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lib/DBD/MVS_FTPSQL.pm  view on Meta::CPAN

This is the default isolation level. It locks only the row (the page) that is currently being returned. As the cursor leaves the row, the lock is released and acquired for the next one, until all the data is returned.
While this maximizes concurrency and prevents dirty reads it does not 
ensure that the data retrieved will not be changed by other transactions, so
if the transaction reads the same row of data more than once odds are it
gets different results each time (nonrepetable read phenomena).

=item Uncommitted Read (UR)

With this isolation levels the transaction (almost) doesn't acquire locks and 
doesn't check if the data that is retrieving is locked.
This, at the price of risking reading non committed data, 
leads to two main advantages:

=over 2

=item *

Better performance if compared with other isolation levels.

=item *

lib/DBD/MVS_FTPSQL.pm  view on Meta::CPAN

only at query level. You can do so by ending the statement 
with a "with" clause whose syntax is:

  (fullselect)  WITH [RR|RS|CS|UR]

as illustrated by the following example:

  SELECT * FROM SYSIBM.SYSDUMMY1 WITH UR

When using IBM FTP CS as a medium to submit queries, there are two main limitations 
that affect your control over the way the data is locked and 
isolated between concurrent processes. These limitations are:

=over 2

=item *

The inability to control transaction boundaries. That is, you can only
issue select statements; every other statement - and this includes also
commit and rollback - are not permitted. To put it briefly, you can't disable 
autocommit.