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

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CHAPTER 12 ■ DATATYPES 511 They are used to approximate numbers; they are not nearly as precise as the built-in Oracle NUMBER type described previously. Floating-point numbers are commonly used in scientific applications and are useful in many types of applications due to the fact that they allow arithmetic to be done in hardware (on the CPU, the chip) rather than in Oracle subroutines. Therefore, the arithmetic is much faster if you are doing real number-crunching in a scientific application, but you would not want to use floating-points to store financial information. The syntax for declaring columns of this type in a table is very straightforward: BINARY_FLOAT BINARY_DOUBLE That is it. There are no options to these types whatsoever. Non-Native Number Types In addition to the NUMBER, BINARY_FLOAT, and BINARY_DOUBLE types, Oracle syntactically supports the following numeric datatypes: ■Note When I say “syntactically supports,” I mean that a CREATE statement may use these datatypes, but under the covers they are all really the NUMBER type. There are precisely three native numeric formats in Oracle 10g Release 1 and later and only one native numeric format in Oracle9i Release 2 and earlier. The use of any other numeric datatype was always mapped to the native Oracle NUMBER type. • NUMERIC(p,s): Maps exactly to a NUMBER(p,s). If p is not specified, it defaults to 38. • DECIMAL(p,s) or DEC(p,s): Maps exactly to a NUMBER(p,s). If p is not specified, it defaults to 38. • INTEGER or INT: Maps exactly to the NUMBER(38) type. • SMALLINT: Maps exactly to the NUMBER(38) type. • FLOAT(b): Maps to the NUMBER type. • DOUBLE PRECISION: Maps to the NUMBER type. • REAL: Maps to the NUMBER type. Performance Considerations In general, the Oracle NUMBER type is the best overall choice for most applications. However, there are performance implications associated with that type. The Oracle NUMBER type is a software datatype—it is implemented in the Oracle software itself. We cannot use native hardware operations to add two NUMBER types together, as it is emulated in the software. The floating-point types, however, do not have this implementation. When we add two floatingpoint numbers together, Oracle will use the hardware to perform the operation.
512 CHAPTER 12 ■ DATATYPES This is fairly easy to see. If we create a table that contains about 50,000 rows and place the same data in there using the NUMBER and BINARY_FLOAT/BINARY_DOUBLE types as follows: ops$tkyte@ORA10G> create table t 2 ( num_type number, 3 float_type binary_float, 4 double_type binary_double 5 ) 6 / Table created. ops$tkyte@ORA10G> insert /*+ APPEND */ into t 2 select rownum, rownum, rownum 3 from all_objects 4 / 48970 rows created. ops$tkyte@ORA10G> commit; Commit complete. We then execute the same query against each type of column, using a complex mathematical function such as LN (natural log). We observe radically different CPU utilization: select sum(ln(num_type)) from t call count cpu elapsed ------- ------ -------- ---------- total 4 2.73 2.73 select sum(ln(float_type)) from t call count cpu elapsed ------- ------ -------- ---------- total 4 0.06 0.12 select sum(ln(double_type)) from t call count cpu elapsed ------- ------ -------- ---------- total 4 0.05 0.11 The Oracle NUMBER type used some 50 times the CPU of the floating-point types. But, you have to remember that we did not receive precisely the same answer from all three queries! The floating-point numbers were an approximation of the number, with between 6 and 13 digits of precision. The answer from the NUMBER type is much more “precise” than from the floats. But when you are performing data mining or complex numerical analysis of scientific data, this loss of precision is typically acceptable and the performance gain to be had can be dramatic.
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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 xxiii Next up are t
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Setting Up Your Environment In this
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338 CHAPTER 10 ■ DATABASE TABLES
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CHAPTER 11 ■ ■ ■ Indexes Inde
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Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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