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

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CHAPTER 10 ■ DATABASE TABLES 351 INITRANS and MAXTRANS Each block in a segment has a block header. Part of this block header is a transaction table. Entries will be made in the transaction table to describe which transactions have what rows/elements on the block locked. The initial size of this transaction table is specified by the INITRANS setting for the object. For tables, this defaults to 2 (indexes default to 2). This transaction table will grow dynamically as needed up to MAXTRANS entries in size (given sufficient free space on the block, that is). Each allocated transaction entry consumes 23 to 24 bytes of storage in the block header. Note that as of Oracle 10g, MAXTRANS is ignored—all segments have a MAXTRANS of 255. Heap Organized Tables A heap organized table is probably used 99 percent (or more) of the time in applications, although that might change over time with the advent of IOTs, since they can themselves be indexed. A heap organized table is the type of table you get by default when you issue the CREATE TABLE statement. If you want any other type of table structure, you need to specify that in the CREATE statement itself. A heap is a classic data structure studied in computer science. It is basically a big area of space, disk, or memory (disk in the case of a database table, of course), which is managed in an apparently random fashion. Data will be placed where it fits best, rather than in any specific sort of order. Many people expect data to come back out of a table in the same order it was put into it, but with a heap, this is definitely not assured. In fact, rather the opposite is guaranteed: the rows will come out in a wholly unpredictable order. This is quite easy to demonstrate. In this example, I will set up a table such that in my database I can fit one full row per block (I am using an 8KB blocksize). You do not need to have the case where you only have one row per block—I am just taking advantage of that to demonstrate a predictable sequence of events. The following sort of behavior (that rows have no order) will be observed on tables of all sizes, in databases with any blocksize: ops$tkyte@ORA10GR1> create table t 2 ( a int, 3 b varchar2(4000) default rpad('*',4000,'*'), 4 c varchar2(3000) default rpad('*',3000,'*') 5 ) 6 / Table created. ops$tkyte@ORA10GR1> insert into t (a) values ( 1); 1 row created. ops$tkyte@ORA10GR1> insert into t (a) values ( 2); 1 row created. ops$tkyte@ORA10GR1> insert into t (a) values ( 3); 1 row created.
352 CHAPTER 10 ■ DATABASE TABLES ops$tkyte@ORA10GR1> delete from t where a = 2 ; 1 row deleted. ops$tkyte@ORA10GR1> insert into t (a) values ( 4); 1 row created. ops$tkyte@ORA10GR1> select a from t; A ---------- 1 4 3 Adjust columns B and C to be appropriate for your blocksize if you would like to reproduce this. For example, if you have a 2KB blocksize, you do not need column C, and column B should be a VARCHAR2(1500) with a default of 1,500 asterisks. Since data is managed in a heap in a table like this, as space becomes available, it will be reused. ■Note When using ASSM or MSSM, you’ll find rows end up in “different places.” The underlying space management routines are very different, and the same operations executed against a table in ASSM and MSSM may well result in different physical order. The data will logically be the same, but it will be stored in different ways. A full scan of the table will retrieve the data as it hits it, never in the order of insertion. This is a key concept to understand about database tables: in general, they are inherently unordered collections of data. You should also note that I do not need to use a DELETE in order to observe this effect; I could achieve the same results using only INSERTs. If I insert a small row, followed by a very large row that will not fit on the block with the small row, and then a small row again, I may very well observe that the rows come out by default in the order “small row, small row, large row.” They will not be retrieved in the order of insertion. Oracle will place the data where it fits, not in any order by date or transaction. If your query needs to retrieve data in order of insertion, you must add a column to the table that you can use to order the data when retrieving it. That column could be a number column, for example, maintained with an increasing sequence (using the Oracle SEQUENCE object). You could then approximate the insertion order using a SELECT that did an ORDER BY on this column. It will be an approximation because the row with sequence number 55 may very well have committed before the row with sequence 54, therefore it was officially “first” in the database. You should think of a heap organized table as a big unordered collection of rows. These rows will come out in a seemingly random order, and depending on other options being used (parallel query, different optimizer modes, and so on), they may come out in a different order with the same query. Do not ever count on the order of rows from a query unless you have an ORDER BY statement on your query!
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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 69 and even dat
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CHAPTER 3 ■ FILES 71 all operatio
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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 4 ■ ■ ■ Memory Struct
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CHAPTER 4 ■ MEMORY STRUCTURES 117
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CHAPTER 6 ■ LOCKING AND LATCHING
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CHAPTER 8 ■ ■ ■ Transactions
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CHAPTER 11 ■ ■ ■ Indexes Inde
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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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Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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