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Transactions are one of the features that set databases apart from file systems. In a file system, if you are in the middle of writing a file and the operating system crashes, that file will probably be corrupted, though there are journaled file systems and the like that may be able to recover your file to some point in time. However, if you need to keep two files synchronized, such a system won t help if you update one file and the system fails before you finish updating the second, your files won t be synchronized. This is the main purpose of transactions they take the database from one consistent state to the next. That is their function. When you commit work in the database, you are assured that either all of your changes, or none of them, have been saved. Furthermore, you are assured that the various rules and checks that protect data integrity were implemented. In the previous chapter, Concurrency and Multi-versioning, we discussed transactions in terms of concurrency control and how, as a result of Oracle s multi-versioning, read-consistent model, Oracle transactions can provide consistent data every time, under highly concurrent data access conditions. Transactions in Oracle exhibit all of the required ACID characteristics: Atomicity: Either all of a transaction happens or none of it happens. Consistency: A transaction takes the database from one consistent state to the next. Isolation: The effects of a transaction may not be visible to other transactions until the transaction has committed. Durability: Once the transaction is committed, it is permanent.

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options are useful for returning the value of a search where you might or might not have a result. You shall see in Defining Discriminated Unions that the option type is defined in the F# library as follows: type 'a option = | None | Some of 'a The following is a data structure that uses options to represent the (optional) parents of some well-known characters: > let people = [ ("Adam", None); ("Eve" , None); ("Cain", Some("Adam","Eve")); ("Abel", Some("Adam","Eve")) ];; val people : (string * (string *string) option) list Pattern matching is frequently used to examine option values: > let showParents (name,parents) = match parents with | Some(dad,mum) -> printfn "%s has father %s, mother %s" name dad mum | None -> printfn "%s has no parents!" name;; val showParents : (string * (string * string) option) -> unit > showParents ("Adam",None);; Adam has no parents val it : unit = () The F# library also includes a module Option that contains some useful functions for programming with options. Table 3-10 shows some of these. Although it is easy to code these by hand using pattern matching, it can also be useful to learn and rely on the standard definitions.

In particular, we discussed how Oracle obtains consistency and isolation in the previous chapter. Here we ll focus most of our attention on the concept of atomicity and how that is applied in Oracle. In this chapter, we ll discuss the implications of atomicity and how it affects statements in Oracle. We ll cover transaction control statements such as COMMIT, SAVEPOINT, and ROLLBACK, and we ll discuss how integrity constraints are enforced in a transaction. We ll also look at why you may have some bad transaction habits if you ve been developing in other databases. We ll look at distributed transactions and the two-phase commit (2PC). Lastly, we ll examine autonomous transactions, what they are, and the role they play.

You don t need a begin transaction statement in Oracle. A transaction implicitly begins with the first statement that modifies data (the first statement that gets a TX lock). You can explicitly begin a transaction using SET TRANSACTION or the DBMS_TRANSACTION package, but it is not a necessary step, unlike in some other databases. Issuing either a COMMIT or ROLLBACK statement explicitly ends a transaction.

Returns the value of a Some option. Returns true for a Some option. Given None, returns None. Given Some(x), returns Some(f x), where f is the given function. Applies the given function to the value of a Some option; otherwise, does nothing.

You should always explicitly terminate your transactions with a COMMIT or ROLLBACK; otherwise, the tool or environment you re using will pick one or the other for you. If you exit your SQL*Plus session normally, without committing or rolling back, SQL*Plus assumes you wish to commit your work and it does so. If you just exit from a Pro*C program, on the other hand, an implicit rollback takes place. Never rely on implicit behavior, as it could change in the future. Always explicitly COMMIT or ROLLBACK your transactions.

Note As an example of something changing in the future, SQL*Plus in Oracle Database 11g Release 2 sports a