In both conventional path and direct path, you can specify a text value for a ROWID column.

This is the same text you get when you perform a SELECT ROWID FROM table_name operation. The character string interpretation of the ROWID is converted into the ROWID type for a column in a table.

SQL*Loader uses the partition-extended syntax of the INSERT statement.

By definition, a conventional path load uses SQL INSERT statements. During a conventional path load of a single partition, SQL*Loader uses the partition-extended syntax of the INSERT statement, which has the following form:

INSERT INTO TABLE T PARTITION (P) VALUES ... 

The SQL layer of the Oracle kernel determines if the row being inserted maps to the specified partition. If the row does not map to the partition, then the row is rejected, and the SQL*Loader log file records an appropriate error message.

How to determine when you should use conventional path load instead of direct path load.

If load speed is most important to you, then you should use direct path load because it is faster than conventional path load. However, certain restrictions on direct path loads may require you to use a conventional path load. You should use a conventional path load in the following situations:

• When accessing an indexed table concurrently with the load, or when applying inserts or updates to a nonindexed table concurrently with the load

  To use a direct path load (except for parallel loads), SQL*Loader must have exclusive write access to the table and exclusive read/write access to any indexes.

• When loading data into a clustered table

  A direct path load does not support loading of clustered tables.

• When loading a relatively small number of rows into a large indexed table

  During a direct path load, the existing index is copied when it is merged with the new index keys. If the existing index is very large and the number of new keys is very small, then the index copy time can offset the time saved by a direct path load.

• When loading a relatively small number of rows into a large table with referential and column-check integrity constraints

  Because these constraints cannot be applied to rows loaded on the direct path, they are disabled for the duration of the load. Then they are applied to the whole table when the load completes. The costs could outweigh the savings for a very large table and a small number of new rows.

• When loading records and you want to ensure that a record is rejected under any of the following circumstances:

  • If the record, upon insertion, causes an Oracle error

  • If the record is formatted incorrectly, so that SQL*Loader cannot find field boundaries

  • If the record violates a constraint or tries to make a unique index non-unique

During a direct path load, data conversion occurs on the client side rather than on the server side.

This means that NLS parameters in the initialization parameter file (server-side language handle) will not be used. To override this behavior, you can specify a format mask in the SQL*Loader control file that is equivalent to the setting of the NLS parameter in the initialization parameter file, or set the appropriate environment variable. For example, to specify a date format for a field, you can either set the date format in the SQL*Loader control file as shown in Example 12-1 or set an NLS_DATE_FORMAT environment variable as shown in Example 12-2.

LOAD DATA
INFILE 'data.dat'
INSERT INTO TABLE emp
FIELDS TERMINATED BY "|"
(
EMPNO NUMBER(4) NOT NULL,
ENAME CHAR(10),
JOB CHAR(9),
MGR NUMBER(4),
HIREDATE DATE 'YYYYMMDD',
SAL NUMBER(7,2),
COMM NUMBER(7,2),
DEPTNO NUMBER(2)
)

% NLS_DATE_FORMAT='YYYYMMDD'
% export NLS_DATE_FORMAT

%setenv NLS_DATE_FORMAT='YYYYMMDD'

When loading a partitioned or subpartitioned table, SQL*Loader partitions the rows and maintains indexes (which can also be partitioned).

Note that a direct path load of a partitioned or subpartitioned table can be quite resource-intensive for tables with many partitions or subpartitions.

During a direct path load of a single partition, SQL*Loader uses the partition-extended syntax of the LOAD statement.

When loading a single partition of a partitioned or subpartitioned table, SQL*Loader partitions the rows and rejects any rows that do not map to the partition or subpartition specified in the SQL*Loader control file. Local index partitions that correspond to the data partition or subpartition being loaded are maintained by SQL*Loader. Global indexes are not maintained on single partition or subpartition direct path loads. During a direct path load of a single partition, SQL*Loader uses the partition-extended syntax of the LOAD statement, which has either of the following forms:

LOAD INTO TABLE T PARTITION (P) VALUES ... 

LOAD INTO TABLE T SUBPARTITION (P) VALUES ... 

While you are loading a partition of a partitioned or subpartitioned table, you are also allowed to perform DML operations on, and direct path loads of, other partitions in the table.

Although a direct path load minimizes database processing, several calls to the Oracle database are required at the beginning and end of the load to initialize and finish the load, respectively. Also, certain DML locks are required during load initialization and are released when the load completes. The following operations occur during the load: index keys are built and put into a sort, and space management routines are used to get new extents when needed and to adjust the upper boundary (high-water mark) for a data savepoint. See Using Data Saves to Protect Against Data Loss for information about adjusting the upper boundary.

This topic describes the advantages of using a direct path load.

A direct path load is faster than the conventional path for the following reasons:

• Partial blocks are not used, so no reads are needed to find them, and fewer writes are performed.

• SQL*Loader need not execute any SQL INSERT statements; therefore, the processing load on the Oracle database is reduced.

• A direct path load calls on Oracle to lock tables and indexes at the start of the load and releases them when the load is finished. A conventional path load calls Oracle once for each array of rows to process a SQL INSERT statement.

• A direct path load uses multiblock asynchronous I/O for writes to the database files.

• During a direct path load, processes perform their own write I/O, instead of using Oracle's buffer cache. This minimizes contention with other Oracle users.

• The sorted indexes option available during direct path loads enables you to presort data using high-performance sort routines that are native to your system or installation.

• When a table to be loaded is empty, the presorting option eliminates the sort and merge phases of index-building. The index is filled in as data arrives.

• Protection against instance failure does not require redo log file entries during direct path loads. Therefore, no time is required to log the load when:

  • The Oracle database has the SQL NOARCHIVELOG parameter enabled

  • The SQL*Loader UNRECOVERABLE clause is enabled

  • The object being loaded has the SQL NOLOGGING parameter set

  See Instance Recovery and Direct Path Loads.

There are restrictions on using direct path loads.

There are restrictions on a direct path load of a single partition.

How to determine if you should use direct path load.

If none of the previous restrictions apply, then you should use a direct path load when:

• You have a large amount of data to load quickly. A direct path load can quickly load and index large amounts of data. It can also load data into either an empty or nonempty table.

• You want to load data in parallel for maximum performance. See Parallel Data Loading Models.

All integrity constraints are enforced during direct path loads, although not necessarily at the same time.

NOT NULL constraints are enforced during the load. Records that fail these constraints are rejected.

UNIQUE constraints are enforced both during and after the load. A record that violates a UNIQUE constraint is not rejected (the record is not available in memory when the constraint violation is detected).

Integrity constraints that depend on other rows or tables, such as referential constraints, are disabled before the direct path load and must be reenabled afterwards. If REENABLE is specified, then SQL*Loader can reenable them automatically at the end of the load. When the constraints are reenabled, the entire table is checked. Any rows that fail this check are reported in the specified error log. See Direct Path Loads_ Integrity Constraints_ and Triggers.

Default column specifications defined in the database are not available when you use direct path loading.

Fields for which default values are desired must be specified with the DEFAULTIF clause. If a DEFAULTIF clause is not specified and the field is NULL, then a null value is inserted into the database.

You can load data into a synonym for a table during a direct path load, but the synonym must point directly to either a table or a view on a simple table.

To prepare the database for direct path loads, you must run the setup script, catldr.sql, to create the necessary views.

To start SQL*Loader in direct path load mode, set the DIRECT parameter to TRUE on the command line or in the parameter file.

DIRECT=TRUE

You can improve performance of direct path loads by using temporary storage. After each block is formatted, the new index keys are put in a sort (temporary) segment.

The old index and the new keys are merged at load finish time to create the new index. The old index, sort (temporary) segment, and new index segment all require storage until the merge is complete. Then the old index and temporary segment are removed.

During a conventional path load, every time a row is inserted the index is updated. This method does not require temporary storage space, but it does add processing time.

• Improving Performance
  To improve performance on systems with limited memory, use the SINGLEROW parameter.
• Temporary Segment Storage Requirements
  Estimating the amount of temporary segment storage requirements.

SQL*Loader leaves indexes in an Index Unusable state when the data segment being loaded becomes more up-to-date than the index segments that index it.

Any SQL statement that tries to use an index that is in an Index Unusable state returns an error. The following conditions cause a direct path load to leave an index or a partition of a partitioned index in an Index Unusable state:

• SQL*Loader runs out of space for the index and cannot update the index.

• The data is not in the order specified by the SORTED INDEXES clause.

• There is an instance failure, or the Oracle shadow process fails while building the index.

• There are duplicate keys in a unique index.

• Data savepoints are being used, and the load fails or is terminated by a keyboard interrupt after a data savepoint occurred.

To determine if an index is in an Index Unusable state, you can execute a simple query:

SELECT INDEX_NAME, STATUS
   FROM USER_INDEXES 
   WHERE TABLE_NAME = 'tablename';

If you are not the owner of the table, then search ALL_INDEXES or DBA_INDEXES instead of USER_INDEXES.

To determine if an index partition is in an unusable state, you can execute the following query:

SELECT INDEX_NAME, 
       PARTITION_NAME,
       STATUS FROM USER_IND_PARTITIONS
       WHERE STATUS != 'VALID';

If you are not the owner of the table, then search ALL_IND_PARTITIONS and DBA_IND_PARTITIONS instead of USER_IND_PARTITIONS.

You can use data saves to protect against loss of data due to instance failure.

• Using the ROWS Parameter
  The ROWS parameter determines when data saves occur during a direct path load.
• Data Save Versus Commit
  In a conventional load, ROWS is the number of rows to read before a commit operation. A direct load data save is similar to a conventional load commit, but it is not identical.

SQL*Loader provides full support for data recovery when using the direct path load method.

There are two main types of recovery:

• Media - recovery from the loss of a database file. You must be operating in ARCHIVELOG mode to recover after you lose a database file.

• Instance - recovery from a system failure in which in-memory data was changed but lost due to the failure before it was written to disk. The Oracle database can always recover from instance failures, even when redo logs are not archived.

• Media Recovery and Direct Path Loads
  If redo log file archiving is enabled (you are operating in ARCHIVELOG mode), then SQL*Loader logs loaded data when using the direct path, making media recovery possible.
• Instance Recovery and Direct Path Loads
  Because SQL*Loader writes directly to the database files, all rows inserted up to the last data save will automatically be present in the database files if the instance is restarted.

Data that is longer than SQL*Loader's maximum buffer size can be loaded on the direct path by using LOBs.

You can improve performance when doing this by using a large STREAMSIZE value.

You could also load data that is longer than the maximum buffer size by using the PIECED parameter, as described in the next section, but Oracle highly recommends that you use LOBs instead.

• Loading Data As PIECED
  The PIECED parameter can be used to load data in sections, if the data is in the last column of the logical record.

You can perform auditing on SQL*Loader direct path loads to monitor and record selected user database actions.

SQL*Loader automatically adds extents to the table if necessary, but this process takes time. For faster loads into a new table, allocate the required extents when the table is created.

You can improve the performance of direct path loads by presorting your data on indexed columns.

Presorting minimizes temporary storage requirements during the load. Presorting also enables you to take advantage of high-performance sorting routines that are optimized for your operating system or application.

If the data is presorted and the existing index is not empty, then presorting minimizes the amount of temporary segment space needed for the new keys. The sort routine appends each new key to the key list.

Instead of requiring extra space for sorting, only space for the keys is needed. To calculate the amount of storage needed, use a sort factor of 1.0 instead of 1.3. For more information about estimating storage requirements, see Temporary Segment Storage Requirements.

If presorting is specified and the existing index is empty, then maximum efficiency is achieved. The new keys are simply inserted into the index. Instead of having a temporary segment and new index existing simultaneously with the empty, old index, only the new index exists. So, temporary storage is not required, and time is saved.

• SORTED INDEXES Clause
  The SORTED INDEXES clause identifies the indexes on which the data is presorted.
• Unsorted Data
  If you specify an index in the SORTED INDEXES clause, and the data is not sorted for that index, then the index is left in an Index Unusable state at the end of the load.
• Multiple-Column Indexes
  If you specify a multiple-column index in the SORTED INDEXES clause, then the data should be sorted so that it is ordered first on the first column in the index, next on the second column in the index, and so on.
• Choosing the Best Sort Order
  For the best overall performance of direct path loads, you should presort the data based on the index that requires the most temporary segment space.

Frequent data saves resulting from a small ROWS value adversely affect the performance of a direct path load.

A small ROWS value can also result in wasted data block space because the last data block is not written to after a save, even if the data block is not full.

Because direct path loads can be many times faster than conventional loads, the value of ROWS should be considerably higher for a direct load than it would be for a conventional load.

During a data save, loading stops until all of SQL*Loader's buffers are successfully written. You should select the largest value for ROWS that is consistent with safety. It is a good idea to determine the average time to load a row by loading a few thousand rows. Then you can use that value to select a good value for ROWS.

For example, if you can load 20,000 rows per minute, and you do not want to repeat more than 10 minutes of work after an interruption, then set ROWS to be 200,000 (20,000 rows/minute * 10 minutes).

One way to speed a direct load dramatically is to minimize use of the redo log.

There are three ways to do this. You can disable archiving, you can specify that the load is unrecoverable, or you can set the SQL NOLOGGING parameter for the objects being loaded. This section discusses all methods.

• Disabling Archiving
  If archiving is disabled, then direct path loads do not generate full image redo.
• Specifying the SQL*Loader UNRECOVERABLE Clause
  To save time and space in the redo log file, use the SQL*Loader UNRECOVERABLE clause in the control file when you load data.
• Setting the SQL NOLOGGING Parameter
  If a data or index segment has the SQL NOLOGGING parameter set, then full image redo logging is disabled for that segment (invalidation redo is generated).

The number of column array rows determines the number of rows loaded before the stream buffer is built. T

If you are performing a direct path load in which the same date or timestamp values are loaded many times, then a large percentage of total load time can end up being used for converting date and timestamp data.

This is especially true if multiple date columns are being loaded. In such a case, it may be possible to improve performance by using the SQL*Loader date cache.

The date cache reduces the number of date conversions done when many duplicate values are present in the input data. It enables you to specify the number of unique dates anticipated during the load.

The date cache is enabled by default. To completely disable the date cache, set it to 0.

The default date cache size is 1000 elements. If the default is used and the number of unique input values loaded exceeds 1000, then the date cache is automatically disabled for that table. This prevents excessive and unnecessary lookup times that could affect performance. However, if instead of using the default, you specify a nonzero value for the date cache and it is exceeded, then the date cache is not disabled. Instead, any input data that exceeded the maximum is explicitly converted using the appropriate conversion routines.

The date cache can be associated with only one table. No date cache sharing can take place across tables. A date cache is created for a table only if all of the following conditions are true:

• The DATE_CACHE parameter is not set to 0

• One or more date values, timestamp values, or both are being loaded that require data type conversion in order to be stored in the table

• The load is a direct path load

Date cache statistics are written to the log file. You can use those statistics to improve direct path load performance as follows:

• If the number of cache entries is less than the cache size and there are no cache misses, then the cache size could safely be set to a smaller value.

• If the number of cache hits (entries for which there are duplicate values) is small and the number of cache misses is large, then the cache size should be increased. Be aware that if the cache size is increased too much, then it may cause other problems, such as excessive paging or too much memory usage.

• If most of the input date values are unique, then the date cache will not enhance performance and therefore should not be used.

  Note:

  Date cache statistics are not written to the SQL*Loader log file if the cache was active by default and disabled because the maximum was exceeded.

If increasing the cache size does not improve performance, then revert to the default behavior or set the cache size to 0. The overall performance improvement also depends on the data types of the other columns being loaded. Improvement will be greater for cases in which the total number of date columns loaded is large compared to other types of data loaded.

During a direct path load, some integrity constraints are automatically disabled.

Others are not. For a description of the constraints, see the information about maintaining data integrity in the Oracle Database Development Guide.

• Enabled Constraints
  During direct path load, some constraints remain enabled.
• Disabled Constraints
  During a direct path load, some constraints are disabled.
• Reenable Constraints
  When the load completes, the integrity constraints will be reenabled automatically if the REENABLE clause is specified.

Table insert triggers are also disabled when a direct path load begins.

After the rows are loaded and indexes rebuilt, any triggers that were disabled are automatically reenabled. The log file lists all triggers that were disabled for the load. There should not be any errors reenabling triggers.

Unlike integrity constraints, insert triggers are not reapplied to the whole table when they are enabled. As a result, insert triggers do not fire for any rows loaded on the direct path. When using the direct path, the application must ensure that any behavior associated with insert triggers is carried out for the new rows.

• Replacing Insert Triggers with Integrity Constraints
  Applications commonly use insert triggers to implement integrity constraints.
• When Automatic Constraints Cannot Be Used
  Sometimes an insert trigger cannot be replaced with Oracle's automatic integrity constraints.
• Preparation
  Before either method can be used, the table must be prepared.
• Using an Update Trigger
  Generally, you can use a database update trigger to duplicate the effects of an insert trigger.
• Duplicating the Effects of Exception Conditions
  If the insert trigger can raise an exception, then more work is required to duplicate its effects.
• Using a Stored Procedure
  This topic describes using a stored procedure.

SQL*Loader needs to acquire several locks on the table to be loaded to disable triggers and constraints.

If a competing process is enabling triggers or constraints at the same time that SQL*Loader is trying to disable them for that table, then SQL*Loader may not be able to acquire exclusive access to the table.

SQL*Loader attempts to handle this situation as gracefully as possible. It attempts to reenable disabled triggers and constraints before exiting. However, the same table-locking problem that made it impossible for SQL*Loader to continue may also have made it impossible for SQL*Loader to finish enabling triggers and constraints. In such cases, triggers and constraints will remain disabled until they are manually enabled.

Although such a situation is unlikely, it is possible. The best way to prevent it is to ensure that no applications are running that could enable triggers or constraints for the table while the direct load is in progress.

If a direct load is terminated due to failure to acquire the proper locks, then carefully check the log. It will show every trigger and constraint that was disabled, and each attempt to reenable them. Any triggers or constraints that were not reenabled by SQL*Loader should be manually enabled with the ENABLE clause of the ALTER TABLE statement described in Oracle Database SQL Language Reference.

If triggers or integrity constraints pose a problem, but you want faster loading, then you should consider using concurrent conventional path loads.

That is, use multiple load sessions executing concurrently on a multiple-CPU system. Split the input data files into separate files on logical record boundaries, and then load each such input data file with a conventional path load session. The resulting load has the following attributes:

• It is faster than a single conventional load on a multiple-CPU system, but probably not as fast as a direct load.

• Triggers fire, integrity constraints are applied to the loaded rows, and indexes are maintained using the standard DML execution logic.

This topic describes using concurrent conventional path loads.

Using multiple conventional path load sessions executing concurrently is discussed in Increasing Performance with Concurrent Conventional Path Loads, you can use this technique to load the same or different objects concurrently with no restrictions.

Intersegment concurrency can be used for concurrent loading of different objects.

SQL*Loader permits multiple, concurrent sessions to perform a direct path load into the same table, or into the same partition of a partitioned table.

Restrictions are enforced on parallel direct path loads.

Each SQL*Loader session takes a different data file as input. In all sessions executing a direct load on the same table, you must set PARALLEL to TRUE.

The syntax is:

Description of the illustration parallel.eps

PARALLEL can be specified on the command line or in a parameter file. It can also be specified in the control file with the OPTIONS clause.

For example, to start three SQL*Loader direct path load sessions on the same table, you would execute each of the following commands at the operating system prompt. After entering each command, you will be prompted for a password.

sqlldr USERID=scott CONTROL=load1.ctl DIRECT=TRUE PARALLEL=TRUE
sqlldr USERID=scott CONTROL=load2.ctl DIRECT=TRUE PARALLEL=TRUE
sqlldr USERID=scott CONTROL=load3.ctl DIRECT=TRUE PARALLEL=TRUE

The previous commands must be executed in separate sessions, or if permitted on your operating system, as separate background jobs. Note the use of multiple control files. This enables you to be flexible in specifying the files to use for the direct path load.

Note:

Indexes are not maintained during a parallel load. Any indexes must be created or re-created manually after the load completes. You can use the parallel index creation or parallel index rebuild feature to speed the building of large indexes after a parallel load.

When you perform a parallel load, SQL*Loader creates temporary segments for each concurrent session and then merges the segments upon completion. The segment created from the merge is then added to the existing segment in the database above the segment's high-water mark. The last extent used of each segment for each loader session is trimmed of any free space before being combined with the other extents of the SQL*Loader session.

When you perform parallel direct path loads, there are options available for specifying attributes of the temporary segment to be allocated by the loader.

These options are specified with the FILE and STORAGE parameters. These parameters are valid only for parallel loads.

• Using the FILE Parameter to Specify Temporary Segments
  To allow for maximum I/O throughput, Oracle recommends that each concurrent direct path load session use files located on different disks.

Constraints and triggers must be enabled manually after all data loading is complete.

Because each SQL*Loader session can attempt to reenable constraints on a table after a direct path load, there is a danger that one session may attempt to reenable a constraint before another session is finished loading data. In this case, the first session to complete the load will be unable to enable the constraint because the remaining sessions possess share locks on the table.

Because there is a danger that some constraints might not be reenabled after a direct path load, you should check the status of the constraint after completing the load to ensure that it was enabled properly.

This topic describes using the PRIMARY KEY and UNIQUE KEY constraints.

PRIMARY KEY and UNIQUE KEY constraints create indexes on a table when they are enabled, and subsequently can take a significantly long time to enable after a direct path loading session if the table is very large. You should consider enabling these constraints manually after a load (and not specifying the automatic enable feature). This enables you to manually create the required indexes in parallel to save time before enabling the constraint.