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Client A sends a SQL SELECT statement over the network to the server process.
2. The server process obtains an SCN for the statement and begins to read the
requested data for the query. For each data block that it reads, it compares the
SCN of the SELECT statement with the SCNs for any transactions for the relevant
rows of the data block. If the server finds a transaction with a later SCN
than the current SELECT statement, the server process uses data in the rollback
segments to create a ???consistent read??? version of the data block, current as of the
time the SELECT was issued. This is what provides the multiversion read consistency
(MVRC) and avoids the need for Oracle to use read locks on data. If a row
has been updated since the transaction started, Oracle simply gets the earlier version
of the data for a consistent view.
3. Client B sends a SQL UPDATE statement for a row in the EMP table that has
not yet been read by Client A??™s SELECT statement. The server process gets an
SCN for the statement and begins the operation.


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