.NET/ASP.NET/C#/VB.NET PDF Document SDK

The only solution to this sticky issue is to have undo segments that are sized correctly for your system. Time and time again, I see people trying to save a few megabytes of disk space by having the smallest possible undo segments ( Why waste space on something I don t really need is the thought). The problem is that the undo segments are a key component of the way the database works, and unless they are sized correctly, you will hit this error. In many years of using Oracle 6, 7, 8, 9, 10, and 11, I can say I have never hit an ORA-01555 error outside of a testing or development system. In such a case, you know you have not sized the undo segments correctly and you fix it. We will revisit this issue in 9 Redo and Undo .

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So far, we ve seen how multi-versioning provides us with non-blocking reads, and I have stressed that this is a good thing: consistent (correct) answers with a high degree of concurrency. What could be wrong with that Well, unless you understand that it exists and what it implies, then you are probably doing some of your transactions incorrectly. Recall from 1 the scheduling resources example whereby we had to employ some manual locking techniques (via SELECT FOR UPDATE to serialize modifications to the SCHEDULES table by resource). But can it affect us in other ways The answer to that is definitely yes. We ll go into the specifics in the sections that follow.

5 The server executes the appropriate page handlers and the control event handler(s) for the button click, and an HTML response is generated (the same page with the label now greeting you by name) 6 This HTML response is sent back to the browser and displayed As this simple but representative example shows, the server controls on a given page are in constant contact with the server: they catch events and pass them onto the server where they are handled and responded to accordingly Page events occur when a page is requested from the server, either via the GET (the first time) or POST (when data is posted to the page) HTTP methods For instance, there are events for page initialization (PreInit, Init), page loading (Load), rendering (Render), and so on These page events are always handled in a given order as defined by the ASP.

A common data warehousing technique I ve seen people employ goes like this: 1. They use a trigger to maintain a LAST_UPDATED column in the source table, much like the method described in the last chapter in the Optimistic Locking section. To initially populate a data warehouse table, they remember what time it is right now by selecting out SYSDATE on the source system. For example, suppose it is exactly 9:00 a.m. right now. They then pull all of the rows from the transactional system a full SELECT * FROM TABLE to get the data warehouse initially populated. To refresh the data warehouse, they remember what time it is right now again. For example, suppose an hour has gone by it is now 10:00 a.m. on the source system. They will remember that fact. They then pull all changed records since 9:00 a.m. (the moment before they started the first pull) and merge them in.

3. 4.

NET page life cycle (see The ASPNET Page Life Cycle sidebar) Although you can programmatically bind a delegate to a given page event, ASPNET makes it even easier As you saw it in Listing 14-5, you can add page event handlers by simply declaring members of the form Page_XXX, where XXX is the name of the page event This works because by default, the AutoEventWireup page attribute is true, and this causes wiring the intended event handler (using this naming convention) to the appropriate event automatically Control events are triggered by the end user: clicking a button, selecting an item in a list box, and so on Contrary to page events, control events cannot be automatically wired, and thus you need to establish the link between the handler and the event manually.

Note This technique may pull the same record twice in two consecutive refreshes. This is unavoidable due to

the granularity of the clock. A MERGE operation will not be affected by this (i.e., update existing record in the data warehouse or insert a new record).

This is quite easy to do: simply set the OnXXX attribute of the server control to the event handler, as we did with the Reload button in the first example Not all control events are immediately posted back to the server; in fact, only a few are, such as button clicks All other control events are so-called change events (because they are triggered as a control undergoes some changes: the selected item in a drop-down list has changed or a check box or radio button has been selected) and as such are not posted to the server automatically for efficiency reasons You can, on the other hand, enable postback in such situations by overriding the control s AutoPostBack property to true.