Additional Considerations for Direct-Path INSERT & NOtes added on
Advantages of Using Direct-Path INSERT
The following are performance benefits of direct-path INSERT:
During direct-path INSERT, you can disable the logging of redo and undo entries. Conventional insert operations, in contrast, must always log such entries, because those operations reuse free space and maintain referential integrity.
To create a new table with data from an existing table, you have the choice of creating the new table and then inserting into it, or executing a CREATE TABLE … AS SELECT statement. By creating the table and then using direct-path INSERT operations, you update any indexes defined on the target table during the insert operation. The table resulting from a CREATE TABLE … AS SELECT statement, in contrast, does not have any indexes defined on it; you must define them later.
Direct-path INSERT operations ensure atomicity of the transaction, even when run in parallel mode. Atomicity cannot be guaranteed during parallel direct-path loads (using SQL*Loader).
If errors occur during parallel direct-path loads, some indexes could be marked UNUSABLE at the end of the load. Parallel direct-path INSERT, in contrast, rolls back the statement if errors occur during index update.
Direct-path INSERT must be used if you want to store the data in compressed form using table compression.
Enabling Direct-Path INSERT
You can implement direct-path INSERT operations by using direct-path INSERT statements, inserting data in parallel mode, or by using the Oracle SQL*Loader utility in direct-path mode. Direct-path inserts can be done in either serial or parallel mode.
To activate direct-path INSERT in serial mode, you must specify the APPEND hint in each INSERT statement, either immediately after the INSERT keyword, or immediately after the SELECT keyword in the subquery of the INSERT statement.
When you are inserting in parallel DML mode, direct-path INSERT is the default. In order to run in parallel DML mode, the following requirements must be met:
You must have Oracle Enterprise Edition installed.
You must enable parallel DML in your session. To do this, run the following statement:
ALTER SESSION { ENABLE | FORCE } PARALLEL DML;
You must specify the parallel attribute for the target table, either at create time or subsequently, or you must specify the PARALLEL hint for each insert operation.
To disable direct-path INSERT, specify the NOAPPEND hint in each INSERT statement. Doing so overrides parallel DML mode.
How Direct-Path INSERT Works
You can use direct-path INSERT on both partitioned and non-partitioned tables.
Serial Direct-Path INSERT into Partitioned or Non-partitioned Tables
The single process inserts data beyond the current high water mark of the table segment or of each partition segment. (The high-water mark is the level at which blocks have never been formatted to receive data.) When a COMMIT runs, the high-water mark is updated to the new value, making the data visible to users.
Parallel Direct-Path INSERT into Partitioned Tables
This situation is analogous to serial direct-path INSERT. Each parallel execution server is assigned one or more partitions, with no more than one process working on a single partition. Each parallel execution server inserts data beyond the current high-water mark of its assigned partition segment(s). When a COMMIT runs, the high-water mark of each partition segment is updated to its new value, making the data visible to users.
Parallel Direct-Path INSERT into Non-partitioned Tables
Each parallel execution server allocates a new temporary segment and inserts data into that temporary segment. When a COMMIT runs, the parallel execution coordinator merges the new temporary segments into the primary table segment, where it is visible to users.
Specifying the Logging Mode for Direct-Path INSERT
Direct-path INSERT lets you choose whether to log redo and undo information during the insert operation.
You can specify logging mode for a table, partition, index, or LOB storage at create time (in a CREATE statement) or subsequently (in an ALTER statement).
If you do not specify either LOGGING or NOLOGGING at these times:
The logging attribute of a partition defaults to the logging attribute of its table.
The logging attribute of a table or index defaults to the logging attribute of the tablespace in which it resides.
The logging attribute of LOB storage defaults to LOGGING if you specify CACHE for LOB storage. If you do not specify CACHE, then the logging attributes defaults to that of the tablespace in which the LOB values resides.
You set the logging attribute of a tablespace in a CREATE TABLESPACE or ALTER TABLESPACE statements.
Note:
If the database or tablespace is in FORCE LOGGING mode, then direct path INSERT always logs, regardless of the logging setting.
Direct-Path INSERT with Logging
In this mode, Oracle Database performs full redo logging for instance and media recovery. If the database is in ARCHIVELOG mode, then you can archive redo logs to tape. If the database is in NOARCHIVELOG mode, then you can recover instance crashes but not disk failures.
Direct-Path INSERT without Logging
In this mode, Oracle Database inserts data without redo or undo logging. (Some minimal logging is done to mark new extents invalid, and data dictionary changes are always logged.) This mode improves performance. However, if you subsequently must perform media recovery, the extent invalidation records mark a range of blocks as logically corrupt, because no redo data was logged for them. Therefore, it is important that you back up the data after such an insert operation.
Additional Considerations for Direct-Path INSERT
The following are some additional considerations when using direct-path INSERT.
Index Maintenance with Direct-Path INSERT
Oracle Database performs index maintenance at the end of direct-path INSERT operations on tables (partitioned or non-partitioned) that have indexes. This index maintenance is performed by the parallel execution servers for parallel direct-path INSERT or by the single process for serial direct-path INSERT. You can avoid the performance impact of index maintenance by dropping the index before the INSERT operation and then rebuilding it afterward.
Space Considerations with Direct-Path INSERT
Direct-path INSERT requires more space than conventional-path INSERT.
All serial direct-path INSERT operations, as well as parallel direct-path INSERT into partitioned tables, insert data above the high-water mark of the affected segment. This requires some additional space.
Parallel direct-path INSERT into non-partitioned tables requires even more space, because it creates a temporary segment for each degree of parallelism. If the non-partitioned table is not in a locally managed tablespace in automatic segment-space management mode, you can modify the values of the NEXT and PCTINCREASE storage parameter and MINIMUM EXTENT tablespace parameter to provide sufficient (but not excess) storage for the temporary segments. Choose values for these parameters so that:
The size of each extent is not too small (no less than 1 MB). This setting affects the total number of extents in the object.
The size of each extent is not so large that the parallel INSERT results in wasted space on segments that are larger than necessary.
After the direct-path INSERT operation is complete, you can reset these parameters to settings more appropriate for serial operations.
Locking Considerations with Direct-Path INSERT
During direct-path INSERT, the database obtains exclusive locks on the table (or on all partitions of a partitioned table). As a result, users cannot perform any concurrent insert, update, or delete operations on the table, and concurrent index creation and build operations are not permitted. Concurrent queries, however, are supported, but the query will return only the information before the insert operation.
NOtes Added On
(1) in 10.x and previous releases the APPEND hint gets ignored when it is present in an INSERT with VALUES clause and that is the expected behavior for previous versions, including 9i and 10g. On 11g Release 1, the APPEND hint it is not ignored and a direct-path load is executed for an INSERT with VALUES clause, therefore such statements will allocate one row/database block.
(2) On 11g Release 2, there is a new hint “APPEND_VALUES”, i.e. use direct path load only when the “APPEND_VALUES” hint is referenced. On 11g Release 2 the “APPEND” hint is ignored for INSERT statements with VALUES clause. This is the expected behavior on 11g and it is currently implemented only in 11g Release 2.