Re: currval() race condition on server?

[Dave Cramer <pg(at)fastcrypt(dot)com>]

On 24-Oct-06, at 3:37 AM, Adriaan Joubert wrote:

> Oliver Jowett wrote:
>
> > The driver should split this query on the semicolons and send  
> > Parse/Bind/Execute for each part, followed by a Sync at the end.  
> > If it ends up using named statements due to JDBC statement reuse,  
> > it should use a different name for each part. Of course there  
> > might be unknown bugs in there, but it has been designed to handle  
> > this sort of compound statement..
> > To the OP: Is autocommit on or off? Can you reproduce the problem  
> > with a URL parameter of loglevel=2 and see what it looks like?  
> > (that'll generate a trace protocol-level messages)
>
> autocommit is off.
>
> I have not been able to reproduce this with logging on - even with  
> logging off the problem only occurs intermittently. Looking at the  
> output from the trace everything seems to happen as expected - see  
> trace below. Yet we are getting a few of these errors in our logs  
> every day - obviously only the first time we ask for the value of  
> the sequence back. Scary thought is that may be getting an older  
> value back in other instances. It feels an awful lot like a timing  
> issue where the sequence number is retrieved, but there is a delay  
> until currval can use it. I'm not sure how currval works.

currval works like this:

When you execute nextval('sequence_name') the result of that is  
stored in your session. A session lives as long as this connection is  
alive. If nextval('sequence_name') has not  been called in this  
session then currval('sequence_name') is undefined.

Hopefully this helps

Dave
> I will continue to try to reproduce this with more logging, but  
> would appreciate any ideas.
>
> Thanks!
>
> Adriaan
>
> <=BE CommandStatus(BEGIN)
>  <=BE ParseComplete [null]
>  <=BE BindComplete [null]
>  <=BE RowDescription(1)
>  <=BE DataRow
>  <=BE CommandStatus(SELECT)
>  <=BE ReadyForQuery(T)
> simple execute, handler=org.postgresql.jdbc2.AbstractJdbc2Statement 
> $StatementResultHandler(at)87e9bf, maxRows=0, fetchSize=0, flags=1
>  FE=> Parse(stmt=null,query="UPDATE prm SET ... AND _audit= 
> $16",oids={23,23,701,23,701,23,23,0,701,701,701,0,0,23,23,23})
>  FE=> Bind(stmt=null,portal=null,$1=<421>,$2=<1>, 
> $3=<0.21927195726630877>,$4=<18>,$5=<0.376345008550404>,$6=<2>, 
> $7=<1>,$8=<NULL>,$9=<0.045037791453060014>, 
> $10=<0.020180805785169714>,$11=<1.0>,$12=<NULL>,$13=<NULL>, 
> $14=<387>,$15=<435>,$16=<3382993>)
>  FE=> Describe(portal=null)
>  FE=> Execute(portal=null,limit=0)
>  FE=> Parse(stmt=null,query=" SELECT currval('ip_audit_seq')",oids={})
>  FE=> Bind(stmt=null,portal=null)
>  FE=> Describe(portal=null)
>  FE=> Execute(portal=null,limit=0)
>  FE=> Sync
>  <=BE ParseComplete [null]
>  <=BE BindComplete [null]
>  <=BE NoData
>  <=BE CommandStatus(UPDATE 1)
>  <=BE ParseComplete [null]
>  <=BE BindComplete [null]
>  <=BE RowDescription(1)
>  <=BE DataRow
>  <=BE CommandStatus(SELECT)
>  <=BE ReadyForQuery(T)
> simple execute, handler=org.postgresql.jdbc2.AbstractJdbc2Connection 
> $TransactionCommandHandler(at)1033450, maxRows=0, fetchSize=0, flags=22
>  FE=> Bind(stmt=S_2,portal=null)
>  FE=> Execute(portal=null,limit=1)
>  FE=> Sync
>  <=BE BindComplete [null]
>  <=BE CommandStatus(COMMIT)
>  <=BE ReadyForQuery(I)
>
> ---------------------------(end of  
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