Обсуждение: Locking entire database
Hello, Is there some way of locking all database tables in a transaction without knowing their names or even better just locking the entire database? I know this is bad tactics but there is a specific case where i need it. Can it be done? Thank you Panagiotis
On 9/14/07, Panagiotis Pediaditis <pped@ics.forth.gr> wrote: > ... there is a specific case where i need it. Don't really know, but, explain what the case is, and maybe someone could help you.
Well the problem is I am working on rdf query engine for persistent RDF data. The data is stored/structured in a specific way in the database. When i perform updates in parallel, because there are cross table dependencies, I end up with inconsistencies, For example One transaction reads to see if there is a resource so as to add a property where it is a subject. Then an other transaction deletes the resource after the first has decided that the resource is there but before it added the property. Thus it would be helpful for me to avoid the difficult task of dependency based locking and just lock the whole database. any ideas? Rodrigo De León wrote: > On 9/14/07, Panagiotis Pediaditis <pped@ics.forth.gr> wrote: > >> ... there is a specific case where i need it. >> > > Don't really know, but, explain what the case is, and maybe someone > could help you. >
On 9/14/07, Panagiotis Pediaditis <pped@ics.forth.gr> wrote: > Hello, > > Is there some way of locking all database tables in a transaction > without knowing their names > or even better just locking the entire database? I know this is bad > tactics but there is a specific > case where i need it. Can it be done? > AFAIK Locking the entire database may not be an option in postgresql. However you can virtually restrict access to everyone to a particular database via pg_hba.conf. More details here http://www.postgresql.org/docs/8.2/static/auth-pg-hba-conf.html regards, -- Sibte Abbas
On Fri, Sep 14, 2007 at 05:45:07PM +0300, Panagiotis Pediaditis wrote: > Well the problem is I am working on rdf query engine for persistent RDF > data. The data is stored/structured in a specific way in the database. > When i perform updates in parallel, because there are cross table > dependencies, I end up with inconsistencies, For example One transaction > reads to see if there is a resource so as to add a property where it is > a subject. Then an other transaction deletes the resource after the > first has decided that the resource is there but before it added the > property. Sounds like what you need is serializable transactions. Then the server will tell you if something conflicts. Have a ncie day, -- Martijn van Oosterhout <kleptog@svana.org> http://svana.org/kleptog/ > From each according to his ability. To each according to his ability to litigate.
Вложения
Panagiotis Pediaditis, 14.09.2007 16:45: > Well the problem is I am working on rdf query engine for persistent RDF > data. The data is stored/structured in a specific way in the database. > When i perform updates in parallel, because there are cross table > dependencies, I end up with inconsistencies, For example One transaction > reads to see if there is a resource so as to add a property where it is > a subject. Then an other transaction deletes the resource after the > first has decided that the resource is there but before it added the > property. > Thus it would be helpful for me to avoid the difficult task of > dependency based locking and just lock the whole database. > any ideas? Hmm. To me this sounds like all those steps should in fact be _one_ transaction and not several transactions. Thomas
A simpler example,
In the context of one transaction i do many queries of the form
INSERT INTO table value WHERE value NOT IN TABLE;
If i have 2 processes running the same 100s of these at the same time i
end up with duplicates.
Even with isolation set to serializable
any ideas?
thnx
Panagiotis
Thomas Kellerer wrote:
> Panagiotis Pediaditis, 14.09.2007 16:45:
>> Well the problem is I am working on rdf query engine for persistent
>> RDF data. The data is stored/structured in a specific way in the
>> database. When i perform updates in parallel, because there are cross
>> table dependencies, I end up with inconsistencies, For example One
>> transaction reads to see if there is a resource so as to add a
>> property where it is a subject. Then an other transaction deletes the
>> resource after the first has decided that the resource is there but
>> before it added the property.
>> Thus it would be helpful for me to avoid the difficult task of
>> dependency based locking and just lock the whole database.
>> any ideas?
>
> Hmm. To me this sounds like all those steps should in fact be _one_
> transaction and not several transactions.
>
> Thomas
>
>
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On 9/14/07, Panagiotis Pediaditis <pped@ics.forth.gr> wrote: > A simpler example, > In the context of one transaction i do many queries of the form > INSERT INTO table value WHERE value NOT IN TABLE; > > If i have 2 processes running the same 100s of these at the same time i > end up with duplicates. > Even with isolation set to serializable > any ideas? Unique index?
Thats the fun part, I actually need to allow duplicates in specific cases but not in this one :) Shouldn't the serializable level prevent these duplicates? As I understand it serializable should give the same result as if the transactions were performed the one after the other. Thnx Panagiotis On Fri, 14 Sep 2007, Scott Marlowe wrote: > On 9/14/07, Panagiotis Pediaditis <pped@ics.forth.gr> wrote: > > A simpler example, > > In the context of one transaction i do many queries of the form > > INSERT INTO table value WHERE value NOT IN TABLE; > > > > If i have 2 processes running the same 100s of these at the same time i > > end up with duplicates. > > Even with isolation set to serializable > > any ideas? > > Unique index? >
-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On 09/15/07 03:28, Panagiwths Pediadiths wrote: > Thats the fun part, I actually need to allow duplicates in specific cases > but not in this one :) Same table? > Shouldn't the serializable level prevent these duplicates? As I understand > it serializable > should give the same result as if the transactions were performed the one > after the other. (Please don't top-post.) Seems to me that you are confused as to the "essence" of relational databases. In other words, the best (heck, even the acceptable) way to design schemas, and how to control the flow of data in order to achieve your ultimate "data" goal. > > On Fri, 14 Sep 2007, Scott Marlowe wrote: > >> On 9/14/07, Panagiotis Pediaditis <pped@ics.forth.gr> wrote: >>> A simpler example, >>> In the context of one transaction i do many queries of the form >>> INSERT INTO table value WHERE value NOT IN TABLE; >>> >>> If i have 2 processes running the same 100s of these at the same time i >>> end up with duplicates. >>> Even with isolation set to serializable >>> any ideas? >> Unique index? - -- Ron Johnson, Jr. Jefferson LA USA Give a man a fish, and he eats for a day. Hit him with a fish, and he goes away for good! -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.6 (GNU/Linux) iD8DBQFG653ZS9HxQb37XmcRAjV+AJsFoJKc79XiGLFWSOT8Kfs0kxQItQCfWcJp syO91mlpB6+P+n5tWh0fGSc= =t8pG -----END PGP SIGNATURE-----
On Fri, Sep 14, 2007 at 04:48:08PM +0300, Panagiotis Pediaditis wrote: > Is there some way of locking all database tables in a transaction > without knowing their names > or even better just locking the entire database? I know this is bad > tactics but there is a specific > case where i need it. Can it be done? there is no such thing, but you can use "advisory locks" to this purpose. depesz -- quicksil1er: "postgres is excellent, but like any DB it requires a highly paid DBA. here's my CV!" :) http://www.depesz.com/ - blog dla ciebie (i moje CV)
On 9/15/07, hubert depesz lubaczewski <depesz@depesz.com> wrote: > On Fri, Sep 14, 2007 at 04:48:08PM +0300, Panagiotis Pediaditis wrote: > > Is there some way of locking all database tables in a transaction > > without knowing their names > > or even better just locking the entire database? I know this is bad > > tactics but there is a specific > > case where i need it. Can it be done? > > there is no such thing, but you can use "advisory locks" to this > purpose. that would require taking a shared advisory lock for every transaction...pretty big price to pay imo, but it would work. I thought about it for a bit and I don't know if there is any other way to do it. merlin
On Sat, 15 Sep 2007, Ron Johnson wrote: > -----BEGIN PGP SIGNED MESSAGE----- > Hash: SHA1 > > On 09/15/07 03:28, Panagiwths Pediadiths wrote: > > Thats the fun part, I actually need to allow duplicates in specific cases > > but not in this one :) > > Same table? Yup > > > Shouldn't the serializable level prevent these duplicates? As I understand > > it serializable > > should give the same result as if the transactions were performed the one > > after the other. > > (Please don't top-post.) > > Seems to me that you are confused as to the "essence" of relational > databases. In other words, the best (heck, even the acceptable) way > to design schemas, and how to control the flow of data in order to > achieve your ultimate "data" goal. > I dont see why the case i suggest is so obscene More specifically consider a table with to columns where the unique index is the two columns together However at some stage of the application I want to insert into the database only if there is no element with a value at column 1 equal to that that i intend to insert. Oddly, in serializable isolation mode, two transactions performing such an insertion in parallel one of the two transaction hits the phantom read case, whereas it should be protected by the isolation level. > > > > On Fri, 14 Sep 2007, Scott Marlowe wrote: > > > >> On 9/14/07, Panagiotis Pediaditis <pped@ics.forth.gr> wrote: > >>> A simpler example, > >>> In the context of one transaction i do many queries of the form > >>> INSERT INTO table value WHERE value NOT IN TABLE; > >>> > >>> If i have 2 processes running the same 100s of these at the same time i > >>> end up with duplicates. > >>> Even with isolation set to serializable > >>> any ideas? > >> Unique index? > > > - -- > Ron Johnson, Jr. > Jefferson LA USA > > Give a man a fish, and he eats for a day. > Hit him with a fish, and he goes away for good! > > -----BEGIN PGP SIGNATURE----- > Version: GnuPG v1.4.6 (GNU/Linux) > > iD8DBQFG653ZS9HxQb37XmcRAjV+AJsFoJKc79XiGLFWSOT8Kfs0kxQItQCfWcJp > syO91mlpB6+P+n5tWh0fGSc= > =t8pG > -----END PGP SIGNATURE----- > > ---------------------------(end of broadcast)--------------------------- > TIP 5: don't forget to increase your free space map settings >
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Hash: SHA1
On 09/15/07 19:59, Panagiwths Pediadiths wrote:
>
> On Sat, 15 Sep 2007, Ron Johnson wrote:
>
> On 09/15/07 03:28, Panagiwths Pediadiths wrote:
>>>> Thats the fun part, I actually need to allow duplicates in specific cases
>>>> but not in this one :)
> Same table?
>> Yup
>
>>>> Shouldn't the serializable level prevent these duplicates? As I understand
>>>> it serializable
>>>> should give the same result as if the transactions were performed the one
>>>> after the other.
> (Please don't top-post.)
>
> Seems to me that you are confused as to the "essence" of relational
> databases. In other words, the best (heck, even the acceptable) way
> to design schemas, and how to control the flow of data in order to
> achieve your ultimate "data" goal.
>
>
>> I dont see why the case i suggest is so obscene
Then you have not explained it to us clearly.
(English language difficulties *are* an acceptable excuse...)
>> More specifically consider a table with to columns where the unique index
>> is the two columns together
>
>> However at some stage of the application I want to insert into the
>> database only if there is no element
>> with a value at column 1 equal to that that i intend to insert.
>
>> Oddly, in serializable isolation mode, two transactions performing such an
>> insertion in parallel one of the
>> two transaction hits the phantom read case, whereas it should be protected
>> by the isolation level.
It should, *if* you do it properly.
IOW, is your program structured like:
BEGIN
SELECT COUNT(*) INTO :cnt
FROM rdf WHERE segment_1 = :some_val;
IF :cnt == 1 THEN
do one thing
ELSE
do another
END IF;
COMMIT;
or is it structured:
BEGIN
SELECT COUNT(*) INTO :cnt
FROM rdf WHERE segment_1 = :some_val;
COMMIT;
BEGIN
IF :cnt == 1 THEN
do one thing
ELSE
do another
END IF;
COMMIT;
>>>> On Fri, 14 Sep 2007, Scott Marlowe wrote:
>>>>
>>>>> On 9/14/07, Panagiotis Pediaditis <pped@ics.forth.gr> wrote:
>>>>>> A simpler example,
>>>>>> In the context of one transaction i do many queries of the form
>>>>>> INSERT INTO table value WHERE value NOT IN TABLE;
>>>>>>
>>>>>> If i have 2 processes running the same 100s of these at the same time i
>>>>>> end up with duplicates.
>>>>>> Even with isolation set to serializable
>>>>>> any ideas?
>>>>> Unique index?
- --
Ron Johnson, Jr.
Jefferson LA USA
Give a man a fish, and he eats for a day.
Hit him with a fish, and he goes away for good!
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On Sat, 15 Sep 2007, Ron Johnson wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
> On 09/15/07 19:59, Panagiwths Pediadiths wrote:
> >
> > On Sat, 15 Sep 2007, Ron Johnson wrote:
> >
> > On 09/15/07 03:28, Panagiwths Pediadiths wrote:
> >>>> Thats the fun part, I actually need to allow duplicates in specific cases
> >>>> but not in this one :)
> > Same table?
> >> Yup
> >
> >>>> Shouldn't the serializable level prevent these duplicates? As I understand
> >>>> it serializable
> >>>> should give the same result as if the transactions were performed the one
> >>>> after the other.
> > (Please don't top-post.)
> >
> > Seems to me that you are confused as to the "essence" of relational
> > databases. In other words, the best (heck, even the acceptable) way
> > to design schemas, and how to control the flow of data in order to
> > achieve your ultimate "data" goal.
> >
> >
> >> I dont see why the case i suggest is so obscene
>
> Then you have not explained it to us clearly.
>
> (English language difficulties *are* an acceptable excuse...)
>
> >> More specifically consider a table with to columns where the unique index
> >> is the two columns together
> >
> >> However at some stage of the application I want to insert into the
> >> database only if there is no element
> >> with a value at column 1 equal to that that i intend to insert.
> >
> >> Oddly, in serializable isolation mode, two transactions performing such an
> >> insertion in parallel one of the
> >> two transaction hits the phantom read case, whereas it should be protected
> >> by the isolation level.
>
> It should, *if* you do it properly.
>
> IOW, is your program structured like:
> BEGIN
> SELECT COUNT(*) INTO :cnt
> FROM rdf WHERE segment_1 = :some_val;
> IF :cnt == 1 THEN
> do one thing
> ELSE
> do another
> END IF;
> COMMIT;
>
> or is it structured:
> BEGIN
> SELECT COUNT(*) INTO :cnt
> FROM rdf WHERE segment_1 = :some_val;
> COMMIT;
> BEGIN
> IF :cnt == 1 THEN
> do one thing
> ELSE
> do another
> END IF;
> COMMIT;
Everything is done in the context of one transaction e.g.
BEGIN
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE
INSERT INTO table SELECT somwhere.value1, somewhere.value2 FROM
somewhere WHERE somewhere.value1 NOT IN ( SELECT table.segment1 FROM
table)
END
Many transactions doing this in parallel end up inserting the value many
times. Could i be missing something regarding how to set up the isolation
level?
Thanks!
>
> >>>> On Fri, 14 Sep 2007, Scott Marlowe wrote:
> >>>>
> >>>>> On 9/14/07, Panagiotis Pediaditis <pped@ics.forth.gr> wrote:
> >>>>>> A simpler example,
> >>>>>> In the context of one transaction i do many queries of the form
> >>>>>> INSERT INTO table value WHERE value NOT IN TABLE;
> >>>>>>
> >>>>>> If i have 2 processes running the same 100s of these at the same time i
> >>>>>> end up with duplicates.
> >>>>>> Even with isolation set to serializable
> >>>>>> any ideas?
> >>>>> Unique index?
>
> - --
> Ron Johnson, Jr.
> Jefferson LA USA
>
> Give a man a fish, and he eats for a day.
> Hit him with a fish, and he goes away for good!
>
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>
> iD8DBQFG7IaUS9HxQb37XmcRAq/bAJwNlJG2BNqfTbXPxd2sa6GsQn3nwQCfXaDo
> BMR4Lple09XnPB5w11geonY=
> =g8lJ
> -----END PGP SIGNATURE-----
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>
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On 9/15/07, Panagiwths Pediadiths <pped@ics.forth.gr> wrote: > Shouldn't the serializable level prevent these duplicates? As I understand > it serializable > should give the same result as if the transactions were performed the one > after the other. http://www.postgresql.org/docs/8.2/static/transaction-iso.html#MVCC-SERIALIZABILITY
On Sun, Sep 16, 2007 at 01:46:44PM +0300, Panagiwths Pediadiths wrote: > Many transactions doing this in parallel end up inserting the value many > times. Could i be missing something regarding how to set up the isolation > level? > Thanks! No, the only easy way you can guarentee you won't insert duplicates is with a unique index. The reason is that even under serialisable mode your algorithm can produce duplicates, because postgres doesn't do predicate locking. The reason unique indexes can do it is because they have special locking requirements that, when met, guarentee the result. If for some reason you can't use a unique index (can't think of one, but perhaps) then you need to implement this locking yourself. The easiest way is to have one lock and take it before running your procedure. For more concurrency you can partition them (unique indexes can be very efficient about this because they can see rows that you can't; they go outside the normal visibility checks). Have a nice day, -- Martijn van Oosterhout <kleptog@svana.org> http://svana.org/kleptog/ > From each according to his ability. To each according to his ability to litigate.