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

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CHAPTER 1 ■ DEVELOPING SUCCESSFUL ORACLE APPLICATIONS 23 scott@ORA10G> select count(*) from emp AS OF SCN :scn; COUNT(*) ---------- 14 Further, this capability works across transactional boundaries. We can even query the same object “as of two points in time” in the same query! That opens some interesting opportunities indeed: scott@ORA10G> commit; Commit complete. scott@ORA10G> select * 2 from (select count(*) from emp), 3 (select count(*) from emp as of scn :scn) 4 / COUNT(*) COUNT(*) ---------- ---------- 0 14 If you are using Oracle 10g and above, you have a command called “flashback” that uses this underlying multi-versioning technology to allow you to return objects to the state they were in at some prior point in time. In this example, we can put EMP back the way it was before we deleted all of the information: scott@ORA10G> flashback table emp to scn :scn; Flashback complete. scott@ORA10G> select * 2 from (select count(*) from emp), 3 (select count(*) from emp as of scn :scn) 4 / COUNT(*) COUNT(*) ---------- ---------- 14 14 ■Note If you receive the error “ORA-08189: cannot flashback the table because row movement is not enabled using the FLASHBACK command,” you must issue ALTER TABLE EMP ENABLE ROW MOVEMENT. This, in effect, gives Oracle the permission to change the rowid assigned to a row. In Oracle, when you insert a row, a rowid is assigned to it and that row will forever have that rowid. The flashback table process will perform a DELETE against EMP and reinsert the rows, hence assigning them a new rowid. You must allow Oracle to do this operation in order to flash back.
24 CHAPTER 1 ■ DEVELOPING SUCCESSFUL ORACLE APPLICATIONS Read Consistency and Non-Blocking Reads Let’s now look at the implications of multi-versioning, read-consistent queries, and nonblocking reads. If you are not familiar with multi-versioning, what you see in the following code might be surprising. For the sake of simplicity, assume that the table we are reading stores one row per database block (the smallest unit of storage in the database) and that we are full-scanning the table in this example. The table we will query is a simple ACCOUNTS table. It holds balances in accounts for a bank. It has a very simple structure: create table accounts ( account_number number primary key, account_balance number ); In reality, the ACCOUNTS table would have hundreds of thousands of rows in it, but for simplicity’s sake we’re just going to consider a table with four rows (we’ll revisit this example in more detail in Chapter 7), as shown in Table 1-1. Table 1-1. ACCOUNTS Table Contents Row Account Number Account Balance 1 123 $500.00 2 234 $250.00 3 345 $400.00 4 456 $100.00 We would like to run an end-of-day report that tells us how much money is in the bank. That is an extremely simple query: select sum(account_balance) from accounts; And, of course, in this example the answer is obvious: $1,250.00. However, what happens if we read row 1, and while we’re reading rows 2 and 3, an automated teller machine (ATM) generates transactions against this table and moves $400.00 from account 123 to account 456? Our query counts $500.00 in row 4 and comes up with the answer of $1,650.00, doesn’t it? Well, of course, this is to be avoided, as it would be an error—at no time did this sum of money exist in the account balance column. Read consistency is the way in which Oracle avoids such occurrences, and you need to understand how Oracle’s methods differ from those of most every other database. In practically every other database, if you want to get a “consistent” and “correct” answer to this query, you either have to lock the whole table while the sum is calculated or you have to lock the rows as you read them. This prevents people from changing the answer as you are getting it. If you lock the table up front, you’ll get the answer that was in the database at the time the query began. If you lock the data as you read it (commonly referred to as a shared read lock, which prevents updates but not other readers from accessing the data), you’ll get the answer that was in the database at the point the query finished. Both methods inhibit concurrency a great deal. The table lock prevents any updates from taking place against the entire table for the duration of your query (for a table of four rows, this would be only a very short
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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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■CONTENTS vii ■CHAPTER 9 Redo a
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■CONTENTS ix ■CHAPTER 12 Dataty
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Foreword “THINK.” In 1914, Thom
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■FOREWORD xiii Tom is an aficiona
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About the Technical Reviewers ■JO
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CHAPTER 11 ■ ■ ■ Indexes Inde
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CHAPTER 13 ■ ■ ■ Partitioning
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