Sqlldr Interview questions

What is SQL*Loader and what is it used for?

SQL*Loader is a bulk loader utility used for moving data from external files into the Oracle database. Its syntax is similar to that of the DB2 Load utility, but comes with more options. SQL*Loader supports various load formats, selective loading, and multi-table loads.

How does one use the SQL*Loader utility?

One can load data into an Oracle database by using the sqlldr (sqlload on some platforms) utility. Invoke the utility without arguments to get a list of available parameters. Look at the following example:

        sqlldr scott/tiger control=loader.ctl

This sample control file (loader.ctl) will load an external data file containing delimited data:

        load data

          infile 'c:\data\mydata.csv'

                               into table emp

          fields terminated by "," optionally enclosed by '"'                          

          ( empno, empname, sal, deptno )

The mydata.csv file may look like this:

              10001,"Scott Tiger", 1000, 40

              10002,"Frank Naude", 500, 20

Another Sample control file with in-line data formatted as fix length records. The trick is to specify "*" as the name of the data file, and use BEGINDATA to start the data section in the control file.

        load data

          infile *

          replace

          into table departments

          (  dept     position (02:05) char(4),

             deptname position (08:27) char(20)

          )

        begindata

          COSC  COMPUTER SCIENCE

          ENGL  ENGLISH LITERATURE

          MATH  MATHEMATICS

          POLY  POLITICAL SCIENCE

Is there a SQL*Unloader to download data to a flat file?

Oracle does not supply any data unload utilities. However, you can use SQL*Plus to select and format your data and then spool it to a file:

        set echo off newpage 0 space 0 pagesize 0 feed off head off trimspool on

        spool oradata.txt

        select col1 || ',' || col2 || ',' || col3

        from   tab1

        where  col2 = 'XYZ';

        spool off

Alternatively use the UTL_FILE PL/SQL package:

        rem Remember to update initSID.ora, utl_file_dir='c:\oradata' parameter

        declare

           fp utl_file.file_type;

        begin

           fp := utl_file.fopen('c:\oradata','tab1.txt','w');

           utl_file.putf(fp, '%s, %s\n', 'TextField', 55);

           utl_file.fclose(fp);

        end;

        /

You might also want to investigate third party tools like SQLWays from Ispirer Systems, TOAD from Quest, or ManageIT Fast Unloader from CA to help you unload data from Oracle.

Can one load variable and fix length data records?

Yes, look at the following control file examples. In the first we will load delimited data (variable length):

   LOAD DATA

   INFILE *

   INTO TABLE load_delimited_data

   FIELDS TERMINATED BY "," OPTIONALLY ENCLOSED BY '"'

   TRAILING NULLCOLS

   (  data1,

      data2

   )

   BEGINDATA

   11111,AAAAAAAAAA

   22222,"A,B,C,D,"

If you need to load positional data (fixed length), look at the following control file example:

   LOAD DATA

   INFILE *

   INTO TABLE load_positional_data

   (  data1 POSITION(1:5),

      data2 POSITION(6:15)

   )

   BEGINDATA

   11111AAAAAAAAAA

   22222BBBBBBBBBB

Can one skip header records load while loading?

Use the "SKIP n" keyword, where n = number of logical rows to skip. Look at this example:

   LOAD DATA

   INFILE *

   INTO TABLE load_positional_data

   SKIP 5

   (  data1 POSITION(1:5),

      data2 POSITION(6:15)

   )

   BEGINDATA

   11111AAAAAAAAAA

   22222BBBBBBBBBB

Can one modify data as it loads into the database?

Data can be modified as it loads into the Oracle Database. Note that this only applies for the conventional load path and not for direct path loads.

   LOAD DATA

   INFILE *

   INTO TABLE modified_data

   (  rec_no                      "my_db_sequence.nextval",

      region                       CONSTANT '31',

      time_loaded             "to_char(SYSDATE, 'HH24:MI')",

      data1        POSITION(1:5)  ":data1/100",

      data2        POSITION(6:15) "upper(:data2)",

      data3        POSITION(16:22)"to_date(:data3, 'YYMMDD')"

   )

   BEGINDATA

   11111AAAAAAAAAA991201

   22222BBBBBBBBBB990112

   LOAD DATA

   INFILE 'mail_orders.txt'

   BADFILE 'bad_orders.txt'

   APPEND

   INTO TABLE mailing_list

   FIELDS TERMINATED BY ","

   (  addr,

      city,

      state,

      zipcode,

      mailing_addr   "decode(:mailing_addr, null, :addr, :mailing_addr)",

      mailing_city   "decode(:mailing_city, null, :city, :mailing_city)",

      mailing_state

   )

Can one load data into multiple tables at once?

Look at the following control file:

   LOAD DATA

   INFILE *

   REPLACE

   INTO TABLE emp

        WHEN empno != ' '

   ( empno  POSITION(1:4)   INTEGER EXTERNAL,

     ename  POSITION(6:15)  CHAR,

     deptno POSITION(17:18) CHAR,

     mgr    POSITION(20:23) INTEGER EXTERNAL

   )

   INTO TABLE proj

        WHEN projno != ' '

   (  projno POSITION(25:27) INTEGER EXTERNAL,

      empno  POSITION(1:4)   INTEGER EXTERNAL

   )

Can one selectively load only the records that one need?

Look at this example, (01) is the first character, (30:37) are characters 30 to 37:

   LOAD DATA

   INFILE  'mydata.dat' BADFILE  'mydata.bad' DISCARDFILE 'mydata.dis'

   APPEND

   INTO TABLE my_selective_table

   WHEN (01) <> 'H' and (01) <> 'T' and (30:37) = '19991217'

   (

      region              CONSTANT '31',

      service_key         POSITION(01:11)   INTEGER EXTERNAL,

      call_b_no           POSITION(12:29)   CHAR

   )

Can one skip certain columns while loading data?

One cannot use POSTION(x:y) with delimited data. Luckily, from Oracle 8i one can specify FILLER columns. FILLER columns are used to skip columns/fields in the load file, ignoring fields that one does not want. Look at this example:

        LOAD DATA

        TRUNCATE INTO TABLE T1

        FIELDS TERMINATED BY ','

        ( field1,

          field2 FILLER,

          field3

        )

How does one load multi-line records?

One can create one logical record from multiple physical records using one of the following two clauses:

• CONCATENATE: - use when SQL*Loader should combine the same number of physical records together to form one logical record.
• CONTINUEIF - use if a condition indicates that multiple records should be treated as one. Eg. by having a '#' character in column 1.

How can get SQL*Loader to COMMIT only at the end of the load file?

One cannot, but by setting the ROWS= parameter to a large value, committing can be reduced. Make sure you have big rollback segments ready when you use a high value for ROWS=.

Can one improve the performance of SQL*Loader?

1. A very simple but easily overlooked hint is not to have any indexes and/or constraints (primary key) on your load tables during the load process. This will significantly slow down load times even with ROWS= set to a high value.
2. Add the following option in the command line: DIRECT=TRUE. This will effectively bypass most of the RDBMS processing. However, there are cases when you can't use direct load. Refer to chapter 8 on Oracle server Utilities manual.
3. Turn off database logging by specifying the UNRECOVERABLE option. This option can only be used with direct data loads.
4. Run multiple load jobs concurrently.

What is the difference between the conventional and direct path loader?

The conventional path loader essentially loads the data by using standard INSERT statements. The direct path loader (DIRECT=TRUE) bypasses much of the logic involved with that, and loads directly into the Oracle data files. More information about the restrictions of direct path loading can be obtained from the Utilities Users Guide. s