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Figure 10-6. IOT with overflow segment, PCTTHRESHOLD clause The gray boxes are the index entries, part of a larger index structure (in 11, you ll see a larger picture of what an index looks like). Briefly, the index structure is a tree, and the leaf blocks (where the data is stored) are, in effect, a doubly linked list to make it easier to traverse the nodes in order once we ve found where we want to start in the index. The white box represents an OVERFLOW segment. This is where data that exceeds our PCTTHRESHOLD setting will be stored. Oracle will work backward from the last column up to but not including the last column of the primary key to find out what columns need to be stored in the overflow segment. In this example, the number column X and the date column Y will always fit in the index block. The last column, Z, is of varying length. When it is less than about 190 bytes or so (10 percent of a 2KB block is about 200 bytes; subtract 7 bytes for the date and 3 to 5 for the number), it will be stored on the index block. When it exceeds 190 bytes, Oracle will store the data for Z in the overflow segment and set up a pointer (a rowid, in fact) to it. The other option is to use the INCLUDING clause. Here we are stating explicitly what columns we want stored on the index block and which should be stored in the overflow. Given a CREATE TABLE statement like this

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Frequently, types will also define methods such as Map that provide a slightly more succinct way of transforming data. For example, we could have written sites |> List.map getStats like so: sites.Map(getStats) Both styles are used in F# programming, depending on the methods and properties that are available for a particular data type.

ops$tkyte@ORA11GR2> create table iot 2 ( x int, 3 y date, 4 z varchar2(2000), 5 constraint iot_pk primary key (x) 6 ) 7 organization index 8 including y 9 overflow 10 / Table created. what we can expect to find is illustrated in Figure 10-7.

Figure 10-7. IOT with OVERFLOW segment, INCLUDING clause In this situation, regardless of the size of the data stored in it, Z will be stored out of line in the overflow segment. Which is better then: PCTTHRESHOLD, INCLUDING, or some combination of both It depends on your needs. If you have an application that always, or almost always, uses the first four columns of a table and rarely accesses the last five columns, using INCLUDING would be appropriate. You would include up to the fourth column and let the other five be stored out of line. At runtime, if you need them, the columns will be retrieved in much the same way as a migrated (also known as a chained) row would be. Oracle will read the head of the row, find the pointer to the rest of the row, and then read that. If, on the other hand, you cannot say that you almost always access these columns and hardly ever access those columns, you should give some consideration to PCTTHRESHOLD. Setting PCTTHRESHOLD is easy once you determine the number of rows you would like to store per index block on average. Suppose you wanted 20 rows per index block. Well, that means each row should be one-twentieth (5 percent). Your PCTTHRESHOLD would be 5, and each chunk of the row that stays on the index leaf block should consume no more than 5 percent of the block. The last thing to consider with IOTs is indexing. You can have an index on IOTs themselves sort of like having an index on an index. These are called secondary indexes. Normally, an index contains the

The |> forward pipe operator is perhaps the most important operator in F# programming. Its definition is deceptively simple: let (|>) x f = f x Here is how to use the operator to compute the cubes of three numbers: [1;2;3] |> List.map (fun x -> x * x * x) This produces [1;8;27], just as if you had written this: List.map (fun x -> x * x * x) [1;2;3] In a sense, |> is just function application in reverse. However, using |> has distinct advantages: Clarity: When used in conjunction with operators such as List.map, the |> operator allows you to perform the data transformations and iterations in a forward-chaining, pipelined style. Type inference: Using the |> operator allows type information to be flowed from input objects to the functions manipulating those objects. F# uses information collected from type inference to resolve some language constructs such as property accesses and method overloading. This relies on information being propagated left to right through the text of a program. In particular, typing information to the right of a position is not taken into account when resolving property access and overloads. For completeness, here is the type of the operator: val (|>) : 'a -> ('a -> 'b) -> 'b