Dirty reads on Sybase

Dirty reads on Sybase

Post by Tony Basog » Sat, 05 Nov 1994 14:13:11




: Sybase allows dirty reads on all its system tables,
: and does not allow only in user
: created tables, although the manual says that.
:(You can try updating syslogins and see.
: That's how we found out !!). It also allows you to
: update system tables, without
: configuring the "allow updates", using stored procedures.

Claim #1 - Sql Server allows dirty reads on system table.
I do not believe this to be true, I have tried various versions of
locking a table and trying to select off that system table with no
success.  What code fragment are you using??

Claim #1 - Sql Server allows updates to system table without setting
           the allow updates flag on(in sp's)
This is true. This is due to the fact that the stored proc was
created when the allow updates was turned on.  The stored proc
retains the update rights when it is executed.

TonyBasoglu

 
 
 

Dirty reads on Sybase

Post by Gayathri Krishnamoorthy,c » Thu, 10 Nov 1994 06:56:55


Tony,
I don't agree with what you've said. We have a case with Sybase also,
 regarding this.
 Claim #1. I tried updating the suid & the dbname column in syslogins, with a begin
 tran, and opened another session to see the old data, and was surprized to note that
 the select, showed me the new values. I went back to the old session, issued a
 rollback tran, and the transaction went to the old status.I tried, this several times,
 before it struck, that Sybase allows dirty reads on system tables.

Claim #2.
 When the same statement was put in a stored proc, and executed, and it executed
 successfully, and the allow updates was never on (neither during creation nor during
 execution).

Thanks
gayathri

 
 
 

Dirty reads on Sybase

Post by Gayathri Krishnamoorthy,c » Fri, 04 Nov 1994 06:07:32


Sybase allows dirty reads on all its system tables, and does not allow only in user
created tables, although the manual says that. (You can try updating syslogins and see.
That's how we found out !!). It also allows you to update system tables, without
configuring the "allow updates", using stored procedures.
 
 
 

1. Sybase with dirty reads

We have a client that have written a  large number of ad-hoc reports
that they run against the  a production server, which then is migrated
to a "backup server)  During this migration, the client wants to make
sure that the reports are written in a way that minimises the load
they place on the system - in particular their impact on Sybase's/the
server's ability to satisfy other requests at the same time.

We have told the client that one of the simplest changes that we have
found is to switch to explicitly use Dirty Reads (at Isolation level
0) in almost all cases. This is completely safe as all these adhoc
reports (queries) are running during times that no updates are being
made to the database.  Now the cliet  has asked two questions:

If there are no updates running, is a Dirty Read any less impactful
than a Read with the default Isolation level?
Is there a way to specify the default Isolation level that is to be
applied at a more global level than on each individual query, while
still controlling the scope of impact - for example by setting an
environment variable for ISQL in each process?

Can someone please help me with this
Thanks
Teresa

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