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

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CHAPTER 15 ■ DATA LOADING AND UNLOADING 689 ALTER TABLE T PROJECT COLUMN REFERENCED | ALL The COUNT(*) earlier in this section made me think about a new feature in Oracle 10g: the ability to optimize external table access by only accessing the fields in the external file that are referenced in the query. That is, if the external table is defined to have 100 number fields, but you select out only one of them, you can direct Oracle to bypass converting the other 99 strings into numbers. It sounds great, but it can cause a different number of rows to be returned from each query. Suppose the external table has 100 lines of data in it. All of the data for column C1 is “valid” and converts into a number. None of the data for column C2 is “valid,” and it does not convert into a number. If you select C1 from that external table, you’ll get 100 rows back. If you select C2 from that external table, you’ll get 0 rows back. You have to explicitly enable this optimization, and you should think about whether it is “safe” for you to use or not (only you know enough about your application and its processing to answer the question “Is it safe?”). Using the earlier example with the bad line of data added, we would expect to see the following output upon querying our external table: ops$tkyte@ORA10G> select dname 2 from SYS_SQLLDR_X_EXT_DEPT 3 / DNAME -------------- Sales Accounting Consulting Finance ops$tkyte@ORA10G> select deptno 2 from SYS_SQLLDR_X_EXT_DEPT 3 / DEPTNO ---------- 10 20 30 40 We know the “bad” record has been logged into the BADFILE. But if we simply ALTER the external table and tell Oracle to only “project” (process) the referenced columns, as follows: ops$tkyte@ORA10G> alter table SYS_SQLLDR_X_EXT_DEPT 2 project column referenced 3 / Table altered.
690 CHAPTER 15 ■ DATA LOADING AND UNLOADING ops$tkyte@ORA10G> select dname 2 from SYS_SQLLDR_X_EXT_DEPT 3 / DNAME -------------- Sales Accounting Consulting Finance XYZ ops$tkyte@ORA10G> select deptno 2 from SYS_SQLLDR_X_EXT_DEPT 3 / DEPTNO ---------- 10 20 30 40 we get different numbers of rows from each query. The DNAME field was valid for every single record in the input file, but the DEPTNO column was not. If we do not retrieve the DEPTNO column, it does not fail the record—the resultset is materially changed. Using an External Table to Load Different Files A common need is to use an external table to load data from differently named files over a period of time. That is, this week we must load file1.dat, and next week it will be file2.dat, and so on. So far, we’ve been loading from a fixed file name, demo1.dat. What if we need subsequently to load from a second file, demo2.dat? Fortunately, that is pretty easy to accommodate. The ALTER TABLE command may be used to repoint the location setting of the external table: ops$tkyte@ORA10G> alter table SYS_SQLLDR_X_EXT_DEPT 2 location( 'demo2.dat' ); Table altered. And that would pretty much be it—the very next query of that external table would have it accessing the file demo2.dat.
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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 4 ■ MEMORY STRUCTURES 117
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Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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