Getting Started with DB2 Express-C

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224 Getting Started with DB2 Express-C Figure 13.8 – Comparing isolation levels for a fetch 13.4.6 Setting the isolation level Isolation levels can be specified at many levels: • Session (application) • Connection • Statement The isolation level is normally defined at the session or Application Level. If no isolation level is specified in your application, it defaults to cursor stability. For example, Table 13.1 shows the possible isolation levels for a .NET or JDBC program and how these properties, when set, match a DB2 isolation level. DB2 .NET JDBC Uncommitted Read (UR) ReadUncommitted TRANSACTION_READ_UNCOMMITTED Cursor Stability (CS) ReadCommitted TRANSACTION_READ_COMMITTED Read Stability (RS) RepeatableRead TRANSACTION_REPEATABLE_READ Repeatable Read (RR) Serializable TRANSACTION_SERIALIZABLE Table 13.1 - Comparison of isolation level terminology
Chapter 13 – Concurrency and Locking 225 Statement isolation level can be set using the WITH {isolation level} clause. For example: SELECT ... WITH {UR | CS | RS | RR} Example scenario: An application needs to get a "rough" count of how many rows are in a table. Performance is of utmost importance. Cursor stability isolation level is required with the exception of one SQL statement: SELECT COUNT(*) FROM tab1 WITH UR For embedded SQL, the level is set at bind time, for dynamic SQL, the level is set at run time. Choosing which isolation level to use depends on your application. If your application does not need exact counts as in the above example, choose UR isolation. If your application requires very tight control on the data it works with, choose RR isolation. To use currently committed semantics at bind time or prepare time, use this syntax: BIND: >--+-------------------------------------------------------------+--> '--CONCURRENTACCESSRESOLUTION--+--USE CURRENTLY COMMITTED--+--' '--WAIT FOR OUTCOME---------' PREPARE: concurrent-access-resolution: |-+-USE CURRENTLY COMMITTED-+--------------------------| '-WAIT FOR OUTCOME--------' On a JDBC application using the IBM Data Server Driver for JDBC and SQLJ, you can use the concurrentAccessResolution property to enable currently committed. 13.5 Lock escalation Every lock made by DB2 consumes some memory. When the optimizer thinks it is better to have one lock on the entire table, rather than multiple row locks, lock escalation occurs. Figure 13.9 illustrates this.
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GETTING STARTED WITH DB2 Express-C
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Contents About this book ..........
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Contents 7 6.3 DB2 storage model ..
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Contents 9 13.4.2 Cursor stability
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About this book Notices and tradema
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About this Book 13 A book for the c
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About this Book 15 Foreword Innovat
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1 Chapter 1 - What is DB2 Express-C
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Chapter 1 - What is DB2 Express-C?
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1.8 Moving up to another DB2 editio
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4 Chapter 4 - DB2 Environment The D
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8 Chapter 8 - Working with Database
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9 Chapter 9 - Data Movement Utiliti
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Chapter 9 - Data Movement Utilities
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Getting started with DB2 9.7 couldn
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Getting Started with DB2 Express-C

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