13 External Tables Concepts

Access Parameters

When you create an external table of a particular type, you can specify access parameters to modify the default behavior of the access driver. Each access driver has its own syntax for access parameters.

Location of Datafiles and Output Files

The access driver runs inside the database server. This is different from SQL*Loader, which is a client program that sends the data to be loaded over to the server. This difference has the following implications:

• The server must have access to any files to be loaded by the access driver.

• The server must create and write the output files created by the access driver: the log file, bad file, and discard file, as well as any dump files created by the ORACLE_DATAPUMP access driver.

The access driver does not allow you to specify a complete specification for files. This is because the server may have access to files that you do not, and allowing you to read this data would affect security. Similarly, you might overwrite a file that you normally would not have privileges to delete.

Instead, you are required to specify directory objects as the locations from which to read files and write files. A directory object maps a name to a directory name on the file system. For example, the following statement creates a directory object named ext_tab_dir that is mapped to a directory located at /usr/apps/datafiles.

CREATE DIRECTORY ext_tab_dir AS '/usr/apps/datafiles';

Directory objects can be created by DBAs or by any user with the CREATE ANY DIRECTORY privilege.

After a directory is created, the user creating the directory object needs to grant READ and WRITE privileges on the directory to other users. These privileges must be explicitly granted, rather than assigned through the use of roles. For example, to allow the server to read files on behalf of user scott in the directory named by ext_tab_dir, the user who created the directory object must execute the following command:

GRANT READ ON DIRECTORY ext_tab_dir TO scott;

The name of the directory object can appear in the following places in a CREATE TABLE...ORGANIZATION EXTERNAL statement:

• The DEFAULT DIRECTORY clause, which specifies the default directory to use for all input and output files that do not explicitly name a directory object.

• The LOCATION clause, which lists all of the datafiles for the external table. The files are named in the form directory:file. The directory portion is optional. If it is missing, the default directory is used as the directory for the file.

• The ACCESS PARAMETERS clause where output files are named. The files are named in the form directory:file. The directory portion is optional. If it is missing, the default directory is used as the directory for the file. Syntax in the access parameters enables you to indicate that a particular output file should not be created. This is useful if you do not care about the output files or if you do not have write access to any directory objects.

The SYS user is the only user that can own directory objects, but the SYS user can grant other users the privilege to create directory objects. Note that READ or WRITE permission to a directory object means only that the Oracle database will read or write that file on your behalf. You are not given direct access to those files outside of the Oracle database unless you have the appropriate operating system privileges. Similarly, the Oracle database requires permission from the operating system to read and write files in the directories.

Example: Creating and Loading an External Table Using ORACLE_LOADER

The steps in this section show an example of using the ORACLE_LOADER access driver to create and load an external table. A traditional table named emp is defined along with an external table named emp_load. The external data is then loaded into an internal table.

1. Assume your .dat file looks as follows:

   56november, 15, 1980  baker             mary       alice     
   87december, 20, 1970  roper             lisa       marie   
   
   

2. Execute the following SQL statements to set up a default directory (which contains the data source) and to grant access to it:

   CREATE DIRECTORY ext_tab_dir AS '/usr/apps/datafiles';
   GRANT READ ON DIRECTORY ext_tab_dir TO SCOTT;
   
   

3. Create a traditional table named emp:

   CREATE TABLE emp (emp_no CHAR(6), last_name CHAR(25), first_name CHAR(20), middle_initial CHAR(1));
   
   

4. Create an external table named emp_load:

   CREATE TABLE emp_load (employee_number CHAR(5), employee_last_name CHAR(20),
                          employee_first_name CHAR(15), 
                          employee_middle_name CHAR(15))
   ORGANIZATION EXTERNAL (TYPE ORACLE_LOADER DEFAULT DIRECTORY ext_tab_dir
                         ACCESS PARAMETERS (RECORDS DELIMITED BY NEWLINE FIELDS 
                         (employee_number CHAR(2),
                         employee_dob CHAR(20),
                         employee_last_name CHAR(18),
                         employee_first_name CHAR(11),
                         employee_middle_name CHAR(11)))
                         LOCATION ('info.dat'));
   
   

5. Load the data from the external table emp_load into the table emp:

   INSERT INTO emp (emp_no, first_name, middle_initial, last_name)
   (SELECT employee_number, employee_first_name, 
           substr(employee_middle_name, 1, 1),
           employee_last_name
   FROM emp_load);
   
   

6. Perform the following select operation to verify that the information in the .dat file was loaded into the emp table:

   SQL> SELECT * FROM emp;
   
   EMP_NO LAST_NAME                 FIRST_NAME           M
   ------ ------------------------- -------------------- -
   56     baker                     mary                 a
   87     roper                     lisa                 m
   
   

Notes about this example:

• The employee_number field in the datafile is converted to a character string for the employee_number field in the external table.

• The datafile contains an employee_dob field that is not loaded into any field in the table.

• The substr function is used on the employee_middle_name column in the external table to generate the value for middle_initial in table emp.

Loading Data

When data is loaded, the data stream is read from the files specified by the LOCATION and DEFAULT DIRECTORY clauses. The INSERT statement generates a flow of data from the external data source to the Oracle SQL engine, where data is processed. As data from the external source is parsed by the access driver and provided to the external table interface, it is converted from its external representation to its Oracle internal datatype.

Unloading Data Using the ORACLE_DATAPUMP Access Driver

To unload data, you use the ORACLE_DATAPUMP access driver. The data stream that is unloaded is in a proprietary format and contains all the column data for every row being unloaded.

An unload operation also creates a metadata stream that describes the contents of the data stream. The information in the metadata stream is required for loading the data stream. Therefore, the metadata stream is written to the datafile and placed before the data stream.

Dealing with Column Objects

When the external table is accessed through a SQL statement, the fields of the external table can be used just like any other field in a normal table. In particular, the fields can be used as arguments for any SQL built-in function, PL/SQL function, or Java function. This enables you to manipulate the data from the external source.

Although external tables cannot contain a column object, you can use constructor functions to build a column object from attributes in the external table. For example, assume a table in the database is defined as follows:

CREATE TYPE student_type AS object (
student_no CHAR(5),
name CHAR(20))
/

CREATE TABLE roster (
  student student_type,
  grade CHAR(2));

Also assume there is an external table defined as follows:

CREATE TABLE roster_data (
  student_no CHAR(5),
  name CHAR(20),
  grade CHAR(2))
  ORGANIZATION EXTERNAL (TYPE ORACLE_LOADER DEFAULT DIRECTORY ext_tab_dir
                         ACCESS PARAMETERS (FIELDS TERMINATED BY ',')
                         LOCATION ('info.dat'));

To load table roster from roster_data, you would specify something similar to the following:

INSERT INTO roster (student, grade)
  (SELECT student_type(student_no, name), grade FROM roster_data);

Parallel Access with ORACLE_LOADER

The ORACLE_LOADER access driver attempts to divide large datafiles into chunks that can be processed separately.

The following file, record, and data characteristics make it impossible for a file to be processed in parallel:

• Sequential data sources (such as a tape drive or pipe)

• Data in any multibyte character set whose character boundaries cannot be determined starting at an arbitrary byte in the middle of a string

  This restriction does not apply to any datafile with a fixed number of bytes per record.

• Records with the VAR format

Specifying a PARALLEL clause is of value only when large amounts of data are involved.

Parallel Access with ORACLE_DATAPUMP

When you use the ORACLE_DATAPUMP access driver to unload data, the data is unloaded in parallel when the PARALLEL clause or parallel hint has been specified and when multiple locations have been specified for the external table.

Each parallel process writes to its own file. Therefore, the LOCATION clause should specify as many files as there are degrees of parallelism. If there are fewer files than the degree of parallelism specified, then the degree of parallelism will be limited to the number of files specified. If there are more files than the degree of parallelism specified, then the extra files will not be used.

In addition to unloading data, the ORACLE_DATAPUMP access driver can also load data. Parallel processes can read multiple dump files or even chunks of the same dump file concurrently. Thus, data can be loaded in parallel even if there is only one dump file, as long as that file is large enough to contain multiple file offsets. This is because when the ORACLE_DATAPUMP access driver unloads data, it periodically remembers the offset into the dump file of the start of a new data chunk and writes that information into the file when the unload completes. For nonparallel loads, file offsets are ignored because only one process at a time can access a file. For parallel loads, file offsets are distributed among parallel processes for multiple concurrent processing on a file or within a set of files.

Performance Hints Specific to the ORACLE_LOADER Access Driver

The information about performance provided in this section is specific to the ORACLE_LOADER access driver.

Performance can sometimes be increased with use of date cache functionality. By using the date cache to specify the number of unique dates anticipated during the load, you can reduce the number of date conversions done when many duplicate date or timestamp values are present in the input data. The date cache functionality provided by external tables is identical to the date cache functionality provided by SQL*Loader. See DATE_CACHE for a detailed description.

In addition to changing the degree of parallelism and using the date cache to improve performance, consider the following information:

• Fixed-length records are processed faster than records terminated by a string.

• Fixed-length fields are processed faster than delimited fields.

• Single-byte character sets are the fastest to process.

• Fixed-width character sets are faster to process than varying-width character sets.

• Byte-length semantics for varying-width character sets are faster to process than character-length semantics.

• Single-character delimiters for record terminators and field delimiters are faster to process than multicharacter delimiters.

• Having the character set in the datafile match the character set of the database is faster than a character set conversion.

• Having datatypes in the datafile match the datatypes in the database is faster than datatype conversion.

• Not writing rejected rows to a reject file is faster because of the reduced overhead.

• Condition clauses (including WHEN, NULLIF, and DEFAULTIF) slow down processing.

• The access driver takes advantage of multithreading to streamline the work as much as possible.

Restrictions Specific to the ORACLE_DATAPUMP Access Driver

In addition to the restrictions just described, the ORACLE_DATAPUMP access driver has the following restrictions:

• Handling of byte-order marks during a load: In an external table load for which the datafile character set is UTF8 or UTF16, it is not possible to suppress checking for byte-order marks. Suppression of byte-order mark checking is necessary only if the beginning of the datafile contains binary data that matches the byte-order mark encoding. (It is possible to suppress byte-order mark checking with SQL*Loader loads.) Note that checking for a byte-order mark does not mean that a byte-order mark must be present in the datafile. If no byte-order mark is present, the byte order of the server platform is used.

• The external tables feature does not support the use of the backslash (\) escape character within strings. See Use of the Backslash Escape Character.

Multiple Primary Input Datafiles

If there are multiple primary input datafiles with SQL*Loader loads, a bad file and a discard file are created for each input datafile. With external table loads, there is only one bad file and one discard file for all input datafiles. If parallel access drivers are used for the external table load, each access driver has its own bad file and discard file.

Syntax and Datatypes

The following are not supported with external table loads:

• Use of CONTINUEIF or CONCATENATE to combine multiple physical records into a single logical record.

• Loading of the following SQL*Loader datatypes: GRAPHIC, GRAPHIC EXTERNAL, and VARGRAPHIC

• Use of the following database column types: LONGs, nested tables, VARRAYs, REFs, primary key REFs, and SIDs

Byte-Order Marks

With SQL*Loader, if a primary datafile uses a Unicode character set (UTF8 or UTF16) and it also contains a byte-order mark (BOM), then the byte-order mark is written at the beginning of the corresponding bad and discard files. With external table loads, the byte-order mark is not written at the beginning of the bad and discard files.

Default Character Sets and Date Masks

For fields in a datafile, the settings of NLS environment variables on the client determine the default character set and date masks. For fields in external tables, the setting of NLS environment variables on the server determine the default character set and date masks.

Use of the Backslash Escape Character

In SQL*Loader, you can use the backslash (\) escape character to mark a single quotation mark as a single quotation mark, as follows:

FIELDS TERMINATED BY ',' OPTIONALLY ENCLOSED BY '\''

In external tables, the use of the backslash escape character within a string will raise an error. The workaround is to use double quotation marks to mark the separation string, as follows:

TERMINATED BY ',' ENCLOSED BY "'"