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

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CHAPTER 10 ■ DATABASE TABLES 341 ops$tkyte@ORA10G> select segment_name, segment_type 2 from user_segments; SEGMENT_NAME SEGMENT_TYPE ------------------------------ ------------------ SYS_IL0000063631C00002$$ LOBINDEX SYS_LOB0000063631C00003$$ LOBSEGMENT SYS_C009783 INDEX SYS_IL0000063631C00003$$ LOBINDEX SYS_LOB0000063631C00002$$ LOBSEGMENT T TABLE 6 rows selected. The table itself created a segment in this example: the last row in the output. The primary key constraint created an index segment in this case in order to enforce uniqueness. ■Note A unique or primary key constraint may or may not create a new index. If there is an existing index on the constrained columns, and these columns are on the leading edge of the index, the constraint can and will use them. Additionally, each of the LOB columns created two segments: one segment to store the actual chunks of data pointed to by the character large object (CLOB) or binary large object (BLOB) pointer, and one segment to “organize” them. LOBs provide support for very large chunks of information, up to many gigabytes in size. They are stored in chunks in the lobsegment, and the lobindex is used to keep track of where the LOB chunks are and the order in which they should be accessed. Segment Space Management Starting in Oracle9i, there are two methods for managing space in segments: • Manual Segment Space Management: You set various parameters such as FREELISTS, FREELIST GROUPS, PCTUSED, and others to control how space is allocated, used, and reused in a segment over time. I will refer to this space management method in this chapter as MSSM, but bear in mind that that is a made-up abbreviation that you will not find in the Oracle documentation. • Automatic Segment Space Management (ASSM): You control one parameter relating to how space is used: PCTFREE. The others are accepted when the segment is created, but they are ignored. MSSM is the legacy implementation in Oracle. It has been around for many years, over many versions. ASSM was first introduced in Oracle9i Release 1 and its design intention was to eliminate the need to fine-tune the myriad parameters used to control space allocation and provide high concurrency. For example, by having the FREELISTS parameter set to the default
342 CHAPTER 10 ■ DATABASE TABLES of 1, you might find that your insert/update-intensive segments may be suffering from contention on free space allocation. When Oracle goes to insert a row into a table, or update an index key entry, or update a row causing the row to migrate (more on that in a moment as well), it may need to get a block from the list of free blocks associated with the segment. If there is only one list, only one transaction at a time may review and modify this list—they would have to wait for each other. Multiple FREELISTS and FREELIST GROUPS serve the purpose of increasing concurrency in such a case, as the transactions may each be looking at different lists and not contending with each other. When I discuss the storage settings shortly, I will mention which are for manual and which are for automatic segment space management, but in the area of storage/segment characteristics, the only storage settings that apply to ASSM segments are as follows: • BUFFER_POOL • PCTFREE • INITRANS • MAXTRANS (only in 9i; in 10g this is ignored for all segments) The remaining storage and physical attribute parameters do not apply to ASSM segments. Segment space management is an attribute inherited from the tablespace in which a segment is contained (and segments never span tablespaces). For a segment to use ASSM, it would have to reside in a tablespace that supported that method of space management. High-Water Mark This is a term used with table segments stored in the database. If you envision a table, for example, as a “flat” structure or as a series of blocks laid one after the other in a line from left to right, the high-water mark (HWM) would be the rightmost block that ever contained data, as illustrated in Figure 10-1. Figure 10-1 shows that the HWM starts at the first block of a newly created table. As data is placed into the table over time and more blocks get used, the HWM rises. If we delete some (or even all) of the rows in the table, we might have many blocks that no longer contain data, but they are still under the HWM, and they will remain under the HWM until the object is rebuilt, truncated, or shrunk (shrinking of a segment is a new Oracle 10g feature that is supported only if the segment is in an ASSM tablespace). The HWM is relevant since Oracle will scan all blocks under the HWM, even when they contain no data, during a full scan. This will impact the performance of a full scan—especially if most of the blocks under the HWM are empty. To see this, just create a table with 1,000,000 rows (or create any table with a large number of rows), and then execute a SELECT COUNT(*) from this table. Now, DELETE every row in it and you will find that the SELECT COUNT(*) takes just as long (or longer, if you need to clean out the block; refer to the “Block Cleanout” section of Chapter 9) to count 0 rows as it did to count 1,000,000. This is because Oracle is busy reading all of the blocks below the HWM to see if they contain data. You should compare this to what happens if you used TRUNCATE on the table instead of deleting each individual row. TRUNCATE will reset the HWM of a table back to “zero” and will truncate the associated indexes on the table as well. If you plan on deleting every row in a table, TRUNCATE—if it can be used— would be the method of choice for this reason.
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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 77 • To maint
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CHAPTER 3 ■ FILES 79 • Resource
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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 113 tkyte@ORA10
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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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284 CHAPTER 9 ■ REDO AND UNDO cri
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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 435 Table 11
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CHAPTER 11 ■ INDEXES 445 Indicate
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CHAPTER 11 ■ INDEXES 447 an index
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CHAPTER 11 ■ INDEXES 449 Table 11
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CHAPTER 13 ■ ■ ■ Partitioning
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