Re: [HACKERS] How to determine that a TransactionId is really aborted?
Eric Ridge wrote: > > Again, you'll probably need to put this low level requirement into > > context if you want sound advice from this list. > > I'm just thinking out lout here, but the context is likely something > along the lines of externally storing all transaction ids, and > periodically asking Postgres if they're > known-to-be-aborted-by-all-transactions -- one at a time. I think if you just check the global xmin, then you're going to maintain a very long list of transactions "potentially running" whenever there are long-lived transactions (pg_dump, for example). You could try to add a TransactionIdIsInProgress() somewhere and discard the xact downright (by DidCommit and DidAbort) if it returns false. A single long-running transaction can keep the global xmin down by hundreds of millions of Xids. -- Álvaro Herrerahttps://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
Re: [HACKERS] How to determine that a TransactionId is really aborted?
On Sun, Oct 22, 2017 at 9:23 PM, Eric Ridge wrote:
> When sitting inside an extension, and given an arbitrary TransactionId, how
> can you determine that it aborted/crashed *and* that no other active
> transaction thinks it is still running?
>
> I've tried to answer this question myself (against the 9.3 sources), and it
> seems like it's just:
>
> {
>TransactionId oldestXmin = GetOldestXmin (false, false);
>TransactionId xid = 42;
>
>if (TransactionIdPrecedes(xid, oldestXmin) &&
> !TransactionIdDidCommit(xid) &&
> !TransactionIdIsInProgress(xid)) /* not even sure this is necessary? */
>{
> /* xid is aborted/crashed and no active transaction cares */
>}
> }
>
> Can anyone confirm or deny that this is correct? I feel like it is correct,
> but I'm no expert.
I think TransactionIdPrecedes(xid, oldestXmin) &&
!TransactionIdDidCommit(xid) is sufficient. If the transaction ID
precedes oldestXmin, then it's either committed or aborted; no other
state is possible. If it didn't commit, it aborted, and it is right
to test that using !TransactionIdDidCommit(xid) since commits are
always recorded but aborts are only usually recorded.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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Re: [HACKERS] How to determine that a TransactionId is really aborted?
On 23 October 2017 at 05:44, Eric Ridge wrote: >> On Oct 22, 2017, at 3:24 PM, Peter Geoghegan wrote: >> Again, you'll probably need to put this low level requirement into >> context if you want sound advice from this list. > > I'm just thinking out lout here, but the context is likely something along > the lines of externally storing all transaction ids, and periodically asking > Postgres if they're known-to-be-aborted-by-all-transactions -- one at a time. I think Peter is asking "why?". -- Craig Ringer http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
Re: [HACKERS] How to determine that a TransactionId is really aborted?
> On Oct 22, 2017, at 3:24 PM, Peter Geoghegan wrote: > > On Sun, Oct 22, 2017 at 2:19 PM, Eric Ridge wrote: >> I'm looking for the status as any concurrent open transaction might see it. >> For example, if any concurrent transaction might see it as "in progress", >> that's what I'd want returned. Does that make sense? > > Maybe, but note that that's fundamentally something that can become > stale immediately. And, just because an MVCC snapshot cannot see a row > does not mean that it cannot affect its transaction/statement in some > other way (e.g. unique index enforcement). Sure, but I don't think I care if it becomes stale immediately. If I ask "now" and PG says "in progress" but it then aborts a few cycles later, I'll just ask again sometime in the future. It's not like a transaction is going to go from "aborted" back to "in progress" -- geez, at least I hope not! I think question I want to answer is "will all active and future transactions see this TransationId as aborted at the time they started"? > Again, you'll probably need to put this low level requirement into > context if you want sound advice from this list. I'm just thinking out lout here, but the context is likely something along the lines of externally storing all transaction ids, and periodically asking Postgres if they're known-to-be-aborted-by-all-transactions -- one at a time. eric -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
Re: [HACKERS] How to determine that a TransactionId is really aborted?
On Sun, Oct 22, 2017 at 2:19 PM, Eric Ridge wrote: > I'm looking for the status as any concurrent open transaction might see it. > For example, if any concurrent transaction might see it as "in progress", > that's what I'd want returned. Does that make sense? Maybe, but note that that's fundamentally something that can become stale immediately. And, just because an MVCC snapshot cannot see a row does not mean that it cannot affect its transaction/statement in some other way (e.g. unique index enforcement). Again, you'll probably need to put this low level requirement into context if you want sound advice from this list. -- Peter Geoghegan -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
Re: [HACKERS] How to determine that a TransactionId is really aborted?
> On Oct 22, 2017, at 2:50 PM, Jaime Casanova > wrote: > > so, what you want is txid_status() [1]... while this is new in v10 you > can use the code as guide or just migrate to v10 ;) Oh neat, thanks. Doesn't that tell you the status relative to the transaction calling txid_status()? I'm looking for the status as any concurrent open transaction might see it. For example, if any concurrent transaction might see it as "in progress", that's what I'd want returned. Does that make sense? That's why I was thinking GetOldestXmin() was the right thing to use rather than GetActiveSnapshot()->xmin and also why I thought to check TransactionIdPrecedes() first. I am curious about the lock on ClogTruncationLock... could any of the TransactionIdDidXXX calls lie without that lock? I haven't seen such a thing used in the 9.3 sources. Maybe it's necessary for 10 or maybe I just missed it in 9.3? Thanks for your time! eric -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
Re: [HACKERS] How to determine that a TransactionId is really aborted?
On 22 October 2017 at 15:00, Eric Ridge wrote: >> On Oct 22, 2017, at 1:50 PM, Peter Geoghegan wrote: >> >> On Sun, Oct 22, 2017 at 12:23 PM, Eric Ridge wrote: >>> Can anyone confirm or deny that this is correct? I feel like it is >>> correct, but I'm no expert. >> >> What are you going to use the code for? I think that that context is >> likely to matter here. > > I'm not exactly sure yet, but I'm thinking about storing transaction ids > externally and then regularly poking Postgres to see which ones are > aborted-and-no-longer-considered-visible so I can clean up my external list. > so, what you want is txid_status() [1]... while this is new in v10 you can use the code as guide or just migrate to v10 ;) [1] https://www.postgresql.org/docs/10/static/functions-info.html#functions-txid-snapshot -- Jaime Casanova www.2ndQuadrant.com PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
Re: [HACKERS] How to determine that a TransactionId is really aborted?
> On Oct 22, 2017, at 1:50 PM, Peter Geoghegan wrote: > > On Sun, Oct 22, 2017 at 12:23 PM, Eric Ridge wrote: >> Can anyone confirm or deny that this is correct? I feel like it is correct, >> but I'm no expert. > > What are you going to use the code for? I think that that context is > likely to matter here. I'm not exactly sure yet, but I'm thinking about storing transaction ids externally and then regularly poking Postgres to see which ones are aborted-and-no-longer-considered-visible so I can clean up my external list. eric -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
Re: [HACKERS] How to determine that a TransactionId is really aborted?
On Sun, Oct 22, 2017 at 12:23 PM, Eric Ridge wrote: > Can anyone confirm or deny that this is correct? I feel like it is correct, > but I'm no expert. What are you going to use the code for? I think that that context is likely to matter here. -- Peter Geoghegan -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
[HACKERS] How to determine that a TransactionId is really aborted?
When sitting inside an extension, and given an arbitrary TransactionId, how can
you determine that it aborted/crashed *and* that no other active transaction
thinks it is still running?
I've tried to answer this question myself (against the 9.3 sources), and it
seems like it's just:
{
TransactionId oldestXmin = GetOldestXmin (false, false);
TransactionId xid = 42;
if (TransactionIdPrecedes(xid, oldestXmin) &&
!TransactionIdDidCommit(xid) &&
!TransactionIdIsInProgress(xid)) /* not even sure this is necessary? */
{
/* xid is aborted/crashed and no active transaction cares */
}
}
Can anyone confirm or deny that this is correct? I feel like it is correct,
but I'm no expert.
Thanks so much for your time!
eric
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