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CHAPTER 2 DATABASE METADATA, PART 1
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of your application. In other words, if you write slow code, the JDBC driver does not throw an exception, but you get a performance hit (which might translate to losing clients). Good performance means that you are satisfying your project s performance requirements, which should be defined precisely in requirements and design documents. To get acceptable performance from JDBC drivers, avoid passing null parameters to most of the methods; for example, passing a null value to the schema parameter might result in a search of all database schemas so if you know the name of your desired schema, then pass the actual schema value. In general, developing performance-oriented JDBC applications is not easy. In every step of the solution, you must make sure that your application will not choke under heavy requirements. For performance reasons, you should avoid excessive metadata calls, because database metadata methods that generate ResultSet objects are relatively slow. JDBC applications may cache information returned from result sets that generate database metadata methods so that multiple executions are not needed. For this purpose you may use Java Caching System (JCS) from the Apache Software Foundation. JCS is a distributed caching system written in Java for server-side Java applications; for more information, see http://jakarta.apache.org/turbine/jcs/. A Java class called DatabaseMetaDataTool (which is defined in the jcb.meta package) will be available for download from the Source Code section of the Apress website. It provides ready-touse methods for answering the database metadata questions. For questions about database metadata, I list portions of these classes in some sections of this book, but you ll find the complete class definitions (including JavaDoc-style comments) at the Apress website. All of the methods in this chapter are static, and each method is written to be as independent as possible. This is so you can cut and paste solutions from this book whenever possible. The methods will return the result as XML (serialized as a String object, which can be easily converted to an XML document such as org.w3c.dom.Document or org.jdom.Document). Also, I provide a utility class, DocumentManager, which can Convert org.w3c.dom.Document to XML as a serialized String object Convert org.jdom.Document to XML as a serialized String object Convert XML as a serialized String object into org.w3c.dom.Document Convert XML as a serialized String object into org.jdom.Document In general, it is efficient to create XML as a serialized String object. The Java and JDBC solutions are grouped in a Java package called jcb (JDBC CookBook). Table 2-1 shows the structure of the package. All of the code will be available from the Source Code section of the Apress website. Table 2-1. JCB Package Structure
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Package name for JDBC CookBook Database and ResultSet metadata-related interfaces and classes Database-related interfaces and classes Utility classes Java servlets All client and test programs using the jcb package
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CHAPTER 2 DATABASE METADATA, PART 1
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When using the DatabaseMetaData object, you should observe two key facts: The MySQL database does not understand schema ; you have to use catalog. The Oracle database does not understand catalog ; you have to use schema.
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2.1. What Is Metadata
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Metadata is data about data (or information about information), which provides structured, descriptive information about other data. According to Wikipedia (http://en.wikipedia.org/ wiki/Metadata): Metadata (Greek: meta-+ Latin: data information ), literally data about data , is information that describes another set of data. A common example is a library catalog card, which contains data about the contents and location of a book: It is data about the data in the book referred to by the card. Other common contents of metadata include the source or author of the described dataset, how it should be accessed, and its limitations. The following quote from the NOAA Coastal Services Center, or CSC (http:// www.csc.noaa.gov/metadata/text/whatismet.htm), illustrates the importance of the concept of metadata: Imagine trying to find a book in a library without the help of a card catalog or computerized search interface. Could you do it Perhaps, but it would be difficult at best. The information contained in such a system is essentially metadata about the books that are housed at that library or at other libraries. It provides you with vital information to help you find a particular book and aids you in making a decision as to whether that book might fit your needs. Metadata serves a similar purpose for geospatial data. The NOAA CSC further adds that metadata is a component of data which describes the data. It is data about data. Metadata describes the content, quality, condition, and other characteristics of data. Metadata describes the who, what, when, where, why, and how of a data set. Without proper documentation, a data set is incomplete. KTWEB (http://www.ktweb.org/rgloss.cfm) defines metadata as data about data, or information about information; in practice, metadata comprises a structured set of descriptive elements to describe an information resource or, more generally, any definable entity. Relational databases (such as MySQL and Oracle) use tables and other means (such as operating system file systems) to store their own data and metadata. Each relational database has its own proprietary methods for storing metadata. Examples of relational database metadata include A list of all the tables in the database, including their names, sizes, and the number of rows A list of the columns in each database, and what tables they are used in, as well as the type of data stored in each column
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