Oracle Database inserts data into a table in one of two ways:（向表中插入数据有两种方法）

• During conventional INSERT operations, the database reuses free space in the table, interleaving newly inserted data with existing data. During such operations, the database also maintains referential integrity constraints.（重用表中的空闲空间，新旧数据交叉存放，维持完整性约束）

• During direct-path INSERT operations, the database appends the inserted data after existing data in the table. Data is written directly into datafiles, bypassing the buffer cache. Free space in the existing data is not reused, and referential integrity constraints are ignored. These procedures combined can enhance performance.（新数据追加到就数据后面、数据直接写入数据文件，绕过buffer cache、不重用表中的空闲空间、不维护完整性约束）

Further, the data can be inserted either in serial mode, where one process executes the statement, or parallel mode, where multiple processes work together simultaneously to run a single SQL statement. The latter is referred to as parallel execution.（插入有串行和并行操作）

This section discusses one aspect of inserting data into tables. Specifically, using the direct-path form of the INSERT statement. It contains the following topics:

• Advantages of Using Direct-Path INSERT

• Enabling Direct-Path INSERT

• How Direct-Path INSERT Works

• Specifying the Logging Mode for Direct-Path INSERT

• Additional Considerations for Direct-Path INSERT

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.（直接路径插入可以disable logging,而常规路径插入总是要logging,因为它重用空闲空间和维护完整性约束）

• 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.（单个进程在hwm上查入数据，执行commit后，hwm才更新（移动到）新数据，这是数据可见）

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.（利用临时段，分配临时段，向临时段插入数据，提交，合并临时段到表段）

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.

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

来自 “ ITPUB博客 ” ，链接：https://blog.itpub.net/9599/viewspace-472908/ ，如需转载，请注明出处，否则将追究法律责任。