Read committed is faster, but I have not done any tests to know how much
faster.  Read committed mode doesn't support concurrent updates either,
but fails silently.  In other words, given to concurrent queries:

update foo set a = a + 1
update foo set a = a + 2

Assuming these overlap, and a starting value of 0 for "a", read
committed mode would result in "a" either being 1 or 2; serializable
mode (with my patch) would result in "a" getting set to 3.

-Randy

> -----Original Message-----
> From: Federico Di Gregorio [mailto:[EMAIL PROTECTED]]
> Sent: Monday, April 09, 2001 1:23 AM
> To: Randall F. Kern
> Cc: [EMAIL PROTECTED]
> Subject: Re: your mail
> 
> 
> Scavenging the mail folder uncovered Randall F. Kern's letter:
> 
> [snip]
> > Concurrent updates are not supported in the serializable transaction
> > mode; if one transaction tries to update (or delete) a row that has
> > already been updated by a concurrent uncommitted 
> transaction, the second
> > transaction will fail with a "concurrent update" error.
> 
> mmm... are they supported in the read commited mode? if yes it will be
> easy to add this mode as on option to psycopg cursors and use 
> it in zope.
> do you know how much faster would it be compared to the 
> serialized mode?
> 
> > Given that the default transaction mode for these DAs is 
> Serializable,
> > and Zope has this nifty support for ConflictErrors and retrying
> > transactions already, I propose Postgres DAs should convert the
> > concurrent update serialization errors into ConflictErrors
> > automatically.
> > 
> > I've been running this way for a few days, and everything 
> seems great.
> > Any reasons why this might be good/bad/ugly very welcome.  
> (Like: is it
> > safe to throw a ConflictError at any random time during processing?
> > >From a quick glance at ZODB it seems they are only thrown 
> at commit time
> > now)
> > 
> > Here is a small patch to ZPsyccopgDA 0.5.1 that implements 
> this change:
> 
> thank you very much for the patch. i am waiting to know what 
> others think
> about it (i am not a ZODB expert) before we incorporate it.
> 
> ciao,
> federico
> 
> -- 
> Federico Di Gregorio
> MIXAD LIVE Chief of Research & Technology              
> [EMAIL PROTECTED]
> Debian GNU/Linux Developer & Italian Press Contact        
> [EMAIL PROTECTED]
>                            Don't dream it. Be it. -- Dr. 
> Frank'n'further
> 

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