Oracle SQL Performance Optimization series I

The ORACLE tutorial is: Oracle SQL Performance Optimization Series 1.

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. various declarations of the OPTIMIZER_MODE parameter in the ora file, such as RULE, COST, CHOOSE, ALL_ROWS, 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 a CBO. Otherwise, the database will adopt a RULE optimizer.

By default, ORACLE uses the CHOOSE optimizer. To avoid unnecessary full table scan, you must avoid using the CHOOSE optimizer, the optimizer based on rules or costs is directly used.

2. Access Table

ORACLE uses two methods to access table records:

A. Full table Scan

Full table scan refers to sequential access to each record in the table. ORACLE optimizes full table scan 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 establish a connection between data and the physical location where data is stored (ROWID. generally, indexes provide a quick way to access ROWID, so those 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 high-speed buffer (cache buffering) for simple tables. This function is not applicable to multi-table join queries.

The database administrator 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 is higher.

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.

Can SQL be shared?

Select max (sal_cap) from sal_limit;

No. Each user has a private synonym-sal_limit, which is a different object.

Select count (* 0 from work_city where sdesc like 'new % ';

Yes. The two users access the same object public synonym-work_city.

Select a. sdesc, B. location from work_city a, plant_detail B where a. city_id = B. city_id

No. User jack accesses plant_detail through private synonym, and jill is the table owner, with different objects.

C. bind variables must be used in the two SQL statements)

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 different bind variables have the same value at runtime)

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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;

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