Question about the db-link SCN flaw

I understand why the SCN is synced between two databases when a DML transaction is made via db-link, but can someone explain to me why a simple SELECT statement via db-link causes the SCN to sync? Will this behaviour still be true after implementing the new patch?

902074 wrote:
I understand why the SCN is synced between two databases when a DML transaction is made via db-link, but can someone explain to me why a simple SELECT statement via db-link causes the SCN to sync? Will this behaviour still be true after implementing the new patch?It's possible that it's a strategy aimed at keeping the code simple.
Remember that the default behaviour of a query is to be read-consistent with the starting SCN for the query, and then consider distirbuted nested loop join where the second (inner) table is at the remote database. For each row in the first row source the local database will execute a query against the remote database - but for distributed read-consistency every execution of that query on the remote database ought to be read-consistent with the start SCN of the first execution. This means that, in general, a distributed query (which could access an arbtrary number of remote databases, and have ann arbitrary number of parts at each remote database) will have to remember the SCN of the first time that it touched every remote datbase in the query. It's far easier, therefore, to bump the SCN at the remote database at the first touch of the query and then go read-consistent to that point.
Regards
Jonathan Lewis
http://jonathanlewis.wordpress.com
Author: Oracle Core

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