.NET/Java PDF, Tiff, Barcode SDK Library

The other side to reflective meta-programming in F# is quotations. These allow you to reflect over expressions in much the same way you ve been reflecting over types in the previous section. It s simple to get going with F# quotations; you simply open the appropriate modules and surround an expression with <@ . . . @> symbols: > open Microsoft.FSharp.Quotations;; > open Microsoft.FSharp.Quotations.Typed;; > let oneExpr = <@ 1 @>;; val oneExpr : Expr<int> > oneExpr;; val it : Expr<int> = <@ (Int32 1) @>

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But after setting up a simulation (I am very much into simulations, testing, and experimenting), we were able to prove it was not only the correct approach but also that it would work very nicely When we created the index, we had to choose between the following approaches: Just create an index on the processed-flag column Create an index only on the processed-flag column when the processed flag is N, that is, only index the values of interest We typically don t want to use an index when the processed flag is Y since the vast majority of the records in the table have the value Y Notice that I did not say We never want to use You might want to very frequently count the number of processed records for some reason, and then an index on the processed records might well come in very handy..

In the chapter on indexing, we ll go into more detail on both types. In the end, we created a very small index on just the records where the processed flag was N. Access to those records was extremely fast and the vast majority of Y records did not contribute to this index at all. We used a function-based index on a function decode( processed_flag, 'N', 'N' ) to return either N or NULL since an entirely NULL key is not placed into a conventional B*Tree index, we ended up only indexing the N records. Was that the end of the story No, not at all. My client still had a less than optimal solution on its hands. They still had to serialize on the dequeue of an unprocessed record. We could easily find the first unprocessed record quickly using select * from queue_table where decode( processed_flag, 'N', 'N') = 'N' FOR UPDATE, but only one session at a time could perform that operation. The project was using Oracle 10g and therefore could not yet make use of the relatively new SKIP LOCKED feature added in Oracle 11g Release 1. SKIP LOCKED would permit many sessions to concurrently find the first unlocked, unprocessed record, lock that record, and process it. Instead, we had to implement code to find the first unlocked record and lock it manually. Such code would in general look like the following in Oracle 10g and before. We begin by creating a table with the requisite index described above and populate it with some data: ops$tkyte%ORA11GR2> create table t 2 ( id number primary key, 3 processed_flag varchar2(1), 4 payload varchar2(20) 5 );

HTTP is a stateless protocol, so special care must be taken to preserve the state of a given page. For instance, your visitor may have just filled out a long form and, as it sometimes happens, he or she made an error. After posting the page, this visitor expects to see all entered data preserved

When we run TKPROF and view the results, we ll see something like this (note that I removed the ELAPSED, CPU, and DISK columns from this report): select * from t call count ------- -----Parse 1 Execute 1 Fetch 2 ------- -----total 4 query current ------ ---------0 0 0 0 7 0 ------ ---------7 0 rows ---------0 0 1 ---------1

update t t1 set x = x+1 call count ------- -----Parse 1 Execute 1 Fetch 0 ------- -----total 2 query current ------ ---------0 0 7 3 0 0 ------ ---------7 3 rows ---------0 1 0 ---------1

with an indication of where the missing or incorrect entry is. Recovering form values after a postback is possible because ASP.NET maintains the so-called view state, that is, the state of all server controls on the page. These values are passed as HTTP form variables on a post and returned as hard-wired HTML. View state is automatically encoded as a hidden <input> element in your pages, and no special handling is needed to enable it. You can, on the other hand, disable it altogether by adding EnableViewState="false" to your Page directive or partially on the control level by setting the same control property for any control for which no view state is to be maintained. This may be necessary if your page displays large amounts of data but with no possibility of a postback or the data on the page cannot be modified, thus making the page source considerably smaller and quicker to download to your clients.