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Oracle10g Sample Table Scans

A sample table scan retrieves a random sample of data. It can be from a simple table or a complex SELECT statement such as a statement involving joins and views.

To perform a sample table scan when sampling by rows with the SAMPLE clause, Oracle reads a specified percentage of rows in the table. To perform a sample table scan when sampling by blocks with the SAMPLE BLOCK clause, Oracle reads a specified percentage of table blocks.


The following statements uses a sample block and sample rows scan on the policyrec table. (Note that there are 50,000 rows in this table.) The first two statements show the sample block scans and the last one shows a sample row scan.

SQL> select count(*) from policyrec sample block(20) ;






SQL> select count(*) from policyrec sample block(5) ;







  2  from policyrec  sample (0.02) ;



---------- ---------- ----------- ---------- --

      2895         10        2525          2   

      3176         10        2525          2   

      9228         10        2525          2   

     11294         11        2535          4   

     19846         11        2535          4   

     25547         12        2545          6   

     29583         12        2545          6   

     40042         13        2555          8   

     47331         14        2565         10   

     45283         14        2565         10   


10 rows selected.

Get the complete Oracle10g story:

The above text is an excerpt from "Oracle Database 10g New Features: Oracle10g Reference for Advanced Tuning and Administration", by Rampant TechPress.  Written by top Oracle experts, this book has a complete online code deport with ready to use scripts. 

To get the code instantly, click here:

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