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

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CHAPTER 12 ■ DATATYPES 509 MSG NUM_COL ---------- ---------- 123.45 100 123.456 100 1234567 1234600 ops$tkyte@ORA10GR1> insert into t (msg,num_col) values ( '12345678', 12345678 ); insert into t (msg,num_col) values ( '12345678', 12345678 ) * ERROR at line 1: ORA-01438: value larger than specified precision allows for this column So, the precision dictates how many digits are permitted in the number after rounding, using the scale to determine how to round. The precision is an integrity constraint, whereas the scale is an “edit.” It is interesting and useful to note that the NUMBER type is, in fact, a variable-length datatype on disk and will consume between 0 and 22 bytes of storage. Many times, programmers consider a numeric datatype to be a fixed-length type, as that is what they typically see when programming with 2- or 4-byte integers and 4- or 8-byte floats. The Oracle NUMBER type is similar to a variable-length character string. We can see what happens with numbers that contain differing amounts of significant digits. We’ll create a table with two NUMBER columns and populate the first column with many numbers that have 2, 4, 6, . . . 28 significant digits. Then, we’ll simply add 1 to each of them: ops$tkyte@ORA10GR1> create table t ( x number, y number ); Table created. ops$tkyte@ORA10GR1> insert into t ( x ) 2 select to_number(rpad('9',rownum*2,'9')) 3 from all_objects 4 where rownum update t set y = x+1; 14 rows updated. Now, if we use the built-in VSIZE function that shows how much storage the column takes, we can review the size differences between the two numbers in each row: ops$tkyte@ORA10GR1> set numformat 99999999999999999999999999999 ops$tkyte@ORA10GR1> column v1 format 99 ops$tkyte@ORA10GR1> column v2 format 99 ops$tkyte@ORA10GR1> select x, y, vsize(x) v1, vsize(y) v2 2 from t order by x; X Y V1 V2 ------------------------------ ------------------------------ --- --- 99 100 2 2 9999 10000 3 2
510 CHAPTER 12 ■ DATATYPES 999999 1000000 4 2 99999999 100000000 5 2 9999999999 10000000000 6 2 999999999999 1000000000000 7 2 99999999999999 100000000000000 8 2 9999999999999999 10000000000000000 9 2 999999999999999999 1000000000000000000 10 2 99999999999999999999 100000000000000000000 11 2 9999999999999999999999 10000000000000000000000 12 2 999999999999999999999999 1000000000000000000000000 13 2 99999999999999999999999999 100000000000000000000000000 14 2 9999999999999999999999999999 10000000000000000000000000000 15 2 14 rows selected. We can see that as we added significant digits to X, the amount of storage required took increasingly more room. Every two significant digits added another byte of storage. But a number just one larger consistently took 2 bytes. When Oracle stores a number, it does so by storing as little as it can to represent that number. It does this by storing the significant digits, an exponent used to place the decimal place, and information regarding the sign of the number (positive or negative). So, the more significant digits a number contains, the more storage it consumes. That last fact explains why it is useful to know that numbers are stored in varying width fields. When attempting to size a table (e.g., to figure out how much storage 1,000,000 rows would need in a table), you have to consider the NUMBER fields carefully. Will your numbers take 2 bytes or 20 bytes? What is the average size? This makes accurately sizing a table without representative test data very hard. You can get the worst-case size and the best-case size, but the real size will likely be some value in between. BINARY_FLOAT/BINARY_DOUBLE Type Syntax and Usage Oracle 10g introduced two new numeric types for storing data; they are not available in any release of Oracle prior to version 10g. These are the IEEE standard floating-points many programmers are used to working with. For a full description of what these number types look like and how they are implemented, I suggest reading http://en.wikipedia.org/wiki/ Floating-point. It is interesting to note the following in the basic definition of a floatingpoint number in that reference (emphasis mine): A floating-point number is a digital representation for a number in a certain subset of the rational numbers, and is often used to approximate an arbitrary real number on a computer. In particular, it represents an integer or fixed-point number (the significand or, informally, the mantissa) multiplied by a base (usually 2 in computers) to some integer power (the exponent). When the base is 2, it is the binary analogue of scientific notation (in base 10).
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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 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 99 “accidenta
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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 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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298 CHAPTER 9 ■ REDO AND UNDO ops
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338 CHAPTER 10 ■ DATABASE TABLES
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CHAPTER 11 ■ ■ ■ Indexes Inde
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CHAPTER 11 ■ INDEXES 423 value of
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CHAPTER 11 ■ INDEXES 431 The data
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CHAPTER 11 ■ INDEXES 445 Indicate
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Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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