.NET/Java PDF, Tiff, Barcode SDK Library

The loading phase involves loading any view state (in case of a postback request) from the page request and loading it in into the server controls. When the view state is restored, the PreLoad event is triggered, followed by the Load event. This is where you normally put your page processing logic that runs on every page request. Following this event, loading the view state is executed again to catch any dynamically added controls (for instance, those created in the Load event handler). Then any control change events and postback events are handled, and finally the LoadComplete event is raised. The rendering phase starts with performing all data binding (which involves the DataBinding and DataBound events for each data-bound control), and then the PreRender event is triggered. Next, any asynchronous tasks (registered via RegisterAsyncTask) are fired off, and the PreRenderComplete event is called. Before the actual rendering takes place, the view state is saved back to the page (by default to the default hidden input field). Finally, the page s Render method is called that produces HTML by recursively invoking the same method on each server control. Before this HTML is passed to the client, the Unload event is triggered and every object involved in the page creation is disposed.

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In this example, that would be 8:59:30 a.m. when the transaction that modified the row started. When we go to refresh the data at 10:00 a.m., we pull all of the changes that had occurred since that time; when we merge these into the data warehouse, we ll have everything we need.

Another situation where it is vital that you understand read consistency and multi-versioning is when you are faced with a query that in production, under a heavy load, uses many more I/Os than you observe in your test or development systems, and you have no way to account for it. You review the I/O performed by the query and note that it is much higher than you have ever seen much higher than seems possible. You restore the production instance on test and discover that the I/O is way down. But in production, it is still very high (but seems to vary: sometimes it is high, sometimes it is low, and sometimes it is in the middle). The reason, as we ll see, is that in your test system, in isolation, you do not have to undo other transactions changes. In production, however, when you read a given block, you might have to undo (roll back) the changes of many transactions, and each rollback could involve I/O to retrieve the undo and apply it. This is probably a query against a table that has many concurrent modifications taking place; you are seeing the reads to the undo segment taking place, the work that Oracle is performing to restore the block back the way it was when your query began. You can see the effects of this easily in a single session, just to understand what is happening. We ll start with a very small table:

Both page and control events are executed on the server, so when they are triggered, the parent page along with its view state is posted to the server, where the event is handled and a response (the same or another page) is sent back to the client A so-called postback occurs if a page posts information to the server requesting the same page back This round-trip scenario is quite common: the Reload button in Listing 14-2 or the Generate button from Listing 14-4 are both examples of triggering a postback In both of these cases, we have a postback because we remain on the same page; for example, the originating page handles the data submission by updating the content of a label (an HTML <span> element) in the button click handler More often, though, data is posted to another page, in which case it is referred to as cross-page posting.

ops$tkyte%ORA11GR2> create table t ( x int ); Table created. ops$tkyte%ORA11GR2> insert into t values ( 1 ); 1 row created. ops$tkyte%ORA11GR2> exec dbms_stats.gather_table_stats( user, 'T' ); PL/SQL procedure successfully completed. ops$tkyte%ORA11GR2> select * from t; X ---------1 Now we ll set our session to use the SERIALIZABLE isolation level, so that no matter how many times we run a query in our session, the results will be as of that transaction s start time: ops$tkyte@ORA11GR2> alter session set isolation_level=serializable; Session altered. Now, we ll query that small table and observe the amount of I/O performed: ops$tkyte%ORA11GR2> set autotrace on statistics ops$tkyte%ORA11GR2> select * from t; X ---------1 Statistics ---------------------------------------------------------0 recursive calls 0 db block gets 7 consistent gets... So, that query took seven I/Os (consistent gets) in order to complete. In another session, we ll modify this table repeatedly: ops$tkyte%ORA11GR2> begin 2 for i in 1 .. 10000 3 loop 4 update t set x = x+1; 5 commit; 6 end loop; 7 end; 8 / PL/SQL procedure successfully completed.