Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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CHAPTER 7 ■ CONCURRENCY AND MULTI-VERSIONING 233 • Phantom read: This means that if you execute a query at time T1 and re-execute it at time T2, additional rows may have been added to the database, which will affect your results. This differs from the nonrepeatable read in that with a phantom read, data you already read has not been changed, but rather that more data satisfies your query criteria than before. ■Note The ANSI/ISO SQL standard defines transaction-level characteristics, not just individual statementby-statement–level characteristics. In the following pages, we’ll examine transaction-level isolation, not just statement-level isolation. The SQL isolation levels are defined based on whether or not they allow each of the preceding phenomena. I find it interesting to note that the SQL standard does not impose a specific locking scheme or mandate particular behaviors, but rather describes these isolation levels in terms of these phenomena, allowing for many different locking/concurrency mechanisms to exist (see Table 7-1). Table 7-1. ANSI Isolation Levels Isolation Level Dirty Read Nonrepeatable Read Phantom Read READ UNCOMMITTED Permitted Permitted Permitted READ COMMITTED Permitted Permitted REPEATABLE READ Permitted SERIALIZABLE Oracle explicitly supports the READ COMMITTED and SERIALIZABLE isolation levels, as they are defined in the standard. However, this doesn’t tell the whole story. The SQL standard was attempting to set up isolation levels that would permit various degrees of consistency for queries performed in each level. REPEATABLE READ is the isolation level that the SQL standard claims will guarantee a read-consistent result from a query. In the SQL standard’s definition, READ COMMITTED does not give you consistent results, and READ UNCOMMITTED is the level to use to get non-blocking reads. However, in Oracle, READ COMMITTED has all of the attributes required to achieve readconsistent queries. In other databases, READ COMMITTED queries can and will return answers that never existed in the database at any point in time. Moreover, Oracle also supports the spirit of READ UNCOMMITTED. The goal of providing a dirty read is to supply a non-blocking read, whereby queries are not blocked by, and do not block, updates of the same data. However, Oracle does not need dirty reads to achieve this goal, nor does it support them. Dirty reads are an implementation other databases must use in order to provide non-blocking reads. In addition to the four defined SQL isolation levels, Oracle provides another level, namely READ ONLY. A READ ONLY transaction is equivalent to a REPEATABLE READ or SERIALIZABLE transaction that cannot perform any modifications in SQL. A transaction using a READ ONLY isolation level only sees those changes that were committed at the time the transaction began, but
234 CHAPTER 7 ■ CONCURRENCY AND MULTI-VERSIONING inserts, updates, and deletes are not permitted in this mode (other sessions may update data, but not the READ ONLY transaction). Using this mode, you can achieve REPEATABLE READ and SERIALIZABLE levels of isolation. Let’s now move on to discuss exactly how multi-versioning and read consistency fits into the isolation scheme, and how databases that do not support multi-versioning achieve the same results. This information is instructive for anyone who has used another database and believes he or she understands how the isolation levels must work. It is also interesting to see how a standard that was supposed to remove the differences between the databases, ANSI/ISO SQL, actually allows for them. The standard, while very detailed, can be implemented in very different ways. READ UNCOMMITTED The READ UNCOMMITTED isolation level allows dirty reads. Oracle does not make use of dirty reads, nor does it even allow for them. The basic goal of a READ UNCOMMITTED isolation level is to provide a standards-based definition that caters for non-blocking reads. As we have seen, Oracle provides for non-blocking reads by default. You would be hard-pressed to make a SELECT query block in the database (as noted earlier, there is the special case of a distributed transaction). Every single query, be it a SELECT, INSERT, UPDATE, MERGE, or DELETE, executes in a read-consistent fashion. It might seem funny to refer to an UPDATE statement as a query—but it is. UPDATE statements have two components: a read component as defined by the WHERE clause and a write component as defined by the SET clause. UPDATE statements read from and write to the database—as do all DML statements. The special case of a single row INSERT using the VALUES clause is the only exception to this, as such statements have no read component, just the write component. In Chapter 1, Oracle’s method of obtaining read consistency was demonstrated by way of a simple single table query, which retrieved rows that were deleted after the cursor was opened. We’re now going to explore a real-world example to see what happens in Oracle using multi-versioning, as well as what happens in any number of other databases. Let’s start with the same basic table and query: create table accounts ( account_number number primary key, account_balance number not null ); select sum(account_balance) from accounts; Before the query begins, we have the data shown in Table 7-2. Table 7-2. ACCOUNTS Table Before Modifications Row Account Number Account Balance 1 123 $500.00 2 456 $240.25 . . . . . . . . . 342,023 987 $100.00
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Expert Oracle Database Architecture
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Contents Foreword . . . . . . . . .
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■CONTENTS v Block Buffer Cache .
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Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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