Github user patricker commented on a diff in the pull request:

    https://github.com/apache/nifi/pull/2162#discussion_r166500502
  
    --- Diff: 
nifi-nar-bundles/nifi-standard-bundle/nifi-standard-processors/src/main/java/org/apache/nifi/processors/standard/AbstractDatabaseFetchProcessor.java
 ---
    @@ -249,34 +260,56 @@ public void setup(final ProcessContext context, 
boolean shouldCleanCache, FlowFi
                     return;
                 }
     
    -            // Try to fill the columnTypeMap with the types of the desired 
max-value columns
    -            final DBCPService dbcpService = 
context.getProperty(DBCP_SERVICE).asControllerService(DBCPService.class);
    -            final String tableName = 
context.getProperty(TABLE_NAME).evaluateAttributeExpressions(flowFile).getValue();
    +        // Try to fill the columnTypeMap with the types of the desired 
max-value columns
    +        final DBCPService dbcpService = 
context.getProperty(DBCP_SERVICE).asControllerService(DBCPService.class);
    +        final String tableName = 
context.getProperty(TABLE_NAME).evaluateAttributeExpressions(flowFile).getValue();
    +        final String sqlQuery = 
context.getProperty(SQL_QUERY).evaluateAttributeExpressions().getValue();
     
                 final DatabaseAdapter dbAdapter = 
dbAdapters.get(context.getProperty(DB_TYPE).getValue());
                 try (final Connection con = dbcpService.getConnection();
                      final Statement st = con.createStatement()) {
     
    -                // Try a query that returns no rows, for the purposes of 
getting metadata about the columns. It is possible
    -                // to use DatabaseMetaData.getColumns(), but not all 
drivers support this, notably the schema-on-read
    -                // approach as in Apache Drill
    -                String query = dbAdapter.getSelectStatement(tableName, 
maxValueColumnNames, "1 = 0", null, null, null);
    -                ResultSet resultSet = st.executeQuery(query);
    -                ResultSetMetaData resultSetMetaData = 
resultSet.getMetaData();
    -                int numCols = resultSetMetaData.getColumnCount();
    -                if (numCols > 0) {
    -                    if (shouldCleanCache) {
    -                        columnTypeMap.clear();
    -                    }
    -                    for (int i = 1; i <= numCols; i++) {
    -                        String colName = 
resultSetMetaData.getColumnName(i).toLowerCase();
    -                        String colKey = getStateKey(tableName, colName);
    -                        int colType = resultSetMetaData.getColumnType(i);
    -                        columnTypeMap.putIfAbsent(colKey, colType);
    +            // Try a query that returns no rows, for the purposes of 
getting metadata about the columns. It is possible
    +            // to use DatabaseMetaData.getColumns(), but not all drivers 
support this, notably the schema-on-read
    +            // approach as in Apache Drill
    +            String query;
    +
    +            if(StringUtils.isEmpty(sqlQuery)) {
    +                query = dbAdapter.getSelectStatement(tableName, 
maxValueColumnNames, "1 = 0", null, null, null);
    +            } else {
    +                StringBuilder sbQuery = getWrappedQuery(sqlQuery, 
tableName);
    --- End diff --
    
    I've checked in fixes for everything except this change. I don't want to 
put in any more SQL building logic than I already have hard coded into QDB. 
What if I added a new method to `DatabaseAdapter` for wrapping a `SELECT` 
statement as a sub query. Input parameters would be similar to the existing 
method for building a SELECT statement; column list, where clause, order by 
clause.


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