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SQL Optimizer for Oracle 10.0 - User Guide

Welcome to SQL Optimizer
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ALL PARTITION ALTER INDEX AND EQUAL ANTI JOIN BITMAP AND BITMAP COMPACTION BITMAP CONSTRUCTION BITMAP CONVERSION BITMAP INDEX BITMAP JOIN INDEX UPDATE BITMAP JOIN INDEX UPDATE STATEMENT BITMAP KEY ITERATION BITMAP MERGE BITMAP MINUS BITMAP OR BUFFER SORT CARTESIAN JOIN COLLECTION ITERATOR CONCATENATION CONNECT BY CONNECT BY PUMP COUNT COUNT STOPKEY CREATE AS SELECT CUBE SCAN DDL STATEMENT DELETE DOMAIN INDEX FAST FULL INDEX SCAN FILTER FIRST ROWS FIXED INDEX FIXED TABLE FOR UPDATE FULL INDEX SCAN FULL INDEX SCAN DESCENDING FULL INDEX SCAN (MIN/MAX) HASH GROUP BY HASH GROUP BY PIVOT HASH JOIN HASH JOIN BUFFERED HASH PARTITION HASH UNIQUE INDEX INDEX BUILD NON UNIQUE INDEX RANGE SCAN INDEX RANGE SCAN DESCENDING INDEX RANGE SCAN (MIN/MAX) INDEX SAMPLE FAST FULL SCAN INDEX SKIP SCAN INDEX SKIP SCAN DESCENDING INDEX UNIQUE SCAN INLIST ITERATOR INLIST PARTITION INSERT INTERSECTION INTO INVALID PARTITION ITERATOR PARTITION LOAD AS SELECT MAT_VIEW ACCESS MAT_VIEW REWRITE ACCESS MERGE JOIN MINUS MULTI-TABLE INSERT NESTED LOOPS OUTER JOIN PARTITION PARTITION HASH EMPTY PARTITION LIST PARTITION RANGE PROJECTION PX BLOCK ITERATOR PX COORDINATOR PX ITERATOR PX PARTITION PX PARTITION HASH ALL PX PARTITION LIST ALL PX PARTITION RANGE ALL PX RECEIVE PX SEND RANGE PARTITION RECURSIVE EXECUTION RECURSIVE WITH PUMP REFERENCE MODEL REMOTE SELECT SEMI JOIN SEQUENCE SINGLE PARTITION SINGLE RANGE PARTITION SORT SORT AGGREGATE SORT GROUP BY SORT GROUP BY CUBE SORT GROUP BY NOSORT SORT GROUP BY ROLLUP SORT JOIN SORT ORDER BY SORT UNIQUE SQL MODEL TABLE ACCESS TABLE ACCESS BY GLOBAL INDEX ROWID TABLE ACCESS BY INDEX ROWID TABLE ACCESS BY LOCAL INDEX ROWID TABLE ACCESS BY ROWID TABLE ACCESS BY USER ROWID TABLE ACCESS CLUSTER TABLE ACCESS FULL TABLE ACCESS HASH TABLE ACCESS SAMPLE TABLE QUEUE TEMP TABLE GENERATION TEMP TABLE TRANSFORMATION UNION UNION ALL UNION ALL (RECURSIVE WITH) UNPIVOT UPDATE VIEW VIEW PUSHED PREDICATE WINDOW
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TABLE ACCESS HASH

Execution Plan image: image\Plan_Table_Access_Hash.gif   

Table Access operation.

TABLE ACCESS HASH is analogous to TABLE ACCESS BY ROWID; instead of accessing rows by ROWID, the optimizer calculates the location of the row inside the table using a hash formula on the key columns of the table and then uses that value to access the rows.

Example

Assume that the COMPANY table is stored in a hash cluster. Execute the following SQL statement: 

select Name
from COMPANY
where Company_ID = 12345
and Active_Flag = 'Y';

Execution Plan

TABLE ACCESS HASH COMPANY

Interpreting the Execution Plan

Since the Company_ID is the primary key for the COMPANY table, the hash formula uses the Company_ID value to determine the ROWID of the sought row. Since the ROWID is determined by the hash formula, there is no need for an index access to retrieve the specified row.

Note: See the Top SQL Tuning Tips topic for tuning guidance related to the use of hash clusters for very large tables.

 

Related Topics

 

TABLE ACCESS SAMPLE

Execution Plan image: image\Plan_Sample.gif   

Table Access operation.

A sample table scan retrieves a random sample of data from a table. This access method is used when the 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 and examines each of these rows to determine whether it satisfies the statement's WHERE clause. To perform a sample table scan when sampling by blocks (the SAMPLE BLOCK clause), Oracle reads a specified percentage of the table's blocks and examines each row in the sampled blocks to determine whether it satisfies the statement's WHERE clause.

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 and 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 cost-based optimizer.

Example

The following statement uses a sample table scan to access 1% of the emp table, sampling by blocks:

SELECT *
FROM emp SAMPLE BLOCK (1);

Execution Plan

Operation

Options

Object_name

SELECT STATEMENT
TABLE ACCESS

SAMPLE

EMP

 

Related Topics

 

TABLE QUEUE

 

The TABLE QUEUE operation indicates use of parallel execution plan.

 

  

 

 

TEMP TABLE GENERATION

Execution Plan image: image\ebx_-1884398177.gif

Miscellaneous operation.

The TEMP TABLE GENERATION operation creates a temporary dimension table for dimension tables that have been joined by a Star Transformation. For each dimension table in the joined set, TEMP TABLE GENERATION creates a temporary table to replace the table in the Execution Plan. A temporary table is created using two steps: create table and insert each. Since there are at least two tables in a join, this operation creates a minimum of four temporary tables.

 

Related Topics

 

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