Thread: the XID question
after i backdb->dropdb->restoredb and then vacuum analy+full -> vacuum freeze the XID had been increased by 4 billion in two weeks...is it noraml? what's the definetion of XID? " select * from mybook" SQL command also increase the XID ? reference: http://www.postgresql.org/docs/9.0/static/routine-vacuuming.html
On 1月19日, 下午5時19分, "Charles.Hou" <giveme...@gmail.com> wrote: > after i backdb->dropdb->restoredb and then vacuum analy+full -> vacuum > freeze > > the XID had been increased by 4 billion in two weeks...is it noraml? > > what's the definetion of XID? > > " select * from mybook" SQL command also increase the XID ? > > reference:http://www.postgresql.org/docs/9.0/static/routine-vacuuming.html sorry... not 4 billion , is 4 hundred million
2011/1/19 Charles.Hou <givemeegn@gmail.com>: > what's the definetion of XID? XID == "Transaction ID". > " select * from mybook" SQL command also increase the XID ? Yes. Single SELECT is a transaction. Hence, it needs a transaction ID. greets, Filip
Filip Rembia*kowski<plk.zuber@gmail.com> wrote: > 2011/1/19 Charles.Hou <givemeegn@gmail.com>: >> " select * from mybook" SQL command also increase the XID ? > > Yes. Single SELECT is a transaction. Hence, it needs a transaction > ID. No, not in recent versions of PostgreSQL. There's virtual transaction ID, too; which is all that's needed unless the transaction writes something. Also, as a fine point, if you use explicit database transactions (with BEGIN or START TRANSACTION) then you normally get one XID for the entire transaction, unless you use SAVEPOINTs. -Kevin
Kevin.Grittner@wicourts.gov ("Kevin Grittner") writes:
> Filip Rembia*kowski<plk.zuber@gmail.com> wrote:
>> 2011/1/19 Charles.Hou <givemeegn@gmail.com>:
>
>>> " select * from mybook" SQL command also increase the XID ?
>>
>> Yes. Single SELECT is a transaction. Hence, it needs a transaction
>> ID.
>
> No, not in recent versions of PostgreSQL. There's virtual
> transaction ID, too; which is all that's needed unless the
> transaction writes something.
>
> Also, as a fine point, if you use explicit database transactions
> (with BEGIN or START TRANSACTION) then you normally get one XID for
> the entire transaction, unless you use SAVEPOINTs.
Erm, "not *necessarily* in recent versions of PostgreSQL."
A read-only transaction won't consume XIDs, but if you don't expressly
declare it read-only, they're still liable to get eaten...
--
(format nil "~S@~S" "cbbrowne" "gmail.com")
http://www3.sympatico.ca/cbbrowne/lisp.html
Parenthesize to avoid ambiguity.
On Wednesday, January 19, 2011 07:06:58 PM Chris Browne wrote:
> Kevin.Grittner@wicourts.gov ("Kevin Grittner") writes:
> > Filip Rembia*kowski<plk.zuber@gmail.com> wrote:
> >> 2011/1/19 Charles.Hou <givemeegn@gmail.com>:
> >>> " select * from mybook" SQL command also increase the XID ?
> >>
> >> Yes. Single SELECT is a transaction. Hence, it needs a transaction
> >> ID.
> >
> > No, not in recent versions of PostgreSQL. There's virtual
> > transaction ID, too; which is all that's needed unless the
> > transaction writes something.
> >
> > Also, as a fine point, if you use explicit database transactions
> > (with BEGIN or START TRANSACTION) then you normally get one XID for
> > the entire transaction, unless you use SAVEPOINTs.
>
> Erm, "not *necessarily* in recent versions of PostgreSQL."
>
> A read-only transaction won't consume XIDs, but if you don't expressly
> declare it read-only, they're still liable to get eaten...
No. The Xid is generally only allocated at the first place a real xid is
needed. See GetCurrentTransactionId, AssignTransactionId in xact.c and the
caller of the former.
Andres
Andres Freund <andres@anarazel.de> wrote: > On Wednesday, January 19, 2011 07:06:58 PM Chris Browne wrote: >> A read-only transaction won't consume XIDs, but if you don't >> expressly declare it read-only, they're still liable to get >> eaten... > No. The Xid is generally only allocated at the first place a real > xid is needed. See GetCurrentTransactionId, AssignTransactionId in > xact.c and the caller of the former. Or just test it in psql. BEGIN, run your query, look at pg_locks. If an xid has been assigned, you'll see it there in the transactionid column. You can easily satisfy yourself which statements grab an xid.... -Kevin
Kevin Grittner wrote: > Or just test it in psql. BEGIN, run your query, look at pg_locks. > If an xid has been assigned, you'll see it there in the > transactionid column. You can easily satisfy yourself which > statements grab an xid... That's a good way to double-check exactly what's happening, but it's not even that hard: gsmith=# select txid_current(); txid_current | 696 gsmith=# select 1; ?column? | 1 gsmith=# select 1; ?column? | 1 gsmith=# select txid_current(); txid_current | 697 Calling txid_current bumps the number up, but if you account for that you can see whether the thing(s) in the middle grabbed a real txid by whether the count increased by 1 or more than that. So here's what one that did get a real xid looks like: gsmith=# select txid_current(); txid_current | 702 gsmith=# insert into t(i) values(1); INSERT 0 1 gsmith=# select txid_current(); txid_current | 704 That proves the INSERT in the middle was assigned one. The commit message that added this feature to 8.3 has a good quick intro to what changed from earlier revs: http://archives.postgresql.org/pgsql-committers/2007-09/msg00026.php Don't have to actually read the source to learn a bit more, because it's actually documented! Mechanics are described at pgsql/src/backend/access/transam/README ; you need to know a bit more about subtransactions to follow all of it, but it gets the general idea across regardless: = Transaction and Subtransaction Numbering = Transactions and subtransactions are assigned permanent XIDs only when/if they first do something that requires one --- typically, insert/update/delete a tuple, though there are a few other places that need an XID assigned. If a subtransaction requires an XID, we always first assign one to its parent. This maintains the invariant that child transactions have XIDs later than their parents, which is assumed in a number of places. The subsidiary actions of obtaining a lock on the XID and and entering it into pg_subtrans and PG_PROC are done at the time it is assigned. A transaction that has no XID still needs to be identified for various purposes, notably holding locks. For this purpose we assign a "virtual transaction ID" or VXID to each top-level transaction. VXIDs are formed from two fields, the backendID and a backend-local counter; this arrangement allows assignment of a new VXID at transaction start without any contention for shared memory. To ensure that a VXID isn't re-used too soon after backend exit, we store the last local counter value into shared memory at backend exit, and initialize it from the previous value for the same backendID slot at backend start. All these counters go back to zero at shared memory re-initialization, but that's OK because VXIDs never appear anywhere on-disk. Internally, a backend needs a way to identify subtransactions whether or not they have XIDs; but this need only lasts as long as the parent top transaction endures. Therefore, we have SubTransactionId, which is somewhat like CommandId in that it's generated from a counter that we reset at the start of each top transaction. The top-level transaction itself has SubTransactionId 1, and subtransactions have IDs 2 and up. (Zero is reserved for InvalidSubTransactionId.) Note that subtransactions do not have their own VXIDs; they use the parent top transaction's VXID. -- Greg Smith 2ndQuadrant US greg@2ndQuadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books
On 1月19日, 下午10時39分, Kevin.Gritt...@wicourts.gov ("Kevin Grittner")
wrote:
> Filip Rembia*kowski<plk.zu...@gmail.com> wrote:
> > 2011/1/19 Charles.Hou <giveme...@gmail.com>:
> >> " select * from mybook" SQL command also increase the XID ?
>
> > Yes. Single SELECT is a transaction. Hence, it needs a transaction
> > ID.
>
> No, not in recent versions of PostgreSQL. There's virtual
> transaction ID, too; which is all that's needed unless the
> transaction writes something.
>
my postgresql version is 8.1.3
you means the newer version has a virtual transaction ID. and what's
the maxmium of this virtual id, also 4 billion ?
should i also vacuum freeze the virtual id in the new version when it
reached the 4 billion?
> Also, as a fine point, if you use explicit database transactions
> (with BEGIN or START TRANSACTION) then you normally get one XID for
> the entire transaction, unless you use SAVEPOINTs.
>
> -Kevin
>
> --
> Sent via pgsql-performance mailing list (pgsql-performa...@postgresql.org)
> To make changes to your subscription:http://www.postgresql.org/mailpref/pgsql-performance
On 1月20日, 上午6時46分, g...@2ndquadrant.com (Greg Smith) wrote: > Kevin Grittner wrote: > > Or just test it in psql. BEGIN, run your query, look at pg_locks. > > If an xid has been assigned, you'll see it there in the > > transactionid column. You can easily satisfy yourself which > > statements grab an xid... > > That's a good way to double-check exactly what's happening, but it's not > even that hard: > > gsmith=# select txid_current(); > txid_current | 696 > > gsmith=# select 1; > ?column? | 1 > > gsmith=# select 1; > ?column? | 1 > > gsmith=# select txid_current(); > txid_current | 697 > > Calling txid_current bumps the number up, but if you account for that > you can see whether the thing(s) in the middle grabbed a real txid by > whether the count increased by 1 or more than that. So here's what one > that did get a real xid looks like: > > gsmith=# select txid_current(); > txid_current | 702 > > gsmith=# insert into t(i) values(1); > INSERT 0 1 > gsmith=# select txid_current(); > txid_current | 704 > > That proves the INSERT in the middle was assigned one. > > The commit message that added this feature to 8.3 has a good quick intro > to what changed from earlier revs:http://archives.postgresql.org/pgsql-committers/2007-09/msg00026.php > > Don't have to actually read the source to learn a bit more, because it's > actually documented! Mechanics are described at > pgsql/src/backend/access/transam/README ; you need to know a bit more > about subtransactions to follow all of it, but it gets the general idea > across regardless: > > = Transaction and Subtransaction Numbering = > > Transactions and subtransactions are assigned permanent XIDs only when/if > they first do something that requires one --- typically, > insert/update/delete > a tuple, though there are a few other places that need an XID assigned. > If a subtransaction requires an XID, we always first assign one to its > parent. This maintains the invariant that child transactions have XIDs > later > than their parents, which is assumed in a number of places. > > The subsidiary actions of obtaining a lock on the XID and and entering > it into > pg_subtrans and PG_PROC are done at the time it is assigned. > > A transaction that has no XID still needs to be identified for various > purposes, notably holding locks. For this purpose we assign a "virtual > transaction ID" or VXID to each top-level transaction. VXIDs are formed > from > two fields, the backendID and a backend-local counter; this arrangement > allows > assignment of a new VXID at transaction start without any contention for > shared memory. To ensure that a VXID isn't re-used too soon after backend > exit, we store the last local counter value into shared memory at backend > exit, and initialize it from the previous value for the same backendID slot > at backend start. All these counters go back to zero at shared memory > re-initialization, but that's OK because VXIDs never appear anywhere > on-disk. > > Internally, a backend needs a way to identify subtransactions whether or not > they have XIDs; but this need only lasts as long as the parent top > transaction > endures. Therefore, we have SubTransactionId, which is somewhat like > CommandId in that it's generated from a counter that we reset at the > start of > each top transaction. The top-level transaction itself has > SubTransactionId 1, > and subtransactions have IDs 2 and up. (Zero is reserved for > InvalidSubTransactionId.) Note that subtransactions do not have their > own VXIDs; they use the parent top transaction's VXID. > > -- > Greg Smith 2ndQuadrant US g...@2ndQuadrant.com Baltimore, MD > PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us > "PostgreSQL 9.0 High Performance":http://www.2ndQuadrant.com/books > > -- > Sent via pgsql-performance mailing list (pgsql-performa...@postgresql.org) > To make changes to your subscription:http://www.postgresql.org/mailpref/pgsql-performance every time, i execute this query string "SELECT datname, age(datfrozenxid), FROM pg_database;" in the sql query of pgAdminIII , the age will be increased by 5 , not 1. why???
"Charles.Hou" <givemeegn@gmail.com> wrote: > my postgresql version is 8.1.3 Ouch! That's getting pretty old; I hope it's not on Windows. http://wiki.postgresql.org/wiki/PostgreSQL_Release_Support_Policy http://www.postgresql.org/about/news.865 > you means the newer version has a virtual transaction ID. and > what's the maxmium of this virtual id, also 4 billion ? > should i also vacuum freeze the virtual id in the new version when > it reached the 4 billion? The point is to reduce maintenance, not increase it -- you don't need to worry about cleaning these up. -Kevin
On Thu, Jan 20, 2011 at 12:04 PM, Kevin Grittner <Kevin.Grittner@wicourts.gov> wrote: > "Charles.Hou" <givemeegn@gmail.com> wrote: > >> my postgresql version is 8.1.3 > > Ouch! That's getting pretty old; I hope it's not on Windows. > > http://wiki.postgresql.org/wiki/PostgreSQL_Release_Support_Policy > > http://www.postgresql.org/about/news.865 > >> you means the newer version has a virtual transaction ID. and >> what's the maxmium of this virtual id, also 4 billion ? >> should i also vacuum freeze the virtual id in the new version when >> it reached the 4 billion? > > The point is to reduce maintenance, not increase it -- you don't > need to worry about cleaning these up. And in fact, in more recent releases - particularly 8.4 and 9.0, the need to worry about vacuum in general is much less. There are many improvements to both vacuum generally and autovacuum in particular that make things much better, including enabling autovacuum by default, multiple autovacuum worker threads, the visibility map, and so on. It's fairly likely that everything that the OP is struggling with on 8.1 would Just Work on 8.4 or 9.0. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company