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

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CHAPTER 10 ■ DATABASE TABLES 349 Figure 10-4. Data block as it would appear after coalescing free space If the row fit into the coalesced space, then this would have happened. This time, however, Oracle will not perform this coalescing and the block will remain as it is. Since row 4 would have to span more than one block if it stayed on this block, Oracle will move, or migrate, the row. However, Oracle cannot just move the row—it must leave behind a “forwarding address.” There may be indexes that physically point to this address for row 4. A simple update will not modify the indexes as well. (Note that there is a special case with partitioned tables that a rowid, the address of a row, will change. We will look at this case in Chapter 13.) Therefore, when Oracle migrates the row, it will leave behind a pointer to where the row really is. After the update, the blocks might look as shown in Figure 10-5. Figure 10-5. Migrated row depiction So, a migrated row is a row that had to move from the block it was inserted into, onto some other block. Why is this an issue? Your application will never know; the SQL you use is no different. It only matters for performance reasons. If you go to read this row via an index, the index will point to the original block. That block will point to the new block. Instead of doing the two or so I/Os to read the index plus one I/O to read the table, you’ll need to do yet one more I/O to get to the actual row data. In isolation, this is no big deal—you won’t even notice it. However, when you have a sizable percentage of your rows in this state, with lots of users accessing them, you’ll begin to notice this side effect. Access to this data will start to slow down (additional I/Os and the associated latching that goes with the I/O add to the access
350 CHAPTER 10 ■ DATABASE TABLES time), your buffer cache efficiency goes down (you need to buffer two blocks instead of just the one you would if the rows were not migrated), and your table grows in size and complexity. For these reasons, you do not want migrated rows. It is interesting to note what Oracle will do if the row that was migrated from the block on the left to the block on the right, in Figure 10-5, has to migrate again at some future point in time. This would be due to other rows being added to the block it was migrated to and then updating this row to make it even larger. Oracle will actually migrate the row back to the original block and, if there is sufficient space, leave it there (the row might become “unmigrated”). If there isn’t sufficient space, Oracle will migrate the row to another block altogether and change the forwarding address on the original block. As such, row migrations will always involve one level of indirection. So, now we are back to PCTFREE and what it is used for: it is the setting that will help you to minimize row chaining when set properly. Setting PCTFREE and PCTUSED Values Setting PCTFREE and PCTUSED is an important—and greatly overlooked—topic. In summary, PCTUSED and PCTFREE are both crucial when using MSSM; with ASSM, only PCTFREE is. On one hand, you need to use them to avoid too many rows from migrating. On the other hand, you use them to avoid wasting too much space. You need to look at your objects and describe how they will be used, and then you can come up with a logical plan for setting these values. Rules of thumb may very well fail you on these settings; they really need to be set based on usage. You might consider the following (keeping in mind that “high” and “low” are relative terms, and that when using ASSM only PCTFREE applies): • High PCTFREE,low PCTUSED: This setting is for when you insert lots of data that will be updated and the updates will increase the size of the rows frequently. This setting reserves a lot of space on the block after inserts (high PCTFREE) and makes it so that the block must almost be empty before getting back onto the freelist (low PCTUSED). • Low PCTFREE, high PCTUSED: This setting is for if you tend to only ever INSERT or DELETE from the table, or if you do UPDATE, the UPDATE tends to shrink the row in size. LOGGING and NOLOGGING Normally objects are created in a LOGGING fashion, meaning all operations performed against them that can generate redo will generate it. NOLOGGING allows certain operations to be performed against that object without the generation of redo; we covered this in the last chapter in some detail. NOLOGGING affects only a few specific operations, such as the initial creation of the object, or direct path loads using SQL*Loader, or rebuilds (see the Oracle SQL Reference manual for the database object you are working with to see which operations apply). This option does not disable redo log generation for the object in general—only for very specific operations. For example, if I create a table as SELECT NOLOGGING and then INSERT INTO ➥ THAT_TABLE VALUES ( 1 ), the INSERT will be logged, but the table creation might not have been (the DBA can force logging at the database or tablespace level).
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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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50 CHAPTER 2 ■ ARCHITECTURE OVERV
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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 83 Trace Files
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CHAPTER 3 ■ FILES 87 6 ( 7 TYPE O
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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 3 ■ FILES 93 (data from m
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CHAPTER 3 ■ FILES 95 dictionary t
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CHAPTER 3 ■ FILES 107 Flashback L
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CHAPTER 4 ■ ■ ■ Memory Struct
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CHAPTER 4 ■ MEMORY STRUCTURES 117
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CHAPTER 5 ■ ■ ■ Oracle Proces
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CHAPTER 5 ■ ORACLE PROCESSES 157
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CHAPTER 8 ■ ■ ■ Transactions
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284 CHAPTER 9 ■ REDO AND UNDO cri
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294 CHAPTER 9 ■ REDO AND UNDO row
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CHAPTER 11 ■ ■ ■ Indexes Inde
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CHAPTER 11 ■ INDEXES 431 The data
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CHAPTER 11 ■ INDEXES 449 Table 11
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CHAPTER 13 ■ ■ ■ Partitioning
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CHAPTER 13 ■ PARTITIONING 561 els
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CHAPTER 13 ■ PARTITIONING 565 Enh
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