#249: Improved CLOB handling in Oracle dialect.

doc/dialects/oracle.md

@@ -137,6 +137,8 @@ CREATE VIRTUAL SCHEMA <virtual schema name>
 
 The Oracle dialect does not support all capabilities. A complete list can be found in [OracleSqlDialect.getCapabilities()](../../jdbc-adapter/virtualschema-jdbc-adapter/src/main/java/com/exasol/adapter/dialects/oracle/OracleSqlDialect.java).
 
+## Type Mappings and Limitations
+
 Oracle data types are mapped to their equivalents in Exasol. The following exceptions apply:
 
 - `NUMBER`, `NUMBER with precision > 36` and `LONG` are casted to `VARCHAR` to prevent a loss of precision. 
@@ -147,8 +149,12 @@ Oracle data types are mapped to their equivalents in Exasol. The following excep
 
     This will cast NUMBER with precision > 36, NUMBER without precision and LONG to DECIMAL(36,20).
     Keep in mind that this will yield errors if the data in the Oracle database does not fit into the specified DECIMAL type.
+
 - `DATE` is casted to `TIMESTAMP`. This data type is only supported for positive year values, i.e., years > 0001.
 - `TIMESTAMP WITH [LOCAL] TIME ZONE` is casted to `TIMESTAMP`. 
 - `INTERVAL` is casted to `VARCHAR`.
-- `CLOB` are `NCLOB` are treated as `VARCHAR`.
+- `CLOB` and `NCLOB` values are supported up to the maximum size of an Exasol [`VARCHAR`](https://docs.exasol.com/sql_references/data_types/datatypedetails.htm#StringDataType).
+
+    By default an error will be thrown if you try to import a value that is larger. If you want to import from tables which contain bigger values, you need to truncate the values.
+
 - `BLOB`, `RAW` and `LONG RAW` are not supported.

jdbc-adapter/virtualschema-jdbc-adapter/src/main/java/com/exasol/adapter/dialects/oracle/OracleSqlGenerationVisitor.java

@@ -19,7 +19,7 @@ public class OracleSqlGenerationVisitor extends SqlGenerationVisitor {
     private boolean requiresSelectListAliasesForLimit = false;
     private static final String TIMESTAMP_FORMAT = "'YYYY-MM-DD HH24:MI:SS.FF3'";
     private static final List<String> TYPE_NAMES_REQUIRING_CAST = List.of("TIMESTAMP", "INTERVAL", "BINARY_FLOAT",
-            "BINARY_DOUBLE", "CLOB", "NCLOB", "ROWID", "UROWID", "BLOB");
+            "BINARY_DOUBLE", "ROWID", "UROWID", "BLOB");
     private final Set<AggregateFunction> aggregateFunctionsCast = EnumSet.noneOf(AggregateFunction.class);
     private final Set<ScalarFunction> scalarFunctionsCast = EnumSet.noneOf(ScalarFunction.class);
 
@@ -62,7 +62,7 @@ Set<ScalarFunction> getScalarFunctionsCast() {
      * SELECT c1, c2, ... FROM ( SELECT LIMIT_SUBSELECT.*, ROWNUM ROWNUM_SUB FROM ( &lt;query-with-aliases&gt; )
      * LIMIT_SUBSELECT WHERE ROWNUM &lt;= 30 ) WHERE ROWNUM_SUB &gt; 20
      *
-     * The rownum filter is evaluated before ORDER BY, which is why we need subselects
+     * The rownum filter is evaluated before ORDER BY, which is why we need sub-selects
      */
     @Override
     public String visit(final SqlStatementSelect select) throws AdapterException {
@@ -261,11 +261,10 @@ private String getProjectionString(final SqlColumn column, final String projecti
         final AbstractSqlDialect dialect = (AbstractSqlDialect) getDialect();
         final String typeName = ColumnAdapterNotes
                 .deserialize(column.getMetadata().getAdapterNotes(), column.getMetadata().getName()).getTypeName();
-        if (typeName.startsWith("INTERVAL") || typeName.equals("BINARY_FLOAT") || typeName.equals("BINARY_DOUBLE")
-                || typeName.equals("CLOB") || typeName.equals("NCLOB")) {
+        if (typeName.startsWith("INTERVAL") || typeName.equals("BINARY_FLOAT") || typeName.equals("BINARY_DOUBLE")) {
             return "TO_CHAR(" + projectionString + ")";
         } else if (typeName.startsWith("TIMESTAMP")
-                && ((OracleSqlDialect) dialect).getImportType() == ImportType.JDBC) {
+                && (((OracleSqlDialect) dialect).getImportType() == ImportType.JDBC)) {
             return "TO_TIMESTAMP(TO_CHAR(" + projectionString + ", " + TIMESTAMP_FORMAT + "), " + TIMESTAMP_FORMAT
                     + ")";
         } else if (typeName.equals("NUMBER")) {

jdbc-adapter/virtualschema-jdbc-adapter/src/test/java/com/exasol/adapter/dialects/oracle/OracleSqlGenerationVisitorTest.java

@@ -122,7 +122,7 @@ void testVisitSqlSelectListSelectStar() throws AdapterException {
         assertSqlNodeConvertedToAsterisk(selectList, this.visitor);
     }
 
-    @CsvSource({ "NUMBER", "INTERVAL", "BINARY_FLOAT", "BINARY_DOUBLE", "CLOB", "NCLOB" })
+    @CsvSource({ "NUMBER", "INTERVAL", "BINARY_FLOAT", "BINARY_DOUBLE" })
     @ParameterizedTest
     void testVisitSqlSelectListSelectStarCastToChar(final String dataType) throws AdapterException {
         final SqlSelectList selectList = createSqlSelectStarListWithOneColumn(