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They believe that they now have all of the records in the data warehouse that were modified since they did the initial pull. They may actually have all of the records, but just as likely they may not. This technique does work on some other databases ones that employ a locking system whereby reads are blocked by writes and vice versa. But in a system where you have non-blocking reads, the logic is flawed.

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To see the flaw in this example, all we need to do is assume that at 9:00 am there was at least one open, uncommitted transaction At 8:59:30 am, it had updated a row in the table we were to copy At 9:00 am, when we started pulling the data and thus reading the data in this table, we would not see the modifications to that row; we would see the last committed version of it If it was locked when we got to it in our query, we would read around the lock If it was committed by the time we got to it, we would still read around it since read consistency permits us to read only data that was committed in the database when our statement began We would not read that new version of the row during the 9:00 am.

This can be quite useful; for instance, a page can respond directly to a change in a drop-down list without the end user having to click a submit button, thus saving valuable clicks and time in the appropriate situations..

initial pull, nor would we read the modified row during the 10:00 am refresh The reason The 10:00 am refresh would only pull records modified since 9:00 am that morning, but this record was modified at 8:59:30 am We would never pull this changed record In many other databases where reads are blocked by writes and a committed but inconsistent read is implemented, this refresh process would work perfectly If at 9:00 am when we did the initial pull of data, we hit that row and it was locked, we would have blocked and waited for it, and read the committed version If it were not locked, we would just read whatever was there, committed So, does this mean the preceding logic just cannot be used No, it means that we need to get the right now time a little differently.

We need to query V$TRANSACTION and find out which is the earliest of the current time and the time recorded in START_TIME column of this view We will need to pull all records changed since the start time of the oldest transaction (or the current SYSDATE value if there are no active transactions): select nvl( min(to_date(start_time,'mm/dd/rr hh24:mi:ss')),sysdate) from v$transaction;.

The ASP.NET page life cycle consists of a few dozen steps and as such is beyond the scope of this book to discuss in detail. However, it is important to understand the main steps involved in this life cycle. For a complete reference, you can refer to books devoted to ASP.NET. Here we give a brief and simplified description, sufficient enough for most situations. Given a request for a page and assuming that the appropriate page class exists in a compiled form on the server (if not, the ASP.NET runtime kicks in and performs a number of code compilation steps), a page instance is created with all server controls declared but not yet initialized. At this point there are three main life cycle phases (initialization, loading, and rendering). Initialization starts with determining whether the page requested is in postback mode (in which case the view state is encoded in the page request) or for the first time. At this point, the PreInit handler is executed, and the page theme and master page are applied. Next, the Init handler is called, initializing all server controls and the page itself, and then the InitCompleted event is triggered.

Note The above query works regardless of the presence of any data in V$TRANSACTION. That is, even if

V$TRANSACTION is empty (because there are no transactions currently), the above query returns a record. A query