hibernate - 如何使用 Hibernate/DB2 按 CLOB 列排序

Question

我正在使用 Hibernate 4 和 DB2 10.1。当我按映射到 CLOB 列的属性对实体 (criteria.addOrder(...)) 进行排序时，出现以下错误:

[...] could not extract ResultSet:.org.hibernate.exception.SQLGrammarException: could not extract ResultSet
    at org.hibernate.exception.internal.SQLExceptionTypeDelegate.convert(SQLExceptionTypeDelegate.java:82)
    at org.hibernate.exception.internal.StandardSQLExceptionConverter.convert(StandardSQLExceptionConverter.java:49)
    at org.hibernate.engine.jdbc.spi.SqlExceptionHelper.convert(SqlExceptionHelper.java:124)
    at org.hibernate.engine.jdbc.spi.SqlExceptionHelper.convert(SqlExceptionHelper.java:109)
    at org.hibernate.engine.jdbc.internal.ResultSetReturnImpl.extract(ResultSetReturnImpl.java:88)
    at org.hibernate.loader.Loader.getResultSet(Loader.java:2062)
    at org.hibernate.loader.Loader.executeQueryStatement(Loader.java:1859)
    at org.hibernate.loader.Loader.executeQueryStatement(Loader.java:1838)
    at org.hibernate.loader.Loader.doQuery(Loader.java:906)
    at org.hibernate.loader.Loader.doQueryAndInitializeNonLazyCollections(Loader.java:348)
    at org.hibernate.loader.Loader.doList(Loader.java:2550)
    at org.hibernate.loader.Loader.doList(Loader.java:2536)
    at org.hibernate.loader.Loader.listIgnoreQueryCache(Loader.java:2366)
    at org.hibernate.loader.Loader.list(Loader.java:2361)
    at org.hibernate.loader.criteria.CriteriaLoader.list(CriteriaLoader.java:124)
    at org.hibernate.internal.SessionImpl.list(SessionImpl.java:1587)
    at org.hibernate.internal.CriteriaImpl.list(CriteriaImpl.java:374)
[...]
Caused by: com.ibm.db2.jcc.am.SqlSyntaxErrorException: DB2 SQL Error: SQLCODE=-134, SQLSTATE=42907, SQLERRMC=TEXT7_0_0_, DRIVER=4.15.100
[...]

原因是 DB2 无法按 CLOB 列排序。所以下面的语句将会失败:

SELECT * FROM MY_TABLE ORDER BY CLOB_COLUMN; 

一种可能的解决方案是将 CLOB 转换为 VARCHAR 进行排序。所以这工作正常:

SELECT * FROM MY_TABLE ORDER BY CAST(CLOB_COLUMN AS VARCHAR(1000));

如何让 Hibernate 进行这样的特殊处理？

Answer 1

我通过子类化org.hibernate.criterion.Order并覆盖toSqlString(...)解决了我的问题。我的实现称为 DB2Order 并将所有调用委托(delegate)给 super，除非它检测到 DB2Dialect 并且其中一个排序列的类型为 CLOB。

在我的代码中我曾经调用:

if (sortProperty.isAscending()) {
     criteria.addOrder(Order.asc(sortPropertyPath));
} else {
     criteria.addOrder(Order.desc(sortPropertyPath));
}

现在我打电话:

if (sortProperty.isAscending()) {
     criteria.addOrder(new DB2Order(sortPropertyPath, true));
} else {
     criteria.addOrder(new DB2Order(sortPropertyPath, false));
}

我的 DB2Order 实现的一些细节:

/**
 * Specialized subclass of Hibernate's {@link Order} that performs special 
 * handling for DB2 databases. The general strategy is to let the superclass 
 * perform all processing except for special cases. Special cases covered are:
 * <ul> 
 * <li>Sorting by {@code CLOB} columns. {@code CLOB} columns cannot be sorted 
 * directly by DB2, e.g. the SQL statement 
 * <p><code>SELECT * FROM MY_TABLE ORDER BY CLOB_COLUMN;</code>
 * <p>will fail with
 * <p><code>DB2 SQL Error: SQLCODE=-134, SQLSTATE=42907</code>
 * <p>This class identifies sorting by CLOB columns and generates an SQL string 
 * that casts {@code CLOB} to a {@code VARCHAR} for sorting, rendering 
 * <p><code>SELECT * FROM MY_TABLE ORDER BY CAST(CLOB_COLUMN AS VARCHAR(1000));</code>
 * <p>(which is successfully executed by DB2).
 * <p><i>Note that this approach will trim {@code CLOB} entries that are larger 
 * than the {@code VARCHAR} casted to. So sorting may not be exact for long 
 * values.</i> 
 * </ul>
 */
public class DB2Order extends Order {
    ...    
    private static final int VARCHAR_SIZE = 1000;
    private static final String CAST_PREFIX = "CAST(";
    private static final String CAST_SUFFIX = " AS VARCHAR(" + VARCHAR_SIZE + "))";
    ...
    @Override
    public String toSqlString(Criteria criteria, CriteriaQuery criteriaQuery)
            throws HibernateException {
        if (isDB2(criteriaQuery) && isClob(criteria, criteriaQuery)) {
            return toSqlStringWithClobTypeCast(criteria, criteriaQuery);
        }
        return super.toSqlString(criteria, criteriaQuery);
    }

    private String toSqlStringWithClobTypeCast(Criteria criteria, CriteriaQuery criteriaQuery) {
        String[] columns = criteriaQuery.getColumnsUsingProjection(criteria, 
            getPropertyName());
        Type type = criteriaQuery.getTypeUsingProjection(criteria, 
                getPropertyName());
        StringBuilder fragment = new StringBuilder();
        for (int i = 0; i < columns.length; i++) {
            final StringBuilder expression = new StringBuilder();
            SessionFactoryImplementor factory = criteriaQuery.getFactory();
            int sqlType = type.sqlTypes(factory)[i];
            boolean lower = false;
            if (isIgnoreCase()) {
                lower = sqlType == Types.VARCHAR
                        || sqlType == Types.CHAR
                        || sqlType == Types.LONGVARCHAR;
            }
            boolean clob = (sqlType == Types.CLOB);
            if (lower) {
                expression.append(factory.getDialect().getLowercaseFunction())
                    .append('(');
            }
            if (clob) {
                expression.append(CAST_PREFIX);
            }
            expression.append(columns[i]);
            if (clob) {
                expression.append(CAST_SUFFIX);
            }
            if (lower) {
                expression.append(')');
            }
            fragment.append(factory.getDialect().renderOrderByElement(
                    expression.toString(), null, isAscending() ? "asc" : "desc",
                    NullPrecedence.NONE));
            if (i < columns.length - 1) {
                fragment.append(", ");
            }
        }
        return fragment.toString();
    }

    private boolean isClob(Criteria criteria, CriteriaQuery criteriaQuery) {
        String[] columns = criteriaQuery.getColumnsUsingProjection(criteria, 
                getPropertyName());
        Type type = criteriaQuery.getTypeUsingProjection(criteria, 
                getPropertyName());
        for (int i = 0; i < columns.length; i++) {
            SessionFactoryImplementor factory = criteriaQuery.getFactory();
            int sqlType = type.sqlTypes(factory)[i];
            if (sqlType == Types.CLOB) {
                return true;
            }
        }
        return false;
    }

    private boolean isDB2(CriteriaQuery criteriaQuery) {
        return criteriaQuery.getFactory().getDialect() instanceof DB2Dialect;
    }
}