Friday, April 27, 2012

Difference between Conventional path Export & Direct path Export


Conventional path Export. 
Conventional path Export uses the SQL SELECT statement to extract data from tables. Data is read from disk into the buffer cache, and rows are transferred to the evaluating buffer. The data, after passing expression evaluation, is transferred to the Export client, which then writes the data into the export file.
 

 Direct path Export. 
When using a direct path Export, the data is read from disk directly into the export session's program global area (PGA): the rows are transferred directly to the Export session's private buffer. This also means that the SQL command-processing layer (evaluation buffer) can be bypassed, because the data is already in the format that Export expects. As a result, unnecessary data conversion is avoided. The data is transferred to the Export client, which then writes the data into the export file.
 

. The parameter DIRECT specifies whether you use the direct path Export (DIRECT=Y) or the conventional path Export (DIRECT=N).

 You may be able to improve performance by increasing the value of the RECORDLENGTH parameter when you invoke a direct path Export.  Your exact performance gain depends upon the following factors: 
- DB_BLOCK_SIZE
 
- the types of columns in your table
 
- your I/O layout (the drive receiving the export file should be separate from the disk drive where the database files reside)
 

For example, invoking a Direct path Export with a maximum I/O buffer of 64kb can improve the performance of the Export with almost 50%. This can be achieved by specifying the additional Export parameters DIRECT and RECORDLENGTH

LIMITATIONS

1)  A Direct path Export does not influence the time it takes to Import the data. That is, an export file created using direct path Export or Conventional path Export, will take the same amount of time to Import. 
2) You cannot use the DIRECT=Y parameter when exporting in transportable tablespace mode.  You can use the DIRECT=Y parameter when exporting in full, user or table mode
3) The parameter QUERY applies ONLY to conventional path Export. It cannot be specified in a direct path export (DIRECT=Y).
4) A Direct path Export can only export the data when the NLS_LANG environment variable of the session who is invoking the export, is equal to the database character set. If NLS_LANG is not set (default is AMERICAN_AMERICA.US7ASCII) and/or NLS_LANG is different, Export will display the warning EXP-41 and abort with EXP-0.

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