How to select an Oracle optimizer

1. select an appropriate Oracle optimizer

There are three optimizer types in Oracle:

A. RULE (RULE-based) B. COST (COST-based) c. CHOOSE (selectivity ).

Set the default optimizer. You can declare the OPTIMIZER_MODE parameter in the init. ora file, such as RULE, COST, CHOOSE, ALL_ROWS, and FIRST_ROWS. Of course, you also overwrite SQL statements or sessions.

To use the Cost-Based Optimizer (CBO, Cost-Based Optimizer), you must run the analyze command frequently to increase the accuracy of object statistics in the database.

If the optimizer mode of the database is set to CHOOSE, the actual optimizer mode is related to whether the analyze command has been run. If the table has been analyze, the optimizer mode will automatically become CBO, and vice versa, the database will adopt the RULE form optimizer.

By default, Oracle uses the CHOOSE optimizer. To avoid unnecessary full table scan, you must avoid using the CHOOSE optimizer, instead, we directly use a rule-based or cost-based optimizer.

2. Access Table

Oracle uses two methods to access table records:

A. Full table scan:

Full table scan accesses each record in the table sequentially. Oracle optimizes full table scanning by reading multiple data blocks at a time.

B. Access the table through ROWID:

You can use ROWID-based access to improve the efficiency of accessing tables. ROWID contains the physical location information recorded in the table. Oracle uses indexes to connect data with the physical location where data is stored (ROWID. Generally, indexes provide a quick way to access ROWID. Therefore, queries based on index columns can improve the performance.

3. Share SQL statements

To avoid repeated parsing of the same SQL statement, Oracle stores the SQL statement in memory after the first parsing. the memory in the shared buffer pool of the SGA (system global area) can be shared by all database users. Therefore, when you execute an SQL statement (sometimes called a cursor), if it is exactly the same as the previously executed statement, oracle can quickly obtain parsed statements and the best execution path. This function greatly improves SQL Execution performance and saves memory usage.

Unfortunately, Oracle only provides cache buffering for simple tables. This function is not applicable to multi-table join queries.

Database administrators must set appropriate parameters for this region in init. ora. When the memory area is larger, more statements can be reserved. Of course, the possibility of sharing will become greater.

When you submit an SQL statement to Oracle, Oracle will first find the same statement in the memory.

It should be noted that Oracle adopts a strict match between the two. To achieve sharing, the SQL statements must be identical (including spaces and line breaks ).

The shared statement must meet three conditions:

A. Character-level comparison:

The statements currently executed must be the same as those in the shared pool.

For example:

SELECT * from emp;

Different from each of the following:

SELECT * from EMP;

Select * From Emp;

SELECT * from emp;

B. The objects referred to by the two statements must be identical:

For example:

How to access user object names

Jack sal_limit private synonym

Work_city public synonym

Plant_detail public synonym

Jill sal_limit private synonym

Work_city public synonym

Plant_detail table owner

Consider whether the following SQL statements can be shared between the two users.

C. bind variables ):

For example:

The two SQL statements in the first group are the same (which can be shared), while the two statements in the second group are different (even if they are assigned the same value for different bind variables at runtime ).

A. select pin, name from people where pin =: blk1.pin;

Select pin, name from people where pin =: blk1.pin;

B. select pin, name from people where pin =: blk1.ot _ ind;

Select pin, name from people where pin =

: Blk1.ov _ ind;