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

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CHAPTER 6 ■ LOCKING AND LATCHING 203 ENQUEUE_RESOURCES system parameter), as each row will create another enqueue (a lock). If this does happen, you’ll need to raise the value of the ENQUEUE_RESOURCES parameter. You might also add a flag to the trigger to allow people to turn the check on and off. If I was going to insert hundreds or thousands of records, for example, I might not want this check enabled. Blocked Merges, Updates, and Deletes In an interactive application—one where you query some data out of the database, allow an end user to manipulate it, and then “put it back” into the database—a blocked UPDATE or DELETE indicates that you probably have a lost update problem in your code (I would call it a bug in your code if you do). You are attempting to UPDATE a row that someone else is already updating (in other words, that someone else already has locked). You can avoid the blocking issue by using the SELECT FOR UPDATE NOWAIT query to • Verify the data has not changed since you queried it out (preventing lost updates). • Lock the row (preventing the UPDATE or DELETE from blocking). As discussed earlier, you can do this regardless of the locking approach you take. Both pessimistic and optimistic locking may employ the SELECT FOR UPDATE NOWAIT query to verify the row has not changed. Pessimistic locking would use that statement the instant the user indicated her intention to modify the data. Optimistic locking would use that statement immediately prior to updating the data in the database. Not only will this resolve the blocking issue in your application, but also it will correct the data integrity issue. Since a MERGE is simply an INSERT and UPDATE (and in 10g with the enhanced MERGE syntax, it’s a DELETE as well), you would use both techniques simultaneously. Deadlocks Deadlocks occur when you have two sessions, each of which is holding a resource that the other wants. For example, if I have two tables, A and B in my database, and each has a single row in it, I can demonstrate a deadlock easily. All I need to do is open two sessions (e.g., two SQL*Plus sessions). In session A, I update table A. In session B, I update table B. Now, if I attempt to update table A in session B, I will become blocked. Session A has this row locked already. This is not a deadlock; it is just blocking. I have not yet deadlocked because there is a chance that session A will commit or roll back, and session B will simply continue at that point. If I go back to session A and then try to update table B, I will cause a deadlock. One of the two sessions will be chosen as a “victim” and will have its statement rolled back. For example, the attempt by session B to update table A may be rolled back, with an error such as the following: update a set x = x+1 * ERROR at line 1: ORA-00060: deadlock detected while waiting for resource Session A’s attempt to update table B will remain blocked—Oracle will not roll back the entire transaction. Only one of the statements that contributed to the deadlock is rolled back. Session B still has the row in table B locked, and session A is patiently waiting for the row to
204 CHAPTER 6 ■ LOCKING AND LATCHING become available. After receiving the deadlock message, session B must decide whether to commit the outstanding work on table B, roll it back, or continue down an alternate path and commit later. As soon as this session does commit or roll back, the other blocked session will continue on as if nothing happened. Oracle considers deadlocks to be so rare, so unusual, that it creates a trace file on the server each and every time one does occur. The contents of the trace file will look something like this: *** 2005-04-25 15:53:01.455 *** ACTION NAME:() 2005-04-25 15:53:01.455 *** MODULE NAME:(SQL*Plus) 2005-04-25 15:53:01.455 *** SERVICE NAME:(SYS$USERS) 2005-04-25 15:53:01.455 *** SESSION ID:(145.208) 2005-04-25 15:53:01.455 DEADLOCK DETECTED Current SQL statement for this session: update a set x = 1 The following deadlock is not an ORACLE error. It is a deadlock due to user error in the design of an application or from issuing incorrect ad-hoc SQL. The following information may aid in determining the deadlock:... Obviously, Oracle considers these application deadlocks a self-induced error on part of the application and, for the most part, Oracle is correct. Unlike in many other RDBMSs, deadlocks are so rare in Oracle they can be considered almost nonexistent. Typically, you must come up with artificial conditions to get one. The number one cause of deadlocks in the Oracle database, in my experience, is unindexed foreign keys (the number two cause is bitmap indexes on tables subject to concurrent updates, which we’ll cover in Chapter 11). Oracle will place a full table lock on a child table after modification of the parent table in two cases: • If you update the parent table’s primary key (a very rare occurrence if you follow the rule of relational databases stating that primary keys should be immutable), the child table will be locked in the absence of an index on the foreign key. • If you delete a parent table row, the entire child table will be locked (in the absence of an index on the foreign key) as well. These full table locks are a short-term occurrence in Oracle9i and above, meaning they need to be taken for the duration of the DML operation, not the entire transaction. Even so, they can and do cause large locking issues. As a demonstration of the first point, if we have a pair of tables set up as follows: ops$tkyte@ORA10G> create table p ( x int primary key ); Table created. ops$tkyte@ORA10G> create table c ( x references p ); Table created. ops$tkyte@ORA10G> insert into p values ( 1 ); 1 row created.
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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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Introduction The inspiration for th
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■INTRODUCTION xix • Exposure to
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■INTRODUCTION xxi Chapter 3: File
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■INTRODUCTION xxiii Next up are t
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Setting Up Your Environment In this
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■SETTING UP YOUR ENVIRONMENT xlii
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CHAPTER 1 ■ ■ ■ Developing Su
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CHAPTER 1 ■ DEVELOPING SUCCESSFUL
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CHAPTER 3 ■ ■ ■ Files In this
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CHAPTER 3 ■ FILES 69 and even dat
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CHAPTER 3 ■ FILES 71 all operatio
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CHAPTER 3 ■ FILES 89 A Brief Revi
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CHAPTER 3 ■ FILES 91 Redundant Ar
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CHAPTER 4 ■ ■ ■ Memory Struct
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CHAPTER 4 ■ MEMORY STRUCTURES 117
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CHAPTER 8 ■ ■ ■ Transactions
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338 CHAPTER 10 ■ DATABASE TABLES
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CHAPTER 11 ■ ■ ■ Indexes Inde
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CHAPTER 13 ■ ■ ■ Partitioning
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