A sample table scan retrieves a random sample of data from a simple table or a complex SELECT statement, such as a statement involving joins and views.
This access path is used when a statement's FROM clause includes:
This clause is used for instance to gathered the table statistics. The estimate percent is the number of the sample clause.
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.
Example below uses a sample table scan to access 1% of the employees table, sampling by blocks.
SELECT * FROM employees SAMPLE BLOCK (1);
The Oracle Database - Explain Plan output for this statement might look like this:
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| Id | Operation | Name | Rows | Bytes | Cost (%CPU)|
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| 0 | SELECT STATEMENT | | 1 | 68 | 3 (34)|
| 1 | TABLE ACCESS SAMPLE | EMPLOYEES | 1 | 68 | 3 (34)|
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