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

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CHAPTER 15 ■ DATA LOADING AND UNLOADING 691 Multiuser Issues In the introduction to this section, I described three situations where external tables might not be as useful as SQLLDR. One of them was a specific multiuser issue. We just saw how to change the location of an external table—how to make it read from file 2 instead of file 1, and so on. The problem arises when multiple users each try to concurrently use that external table and have it point to different files for each session. This cannot be done. The external table will point to a single file (or set of files) at any given time. If I log in and alter the table to point to file 1 and you do the same at about the same time, and then we both query that table, we’ll both be processing the same file. Generally, this issue is not one that you should encounter. External tables are not a replacement for “database tables”; they are a means to load data, and as such you would not use them on a daily basis as part of your application. They are generally a DBA or developer tool used to load information, either as a one-time event or on a recurring basis, as in a data warehouse load. If the DBA has ten files to load into the database using the same external table, she would not do them sequentially—that is, pointing the external file to file 1 and processing it, then file 2 and processing it, and so on. Rather, she would simply point the external table to both files and let the database process both of them: ops$tkyte@ORA10G> alter table SYS_SQLLDR_X_EXT_DEPT 2 location( 'file1.dat', 'file2.dat') 3 / Table altered. If “parallel processing” is required, then the database already has the built-in ability to do this, as demonstrated in the last chapter. So the only multiuser issue would be if two sessions both tried to alter the location at about the same time—and this is just a possibility to be aware of, not something I believe you’ll actually run into very often. Another multiuser consideration is that of the bad and log file names. What if you have many sessions concurrently looking at the same external table, or using parallel processing (which in some respects is a multiuser situation)? It would be nice to be able to segregate these files by session, and fortunately you can do that. You may incorporate the following special strings: • %p: PID. • %a: Parallel execution servers agent ID. The parallel execution servers have the numbers 001, 002, 003, and so on assigned to them. In this fashion, each session will tend to generate its own bad and log files. For example, if you used the following BADFILE syntax in the CREATE TABLE from earlier: RECORDS DELIMITED BY NEWLINE CHARACTERSET WE8ISO8859P1 BADFILE 'SYS_SQLLDR_XT_TMPDIR_00000':'demo1_%p.bad' LOGFILE 'demo1.log_xt' you would expect to find a file named similarly to the following: $ ls *.bad demo1_7108.bad
692 CHAPTER 15 ■ DATA LOADING AND UNLOADING However, you still might have issues over lengthy periods of time. The PIDs will be reused on most operating systems. So the techniques outlined in dealing with errors may well still be relevant—you’ll need to reset your bad file or rename it if it exists and if you determine this to be an issue. External Tables Summary In this section, we explored external tables. They are a new feature of Oracle9i and later that may, for the most part, replace SQLLDR. We investigated the quickest way to get going with external tables: the technique of using SQLLDR to convert the control files we have from past experiences. We demonstrated some techniques for detecting and handling errors via the bad files and, lastly, we explored some multiuser issues regarding external tables. We are now ready to get into the last sections in this chapter, which deal with unloading data from the database. Flat File Unload One thing SQLLDR does not do, and that Oracle supplies no command-line tools for, is unload data in a format understandable by SQLLDR or other programs. This would be useful for moving data from system to system without using EXP/IMP or EXPDP/IMPDP (the new Data Pump replacements for EXP and IMP). Using EXP(DP)/IMP(DP) to move data from system to system works fine—as long as both systems are Oracle. ■Note HTML DB provides a data export feature as part of its SQL Workshop. You may export the information in a CSV format easily. This works well for a few megabytes of information, but it is not appropriate for many tens of megabytes or more. We will develop a small PL/SQL utility that may be used to unload data on a server in a SQLLDR-friendly format. Also, equivalent tools for doing so in Pro*C and SQL*Plus are provided on the Ask Tom web site at http://asktom.oracle.com/~tkyte/flat/index.html. The PL/SQL utility will work fine in most small cases, but better performance will be had using Pro*C. Pro*C and SQL*Plus are also useful if you need the files to be generated on the client and not on the server, which is where PL/SQL will create them. The specification of the package we will create is as follows: ops$tkyte@ORA10G> create or replace package unloader 2 AUTHID CURRENT_USER 3 as 4 /* Function run -- unloads data from any query into a file 5 and creates a control file to reload that 6 data into another table 7 8 p_query = SQL query to "unload". May be virtually any query. 9 p_tname = Table to load into. Will be put into control file.
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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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Foreword “THINK.” In 1914, Thom
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About the Technical Reviewers ■JO
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Introduction The inspiration for th
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