakumar: >> Just create a unique index on EMAIL column and handle error if it comes > > Thanks for your suggestion. Of course, I do understand that this could be > enforced/imposed at the database-level at any time. But I'm trying to find > out whether this could be solved at the application layer itself. Any > thoughts/ideas? If you use serializable transactions in PostgreSQL 9.1, you can implement such constraints in the application without additional locking. However, with concurrent writes and without an index, the rate of detected serialization violations and resulting transactions aborts will be high. -- Florian Weimer <fweimer@bfk.de> BFK edv-consulting GmbH http://www.bfk.de/ Kriegsstraße 100 tel: +49-721-96201-1 D-76133 Karlsruhe fax: +49-721-96201-99
On Thu, Jan 19, 2012 at 7:54 AM, Florian Weimer <fweimer@bfk.de> wrote: > * Gnanakumar: > >>> Just create a unique index on EMAIL column and handle error if it comes >> >> Thanks for your suggestion. Of course, I do understand that this could be >> enforced/imposed at the database-level at any time. But I'm trying to find >> out whether this could be solved at the application layer itself. Any >> thoughts/ideas? > > If you use serializable transactions in PostgreSQL 9.1, you can > implement such constraints in the application without additional > locking. However, with concurrent writes and without an index, the rate > of detected serialization violations and resulting transactions aborts > will be high. No, you sadly can't. PostgreSQL doesn't yet support proper predicate locking to allow the application to be sure that the OP's original statement, and ones like it, don't have a race condition. A unique index is the only way to be sure.
On Thu, Jan 19, 2012 at 9:49 AM, Scott Marlowe <scott.marlowe@gmail.com> wrote: > On Thu, Jan 19, 2012 at 7:54 AM, Florian Weimer <fweimer@bfk.de> wrote: >> * Gnanakumar: >> >>>> Just create a unique index on EMAIL column and handle error if it comes >>> >>> Thanks for your suggestion. Of course, I do understand that this could be >>> enforced/imposed at the database-level at any time. But I'm trying to find >>> out whether this could be solved at the application layer itself. Any >>> thoughts/ideas? >> >> If you use serializable transactions in PostgreSQL 9.1, you can >> implement such constraints in the application without additional >> locking. However, with concurrent writes and without an index, the rate >> of detected serialization violations and resulting transactions aborts >> will be high. > > No, you sadly can't. PostgreSQL doesn't yet support proper predicate > locking to allow the application to be sure that the OP's original > statement, and ones like it, don't have a race condition. A unique > index is the only way to be sure. Wait, did 9.1 implement proper predicate locking to allow this? If so I apologize for being out of the loop on the new versions.
* Scott Marlowe: > On Thu, Jan 19, 2012 at 7:54 AM, Florian Weimer <fweimer@bfk.de> wrote: >> * Gnanakumar: >> >>>> Just create a unique index on EMAIL column and handle error if it comes >>> >>> Thanks for your suggestion. Of course, I do understand that this could be >>> enforced/imposed at the database-level at any time. But I'm trying to find >>> out whether this could be solved at the application layer itself. Any >>> thoughts/ideas? >> >> If you use serializable transactions in PostgreSQL 9.1, you can >> implement such constraints in the application without additional >> locking. However, with concurrent writes and without an index, the rate >> of detected serialization violations and resulting transactions aborts >> will be high. > > No, you sadly can't. PostgreSQL doesn't yet support proper predicate > locking to allow the application to be sure that the OP's original > statement, and ones like it, don't have a race condition. A unique > index is the only way to be sure. Huh? This was one of the major new features in PostgreSQL 9.1. -- Florian Weimer <fweimer@bfk.de> BFK edv-consulting GmbH http://www.bfk.de/ Kriegsstraße 100 tel: +49-721-96201-1 D-76133 Karlsruhe fax: +49-721-96201-99
At 10:54 PM 1/19/2012, Florian Weimer wrote: >* Gnanakumar: > > >> Just create a unique index on EMAIL column and handle error if it comes > > > > Thanks for your suggestion. Of course, I do understand that this could be > > enforced/imposed at the database-level at any time. But I'm trying to find > > out whether this could be solved at the application layer itself. Any > > thoughts/ideas? > >If you use serializable transactions in PostgreSQL 9.1, you can >implement such constraints in the application without additional >locking. However, with concurrent writes and without an index, the rate >of detected serialization violations and resulting transactions aborts >will be high. Would writing application-side code to handle those transaction aborts in 9.1 be much easier than writing code to handle transaction aborts/DB exceptions due to unique constraint violations? These transaction aborts have to be handled differently (e.g. retried for X seconds/Y tries) from other sort of transaction aborts (not retried). Otherwise I don't see the benefit of this feature for this scenario. Unless of course you get significantly better performance by not having a unique constraint. If insert performance is not an issue and code simplicity is preferred, one could lock the table (with an exclusive lock mode), then do the selects and inserts, that way your code can assume that any transaction aborts are due to actual problems rather than concurrency. Which often means less code to write :). Regards, Link.
* Lincoln Yeoh: >>If you use serializable transactions in PostgreSQL 9.1, you can >>implement such constraints in the application without additional >>locking. However, with concurrent writes and without an index, the rate >>of detected serialization violations and resulting transactions aborts >>will be high. > > Would writing application-side code to handle those transaction aborts > in 9.1 be much easier than writing code to handle transaction > aborts/DB exceptions due to unique constraint violations? These > transaction aborts have to be handled differently (e.g. retried for X > seconds/Y tries) from other sort of transaction aborts (not retried). There's a separate error code, so it's easier to deal with in theory. However, I don't think that's sufficient justification for removing the unique constraints. > Otherwise I don't see the benefit of this feature for this > scenario. Unless of course you get significantly better performance by > not having a unique constraint. Performance is worse. > If insert performance is not an issue and code simplicity is > preferred, one could lock the table (with an exclusive lock mode), > then do the selects and inserts, that way your code can assume that > any transaction aborts are due to actual problems rather than > concurrency. Which often means less code to write :). Choosing the right lock is a bit tricky because you usually want to block INSERTs only. Explicit locks on a hash of the unique column, using pg_advisory_xact_lock, are often an alternative. -- Florian Weimer <fweimer@bfk.de> BFK edv-consulting GmbH http://www.bfk.de/ Kriegsstraße 100 tel: +49-721-96201-1 D-76133 Karlsruhe fax: +49-721-96201-99
At 04:27 PM 1/20/2012, Florian Weimer wrote: >* Lincoln Yeoh: > > >>If you use serializable transactions in PostgreSQL 9.1, you can > >>implement such constraints in the application without additional > >>locking. However, with concurrent writes and without an index, the rate > >>of detected serialization violations and resulting transactions aborts > >>will be high. > > > > Would writing application-side code to handle those transaction aborts > > in 9.1 be much easier than writing code to handle transaction > > aborts/DB exceptions due to unique constraint violations? These > > transaction aborts have to be handled differently (e.g. retried for X > > seconds/Y tries) from other sort of transaction aborts (not retried). > >There's a separate error code, so it's easier to deal with in theory. Is there a simple way to get postgresql to retry a transaction, or does the application have to actually reissue all the necessary statements again? I'd personally prefer to use locking and selects to avoid transaction aborts whether due to unique constraint violations or due to serialization violations. But I'm lazy ;). Regards, Link.
* Lincoln Yeoh: > Is there a simple way to get postgresql to retry a transaction, or > does the application have to actually reissue all the necessary > statements again? The application has to re-run the transaction, which might result in the execution of different statements. In the INSERT-or-UPDATE case, the new attempt will have to use an UPDATE instead of an INSERT, so replying the statements verbatim will not work. > I'd personally prefer to use locking and selects to avoid transaction > aborts whether due to unique constraint violations or due to > serialization violations. Once you address the restart issue, transactional code is simpler and easier to check for correctness. Restarting transactions has other benefits, too. For instance, you can restart your PostgreSQL server process, and your applications will just keep running. -- Florian Weimer <fweimer@bfk.de> BFK edv-consulting GmbH http://www.bfk.de/ Kriegsstraße 100 tel: +49-721-96201-1 D-76133 Karlsruhe fax: +49-721-96201-99