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type SchemaReader<'schema> = new : unit -> SchemaReader<'schema> member ReadFile : string -> seq<'schema> member ReadLine : System.IO.TextReader -> 'schema First we show how the SchemaReader is used in practice. Let s say you have a text file containing lines such as this: Steve, 12 March 2007, Cheddar Sally, 18 Feb 2007, Brie ... Now it s reasonable to want to convert this data to a typed data representation. You can do this simply by defining an appropriate record type along with enough information to indicate how the data in the file maps into this type. This information is expressed using custom attributes, which are a way to add extra meta-information to assembly, type, member, property, and parameter definitions. Each custom attribute is specified as an instance of a typed object, here ColumnAttribute, defined in Listing 9-10. The suffix Attribute can be dropped when using the custom attribute. type CheeseClub = { [<Column(0)>] Name : string [<Column(2)>] FavouriteCheese : string [<Column(1)>] LastAttendance : System.DateTime } You can now instantiate the SchemaReader type and use it to read the data from the file into this typed format: > let reader = new SchemaReader<CheeseClub>();; val reader : SchemaReader<CheeseClub> > fsi.AddPrinter(fun (c:System.DateTime) -> c.ToString());; val it : unit = () > System.IO.File.WriteAllLines("data.txt", [| "Steve, 12/03/2007, Cheddar"; "Sally, 18/02/2007, Brie"; |]);; val it : unit = () > reader.ReadFile("data.txt");; val it : seq<CheeseClub> = seq [{Name = "Steve"; FavouriteCheese = "Cheddar"; LastAttendance = 12/03/2007 00:00:00;}; {Name = "Sally"; FavouriteCheese = "Brie"; LastAttendance = 18/02/2007 00:00:00;}]

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So far, we ve looked at read consistency: Oracle s ability to use undo information to provide nonblocking query and consistent (correct) reads. We understand that as Oracle reads blocks for queries out of the buffer cache, it will ensure that the version of the block is old enough to be seen by that query. But that begs the following questions: What about writes/modifications What happens when you run an UPDATE statement Update t set x = 2 where y = 5; and while that statement is running, someone updates a row it has yet to read from Y=5 to Y=6 and commits That is, when your UPDATE began, some row had the value Y=5. As your UPDATE reads the table using consistent reads, it sees that the row was Y=5 when the UPDATE began. But, the current value for Y is now 6 (it s not 5 anymore) and before updating the value of X, Oracle will check to see that Y is still 5. Now what happens How are the updates affected by this Obviously, we can t modify an old version of a block; when we go to modify a row, we must modify the current version of that block. Additionally, Oracle can t just simply skip this row, as that would be an inconsistent read and unpredictable. What we ll discover is that in such cases, Oracle will restart the write modification from scratch.

Oracle does do two types of block gets when processing a modification statement. It performs Consistent reads: When finding the rows to modify Current reads: When getting the block to actually update the row of interest

There is something somewhat magical about this; you have been able to build a layer that has automatically done the impedance matching between the untyped world of a text file format into the typed world of F# programming Amazingly, the SchemaReader type itself is only about 50 lines of code The comments in Listing 9-10 show the basic steps being performed The essential features of this technique are as follows: 1 The schema information is passed to the SchemaReader as a type variable The SchemaReader then uses the typeof operator to extract a SystemType representation of the schema type 2 The information needed to drive the transformation process comes from custom attributes Extra information could also be supplied to the constructor of the SchemaReader type if necessary 3.

We can see this easily using TKPROF. Consider this small one row example, which reads and updates the single row in table T from earlier: ops$tkyte%ORA11GR2> exec dbms_monitor.session_trace_enable PL/SQL procedure successfully completed. ops$tkyte%ORA11GR2> select * from t; X ---------10001 ops$tkyte%ORA11GR2> update t t1 set x = x+1; 1 row updated. ops$tkyte%ORA11GR2> update t t2 set x = x+1; 1 row updated.

Note I will use autonomous transactions throughout this book to demonstrate locking, blocking, and