http://aguppi.blogspot.com/2011/06/oracle-access-path.html
A sample table scan retrieves a random sample of data from a table. This access path is used when a statement's
FROM clause includes the SAMPLE clause or the SAMPLE BLOCK clause. To perform a sample table scan when sampling by rows (the SAMPLE clause), Oracle reads a specified percentage of rows in the table. To perform a sample table scan when sampling by blocks (the SAMPLE BLOCK clause), Oracle reads a specified percentage of table blocks.Oracle does not support sample table scans when the query involves a join or a remote table. However, you can perform an equivalent operation by using a
CREATE TABLE AS SELECT query to materialize a sample of an underlying table. You then rewrite the original query to refer to the newly created table sample. Additional queries can be written to materialize samples for other tables. Sample table scans require the CBO.Example 1-13 uses a sample table scan to access 1% of the
employees table, sampling by blocks.Example 1-13 Sample Table Scan
SELECT * FROM employees SAMPLE BLOCK (1);
The
EXPLAIN PLAN output for this statement might look like this:------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| ------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 68 | 3 (34)| | 1 | TABLE ACCESS SAMPLE | EMPLOYEES | 1 | 68 | 3 (34)| -------------------------------------------------------------------------
