187 lines
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HTML
187 lines
12 KiB
HTML
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<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE html
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PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<meta name="DC.Type" content="concept" />
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<meta name="DC.Title" content="DataTruncation and silent truncation" />
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<meta name="abstract" content="DataTruncation is a subclass of SQLWarning. While SQLWarnings are not thrown, DataTruncation objects are sometimes thrown and attached like other SQLWarning objects. Silent truncation occurs when the size of a column exceeds the size specified by the setMaxFieldSize statement method, but no warning or exception is reported." />
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<meta name="description" content="DataTruncation is a subclass of SQLWarning. While SQLWarnings are not thrown, DataTruncation objects are sometimes thrown and attached like other SQLWarning objects. Silent truncation occurs when the size of a column exceeds the size specified by the setMaxFieldSize statement method, but no warning or exception is reported." />
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<meta name="DC.Relation" scheme="URI" content="exceptions.htm" />
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<meta name="DC.Relation" scheme="URI" content="sqlwarng.htm" />
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<meta name="copyright" content="(C) Copyright IBM Corporation 2006" />
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<!-- All rights reserved. Licensed Materials Property of IBM -->
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<link rel="stylesheet" type="text/css" href="./ibmdita.css" />
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<link rel="stylesheet" type="text/css" href="./ic.css" />
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<title>DataTruncation and silent truncation</title>
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</head>
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<body id="datatruk"><a name="datatruk"><!-- --></a>
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<!-- Java sync-link --><script language="Javascript" src="../rzahg/synch.js" type="text/javascript"></script>
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<h1 class="topictitle1">DataTruncation and silent truncation</h1>
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<div><p>DataTruncation is a subclass of SQLWarning. While SQLWarnings are
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not thrown, DataTruncation objects are sometimes thrown and attached like
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other SQLWarning objects. Silent truncation occurs when the size of a column
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exceeds the size specified by the setMaxFieldSize statement method, but no
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warning or exception is reported.</p>
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<p>DataTruncation objects provide additional information beyond what is returned
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by an SQLWarning. The available information includes the following:</p>
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<ul><li>The number of bytes of data that have been transferred.</li>
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<li>The column or parameter index that was truncated.</li>
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<li>Whether the index is for a parameter or a ResultSet column.</li>
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<li>Whether the truncation happened when reading from the database or writing
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to it.</li>
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<li>The amount of data that was actually transferred.</li>
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</ul>
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<p> In some instances, the information can be deciphered, but situations
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arise that are not completely intuitive. For example, if the PreparedStatement's
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setFloat method is used to insert a value into a column that holds integer
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values, a DataTruncation may result because the float may be larger than the
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largest value that the column can hold. In these situations, the byte counts
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for truncation do not make sense, but it is important for the driver to provide
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the truncation information. </p>
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<div class="section"><h4 class="sectiontitle">Report set() and update() methods</h4><p>There is a subtle
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difference between JDBC drivers. Some drivers such as the native and IBM<sup>®</sup> Toolbox for Java™ JDBC
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drivers catch and report data truncation issues at the time of the parameter
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setting. This is done either on the PreparedStatement set method or the ResultSet
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update method. Other drivers report the problem at the time of processing
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the statement and is accomplished by the execute, executeQuery, or updateRow
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methods.</p>
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<p>Failing to report the problem at the time that you provide
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incorrect data instead of at the time that processing cannot continue any
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further offers a couple of advantages:</p>
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<ul><li>The failure can be addressed in your application when you have a problem
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instead of addressing the problem at processing time.</li>
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<li>By checking when setting the parameters, the JDBC driver can ensure that
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the values that are handed to the database at statement processing time are
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valid. This allows the database to optimize its work and processing can be
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completed faster.</li>
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</ul>
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</div>
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<div class="section"><h4 class="sectiontitle">ResultSet.update() methods throw DataTruncation exceptions</h4><p>In
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some past releases, ResultSet.update() methods posted warnings when truncation
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conditions existed. This case occurs when the data value is going to be inserted
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into the database. The specification dictates that JDBC drivers throw exceptions
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in these cases. As a result, the JDBC driver works in this manner.</p>
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<p>There
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are no significant difference between handling a ResultSet update function
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that receives a data truncation error and handling a prepared statement parameter
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set for an update or insert statement that receives an error. In both cases,
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the problem is identical; you provided data that does not fit where you wanted
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it.</p>
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<p>NUMERIC and DECIMAL truncate to the right side of a decimal point
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silently. This is how both JDBC for UDB NT works and how interactive SQL on
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an iSeries™ server
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works.</p>
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<p><strong>Note:</strong> No value is rounded when a data truncation occurs.
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Any fractional portion of a parameter that does not fit in a NUMERIC or DECIMAL
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column is simply lost without warning.</p>
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<p>The following are examples, assuming
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that the value in the values clause is actually a parameter being set on a
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prepared statement:</p>
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<pre>create table cujosql.test (col1 numeric(4,2))
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a) insert into cujosql.test values(22.22) // works - inserts 22.22
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b) insert into cujosql.test values(22.223) // works - inserts 22.22
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c) insert into cujosql.test values(22.227) // works - inserts 22.22
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d) insert into cujosql.test values(322.22) // fails - Conversion error on assignment to column COL1.</pre>
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<strong>Difference
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between a data truncation warning and a data truncation exception</strong><p>The
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specification states that data truncation on a value to be written to the
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database throws an exception. If data truncation is not performed on the value
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being written to the database, a warning is generated. This means that the
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point at which a data truncation situation is identified, you must also be
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aware of the statement type that the data truncation is processing. Given
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this as a requirement, the following lists the behavior of several SQL statement
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types:</p>
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<ul><li>In a SELECT statement, query parameters never damage database content.
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Therefore, data truncation situations are always handled by posting warnings.</li>
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<li>In VALUES INTO and SET™ statements, the input values are only
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used to generate output values. As a result, warnings are issued.</li>
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<li>In a CALL statement, the JDBC driver cannot determine what a stored procedure
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does with a parameter. Exceptions are always thrown when a stored procedure
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parameter truncates.</li>
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<li>All other statement types throw exceptions rather than post warnings.</li>
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</ul>
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</div>
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<div class="section"><h4 class="sectiontitle">Data truncation property for Connection and DataSource</h4><p>There
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has been a data truncation property available for many releases. The default
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for that property is true, meaning that data truncation issues are checked
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and warnings are posted or exceptions are thrown. The property is provided
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for convenience and performance in cases where you are not concerned that
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a value does not fit into the database column. You want the driver to put
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as much of the value as it can into the column.</p>
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</div>
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<div class="section"><h4 class="sectiontitle">Data truncation property only affects character and binary-based
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data types</h4><p>A couple releases ago, the data truncation property determined
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whether data truncation exceptions could be thrown. The data truncation property
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was put in place to have JDBC applications not worry about a value getting
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truncated when the truncation was not important to them. There are few cases
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where you would want either the value 00 or 10 stored in the database when
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applications attempted to insert 100 into a DECIMAL(2,0). Therefore, the JDBC
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driver's data truncation property was changed to only honor situations where
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the parameter is for character-based types such as CHAR, VARCHAR, CHAR FOR
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BIT DATA, and VARCHAR FOR BIT DATA.</p>
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</div>
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<div class="section"><h4 class="sectiontitle">Data truncation property is only applied to parameters</h4><p>The
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data truncation property is a setting of the JDBC driver and not of the database.
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As a result, it has no effect on statement literals. For example, the following
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statements that are processed to insert a value into a CHAR(8) column in the
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database still fail with the data truncation flag set to false (assume that
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connection is a java.sql.Connection object allocated elsewhere).</p>
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<pre>Statement stmt = connection.createStatement();
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Stmt.executeUpdate("create table cujosql.test (col1 char(8))");
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Stmt.executeUpdate("insert into cujosql.test values('dettinger')");
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// Fails as the value does not fit into database column.</pre>
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</div>
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<div class="section"><h4 class="sectiontitle">Native JDBC driver throws exceptions for insignificant data
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truncation</h4><p>The native JDBC driver does not look at the data that
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you provide for parameters. Doing so only slows down processing. However,
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there can be situations where it does not matter to you that a value truncates,
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but you have not set the data truncation connection property to false.</p>
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<p>For
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example, 'dettinger ', a char(10) that is passed, throws an exception even
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though everything important about the value fits. This does happen to be how
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JDBC for UDB NT works; however, it is not the behavior you would get if you
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passed the value as a literal in an SQL statement. In this case, the database
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engine would throw out the additional spaces quietly.</p>
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<p>The problems with
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the JDBC driver not throwing an exception are the following:</p>
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<ul><li>Performance overhead is extensive on every set method, whether needed
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or not. For the majority of cases where there would be no benefit, there is
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considerable performance overhead on a function as common as setString().</li>
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<li>Your workaround is trivial, for example, calling the trim function on
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the string value passed in.</li>
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<li>There are issues with the database column to take into account. A space
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in CCSID 37 is not at all a space in CCSID 65535, or 13488.</li>
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</ul>
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</div>
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<div class="section"><h4 class="sectiontitle">Silent truncation</h4><p>The setMaxFieldSize statement
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method allows a maximum field size to be specified for any column. If data
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truncates because its size has exceeded the maximum field size value, no warning
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or exception is reported. This method, like the data truncation property previously
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mentioned, only affects character-based types such as CHAR, VARCHAR, CHAR
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FOR BIT DATA, and VARCHAR FOR BIT DATA. </p>
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</div>
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</div>
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<div>
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<div class="familylinks">
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<div class="parentlink"><strong>Parent topic:</strong> <a href="exceptin.htm" title="The Java language uses exceptions to provide error-handling capabilities for its programs. An exception is an event that occurs when you run your program that disrupts the normal flow of instructions.">Exceptions</a></div>
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</div>
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<div class="relconcepts"><strong>Related concepts</strong><br />
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<div><a href="exceptions.htm" title="The SQLException class and its subtypes provide information about errors and warnings that occur while a data source is being accessed.">SQLException</a></div>
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<div><a href="sqlwarng.htm" title="Methods in some interfaces generate an SQLWarning object if the methods cause a database access warning.">SQLWarning</a></div>
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</div>
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</div>
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</body>
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</html>
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