Learning Visually with Examples [Electronic resources] نسخه متنی

12.4. The DB2 IMPORT Utility

The import utility inserts data from an input file into a table or a view. The utility performs inserts as if it was executing

INSERT statements. Just like normal insert operations, DB2 validates the data and checks against the table definitions, constraints (such as referential integrity and check constraints), and index definitions. Triggers with satisfying conditions are also activated.

The utility supports options and import modes that let you customize its behavior. The syntax diagram of the

import command is very long; Figure 12.10 shows only a portion of it. Please refer to the

DB2 Command Reference for the complete syntax diagram.

Figure 12.10. Simplified syntax diagram of the import command

>>-IMPORT FROM--filename--OF--filetype-------------------------->
>--+-------------------------+---------------------------------->
|            .-,--------. |
|            V          | |
'-LOBS FROM----lob-path-+-'
>--+-------------------------------+---------------------------->
|              .--------------. |
|              V              | |
'-MODIFIED BY----filetype-mod-+-'
.-ALLOW NO ACCESS----.
>--+--------------------+--+----------------------------+------->
'-ALLOW WRITE ACCESS-'  '-COMMITCOUNT--+-n---------+-'
'-AUTOMATIC-'
>--+---------------------+--+-------------+--------------------->
'-+-RESTARTCOUNT-+--n-'  '-ROWCOUNT--n-'
'-SKIPCOUNT----'
>--+-----------------+--+-----------+--------------------------->
'-WARNINGCOUNT--n-'  '-NOTIMEOUT-'
>--+------------------------+----------------------------------->
'-MESSAGES--message-file-'
>--+-+-INSERT---------+--INTO--+-table-name--+-------------------+-+------+-->
| +-INSERT_UPDATE--+        |             |.-,-------------.  | |      |
| +-REPLACE--------+        |             |    V              | | |    |
| '-REPLACE_CREATE-'        |             '-(-insert-column-+-)-' |    |
'-CREATE--INTO--+-table-name+-------------| tblspace-specs |----'
tblspace-specs:
|--+-----------------------------------------------------------------------+--|
'-IN--tablespace-name--+----------------------+--+----------------------+-'
'-INDEX INtspace-name-'  '-LONG IN--tspace-name-'

Although the syntax diagram may seem complex, it is quite easy to understand and follow. Let's start with a simple

import command and discuss the mandatory options. To a certain degree, the

import command is structured much like the

export command: you have to specify the input file name, format of the file, and the target table name. For example:

import from employee.ixf of ixf 
messages employee.out 
insert into employee 

This command takes the file

employee.ixf, which is in IXF format, as the input and inserts data into the

employee table. The import utility supports input files in ASC, DEL, IXF, and WSF formats. We also recommend you to specify the optional clause

messages to save the errors and warning messages and the import status. In section 12.4.4, Restarting a Failed Import, you will see that the message file can be used to identify where to restart an import operation.

12.4.1. Import Mode

The previous example uses

insert to indicate that new data is to be appended to the existing

employee table. Table 12.1 lists the modes supported by the import utility.

Figures 12.11, 12.12, and 12.13 demonstrate some of the import modes and other options.

In Figure 12.11, the input data of specific columns are selected from the DEL input file and imported into the

empsalary table. The

warningcount option indicates that the utility will stop after 10 warnings are received.

Figure 12.11. Example 1: import command

import from employee.del of del 
messages empsalary.out 
warningcount 10 
replace into empsalary (salary, bonus, comm) 

In Figure 12.12, the

import command deletes all the rows in the table (if table

newemployee exists) and inserts the row contents. If the

newemployee table does not exist, the command creates the table with definitions stored in the IXF input file and inserts the row contents. In addition to specifying the columns you want to import as demonstrated in Figure 12.11, you can also limit the number of rows to be imported using the

rowcount option. In Figure 12.12, the number of rows to import is limited to the first 1000 rows.

Figure 12.12. Example 2: import command

import from employee.ixf of ixf 
messages employee.out 
rowcount 1000 
replace_create into newemployee 

If the

create option is used as in Figure 12.13, you can also specify which table space the new table is going to be created in. The

in clause tells DB2 to store the table data in a particular table space, and the

index in clauses indicates where the index is to be stored.

Figure 12.13. Example 3: import command

import from employee.ixf of ixf 
messages newemployee.out 
create into newemployee in datats index in indexts 

12.4.2. Allow Concurrent Write Access

While the import utility is adding new rows to the table, the table by default is locked exclusively to block any read/write activities from other applications. This is the behavior of the

allow no access option. Alternatively, you can specify

allow write access in the command to allow concurrent read/write access to the target table. A less restrictive lock is acquired at the beginning of the import operation.

Both the

allow write access and

allow no access options require some type of table lock. It is possible that the utility will be placed in lock-wait state and eventually will be terminated due to a lock timeout. You can specify the

notimeout option so that the utility will not time out while waiting for locks. This option supersedes the LOCKTIMEOUT database configuration parameter.

12.4.3. Regular Commits During an Import

The import utility inserts data into a table through normal insert operations. Therefore, changes made during the import are logged, and they are committed to the database upon successful completion of the import operation. By default, an import, behaves like a non-atomic compound statement for which more than one insert is grouped into a transaction. If any insert fails, the rest of the inserts will still be committed to the database. Atomic and non-atomic compound statements are discussed in detail in Chapter 9, Leveraging the Power of SQL.

If you were to import a few million rows into a table, you would need to make sure there was enough log space to hold the insertions because they are treated as one transaction. However, sometimes it is not feasible to allocate large log space just for the import. You can specify the commitcount n option to force a commit after every n records are imported. With commitcount automatic , the utility will commit automatically at an appropriate time to avoid running out of active log space and avoid lock escalation.

Figure 12.14 shows the messages captured during the following

import command. Note that a COMMIT is issued every 1,000 rows. The message file also serves as a very good progress indicator, because you can access this file while the utility is running.

Figure 12.14. Importing with intermediate commits

import from employee.ixf of ixf 
commitcount 1000 
messages newemployee.out 
create into newemployee in datats index in indexts 

12.4.4. Restarting a Failed Import

If you have import failures due to invalid input, for example, you can use the message file generated from an

import command that uses the

commitcount and

messages options to identify which record failed. Then you could issue the same

import command with

restartcount

n or

skipcount

n to start the import from record

n+ 1. This is a very handy method to restart a failed import. Here is an example:

import from employee.ixf of ixf 
commitcount 1000 
skipcount 550 
messages newemployee.out 
create into newemployee in datats index in indexts 

12.4.5. File Type Modifiers Supported in the Import Utility

The import utility also has the

modified by clause to allow customization. Some modifiers supported in the export utility also apply to the import utility. Refer to the

DB2 Data Movement Utilities Guide and Reference for a complete listing specific to the import utility. The following sections describe some of the more useful modifiers.

12.4.5.1 Handling Target Tables with Generated and Identity Columns

Tables with generated columns or identity columns are defined in a way that column values will be automatically generated when records are inserted into the tables. Since import operations perform inserts in the background, new values will be generated at the target server. Therefore, you need to decide whether values stored in the source input file should be used or if new values should be generated. The import utility supports a few file type modifiers to take care of that.

The file modifier

generatedignore forces the import utility to ignore data for all generated columns presented in the data file. The utility generates the values of those columns. The file modifier

identityignore behaves the same way as

generatedignore .

You can use the

generatemissing modifier to inform the import utility that the input data file contains no data for the generated columns (not even NULLs), and the import utility will therefore generate a value for each row. This behavior also applies to

identitymissing modifier.

12.4.6. Importing Large Objects

If you are exporting LOB data in separate files (as described in Section 12.3.2, Exporting Large Objects), you need to tell the import utility the location and name of the files. Consider the following

import command.

import from mgrresume.ixf of ixf 
lobs from c:\lobs1, c:\lobs2, c:\lobs3 
modified by lobsinfile 
commitcount 1000 
messages mgrresume.out 
create into newemployee in datats index in indexts long in lobts 

This command takes

mgrresume.del as the input file. With the

lobsinfile modifier, the utility searches the paths specified in the

lobs from clause for the LOB location specifier (LLS). Recall that each LOB data has a LLS that represents the location of a LOB in a file stored in the LOB file path.

Notice that an additional clause,

long in lobts , is added to the

create into option. It indicates that all LOB data will be created and stored in

lobts table space. If this clause is omitted, LOB data will be stored in the same table space with the other data. Typically, we recommend that you use DMS table space and keep regular data, LOB data, and indexes in different table spaces.

12.4.7. Selecting Columns to Import

There are three ways to select particular columns you want to import.

method l uses the starting and ending position (in bytes) for all columns to be imported. This method only supports ASC files. For example:

import from employee.asc of asc 
messages employee.out 
method l (1 5, 6 14, 24 30) 
insert into employee 

This command imports three selected columns of data into the

employee table: bytes 1 to 5 from the first column, bytes 6 to 14 from the second column, and bytes 24 to 30 from the third column.

The other two methods specify the names of the columns (

method n ) or the field numbers of the input data (

method p ).

method n is only valid for IXF files and

method p can be used with IXF or DEL files. The following shows an example of an

import command with

method n and

method p clauses.

import from employee.ixf of ixf 
messages employee.out 
method n (empno, firstnme, lastname) 
insert into employee (empno, firstnme, lastname) 
import from employee.ixf of ixf 
messages employee.out 
method p (1, 2, 4) 
insert into employee (empno, firstnme, lastname) 

12.4.8. Authorities Required to Perform an Import

Depending on the options you have chosen for the import, specific authorization and privileges are required. Since SYSADM and DBADM hold the highest authority for an instance and a database respectively, both of them can issue

import commands with all of the options discussed above. For users who do not have SYSADM and DBADM privileges, refer to Chapter 10, Implementing Security.

12.4.9. Importing a Table Using the Control Center

You can invoke the import utility from the Control Center by right-clicking on the target table and selecting Import. This displays the Import table dialog (shown in Figure 12.15).