Oracle Query optimization

Oracle Query optimization

(1) Select the most efficient table name order (only valid in the rule-based optimizer): The Oracle parser processes the table name from the FROM clause in a right-to-left order, and the last table written in the FROM clause (base table driving tables) is processed first. In cases where the FROM clause contains more than one table, you must select the table with the lowest number of record bars as the underlying table. If you have more than 3 tables connected to the query, you need to select the crosstab (intersection table) as the underlying table, which refers to the table that is referenced by the other table.
(2) The connection order in the WHERE clause. : Oracle parses the WHERE clause in a bottom-up order, and according to this principle, the connection between tables must be written before other where conditions, and those conditions that can filter out the maximum number of records must be written at the end of the WHERE clause.
(3) Avoid using ' * ' in the SELECT clause: Oracle will convert ' * ' to all column names in the process of parsing, which is done by querying the data dictionary, which means more time is spent
(4) Reduce access to the database: Oracle has done a lot of work internally: Parsing SQL statements, estimating index utilization, binding variables, reading data blocks, etc.;
(5) Reset the ArraySize parameter in Sql*plus, sql*forms and pro*c to increase the amount of data retrieved per database access, with a recommended value of 200
(6) Use the Decode function to reduce processing time: Use the Decode function to avoid duplicate scans of the same record or duplicate connections to the same table.
(7) Integrated simple, unrelated database access: If you have a few simple database query statements, you can integrate them into a single query (even if they don't have a relationship)
(8) Delete duplicate records: The most efficient way to delete duplicate records
(Because of the use of ROWID) example: DELETE from EMP E WHERE e.rowid >
(SELECT MIN (x.rowid) from EMP X WHERE x.emp_no = e.emp_no);
(9) Replace Delete with truncate: When you delete a record in a table, in general, the rollback segment (rollback segments) is used to hold information that can be recovered. If you do not have a COMMIT transaction, Oracle restores the data to the state before it was deleted ( It is accurate to revert to the condition before executing the delete command) and when Truncate is applied, the rollback segment no longer holds any recoverable information. When the command is run, the data cannot be restored. Therefore, very few resources are invoked and execution times are short. (Translator Press: truncate only in Delete full table applies, truncate is DDL is not DML)
(10) Use commit as much as possible: whenever possible, use commit as much as you can in the program, so that the performance of the program is improved and the requirements are reduced by the resources freed by the commit: the resources freed by commit:
A. Information for recovering data on a rollback segment.
B. Locks acquired by program statements
C. Redo space in the log buffer
D. Oracle manages internal spending on 3 of these resources
(11) Replace the HAVING clause with a WHERE clause: to avoid having a HAVING clause, the having will only filter the result set after retrieving all records. This processing requires sorting, totals, and so on.
If you can limit the number of records through the WHERE clause, you can reduce this overhead.
(Non-Oracle) on, where, have the three clauses that can be added conditionally, on is the first execution, where the second, having the last, because on is the non-qualifying records filtered before the statistics, it can reduce the intermediate operation to process the data, It should be said that the speed is the fastest, where should also be faster than having to, because it filters the data before the sum, in two table joins only use on, so in a table, the left where and have compared. In the case of this single-table query statistics, if the conditions to be filtered do not involve the fields to be calculated, then they will be the same result, but where you can use the Rushmore technology, and have not, at the speed of the latter slow if you want to relate to the calculated field, it means that before the calculation, The value of this field is indeterminate, according to the workflow of the previous write, where the action time is done before the calculation, and having is calculated after the function, so in this case, the results will be different. On a multi-table join query, on has an earlier effect than where. The system first synthesizes a temporary table based on the conditions of the joins between the tables, then the where is filtered, then calculated, and then filtered by having. Thus, to filter the conditions to play the right role, first of all to understand when this condition should play a role, and then decided to put it there
(12) Reduce the query on the table: in the SQL statement that contains the subquery, pay special attention to reducing the query on the table. Example: SELECT tab_name from TABLES WHERE
(tab_name,db_ver) = (SELECT tab_name,db_ver from tab_columns WHERE VERSION = 604)
(13) Improve SQL efficiency with internal functions. Complex SQL often sacrifices execution efficiency. The method of solving the problem by using the function above is very meaningful in the actual work.
(14) using the alias of the table: when you concatenate multiple tables in an SQL statement, use the alias of the table and prefix the alias to each column. This reduces the time to parse and reduces the syntax errors caused by column ambiguity.
(15) Replace in with EXISTS with not EXISTS instead of in: For many base table-based queries, it is often necessary to join another table in order to satisfy one condition. In this case, using EXISTS (or not EXISTS) will usually improve the efficiency of the query.
In a subquery, the NOT IN clause performs an internal sort and merge.
In either case, not in is the least effective
(because it performs a full-table traversal of a table in a subquery).
To avoid using not, we can change it to an outer join (Outer Joins) or not EXISTS.
Example: (efficient) SELECT * from EMP
(base table) WHERE EMPNO > 0 and EXISTS
(SELECT ' X ' from DEPT WHERE DEPT. DEPTNO = EMP. DEPTNO and LOC = ' Melb ') (inefficient) SELECT * from EMP
(base table) where EMPNO > 0 and DEPTNO in (SELECT DEPTNO from DEPT WHERE LOC = ' Melb ')
(16) Identify the SQL statement for ' inefficient execution ': although there are many graphical tools for SQL optimization, it is always a good idea to write your own SQL tools to solve the problem: SELECT executions, disk_reads, buffer_gets , ROUND ((buffer_gets-disk_reads)/buffer_gets,2) Hit_radio, ROUND (disk_reads/executions,2) Reads_per_run, SQL_TEXT From V$sqlarea WHERE executions>0 and buffer_gets > 0 and
(buffer_gets-disk_reads)/buffer_gets < 0.8 ORDER by 4 DESC;
(17) Improve efficiency with indexes: An index is a conceptual part of a table used to improve the efficiency of retrieving data, and Oracle uses a complex self-balancing b-tree structure.
In general, querying data through an index is faster than a full table scan.
When Oracle finds the best path to execute queries and UPDATE statements, the Oracle Optimizer uses the index.
Also, using indexes when joining multiple tables can improve efficiency.
Another advantage of using an index is that it provides uniqueness validation of the primary key (primary key).
。 Those long or long raw data types, you can index almost all the columns.
In general, using indexes in large tables is particularly effective.
Of course, you will also find that using indexes can also improve efficiency when scanning small tables.
Although the use of indexes can improve the efficiency of query, but we must also pay attention to its cost.
Indexes require space to store, and they need to be maintained regularly, and the index itself is modified whenever a record is added to a table or the index column is modified.
This means that each record's insert, DELETE, and update will pay more than 4, 5 disk I/O.
Because indexes require additional storage space and processing, those unnecessary indexes can slow query response time.
。 It is necessary to periodically refactor the index.
: ALTER INDEX <INDEXNAME> REBUILD <TABLESPACENAME>
(18) Replace distinct with exists: avoid using DISTINCT in the SELECT clause when submitting a query that contains one-to-many table information, such as a departmental table and an employee table.
It is generally possible to consider replacing with exist, EXISTS makes the query faster because the RDBMS core module will return the results immediately after the conditions of the subquery have been met.
Example:
(inefficient): SELECT DISTINCT dept_no,dept_name from DEPT D, EMP E WHERE d.dept_no = e.dept_no
(efficient): SELECT dept_no,dept_name from DEPT D WHERE EXISTS
(SELECT ' X ' from EMP E WHERE e.dept_no = d.dept_no);
SQL statements are capitalized, because Oracle always parses the SQL statements first, converting lowercase letters to uppercase and then executing
(20) Use the connector "+" connection string sparingly in Java code!
(21) Avoid using not on indexed columns typically, we want to avoid using not on indexed columns, which does not produce the same effect as using functions on indexed columns.
When Oracle "encounters" not, he stops using the index instead of performing a full-table scan.
(22) Avoid using calculations on indexed columns. Where clause, if the index column is part of a function. The optimizer will use a full table scan without using an index. Example: Inefficiency: SELECT ... From DEPT WHERE SAL * > 25000; Efficient: SELECT ... From DEPT WHERE SAL > 25000/12;
(23) Replace > Efficient with >=: SELECT * from emp where DEPTNO >=4 inefficient: SELECT * from emp where DEPTNO >3 The difference is that the former DBMS will jump directly to the first A record with a dept equal to 4 and the latter will first locate the Deptno=3 record and scan forward to the first record with a dept greater than 3.
(24) Replace or with union
(for indexed columns) in general, replacing or in the WHERE clause with union will have a good effect.
Using or on an indexed column causes a full table scan.
Note that the above rules are valid only for multiple indexed columns.
If a column is not indexed, the query efficiency may be reduced because you did not select or.
In the following example, indexes are built on both loc_id and region.
Efficient: Select loc_id, Loc_desc, region from location WHERE loc_id = Ten UNION SELECT loc_id, Loc_desc, region from Locatio N WHERE region = "MELBOURNE" inefficient: Select loc_id, Loc_desc, region from location WHERE loc_id = ten OR region = "MELBOURNE" If you persist in using or, you need to return the least logged index column to the front.
(25) Replace or with in. This is a simple and easy-to-remember rule, but the actual execution effect has to be tested, and the execution path of the two appears to be the same under Oracle8i. Inefficient: SELECT ....
From location WHERE loc_id = ten or loc_id = or loc_id = 30 efficient SELECT ... From location WHERE loc_in in (10,20,30);
(26) Avoid using is null on the index column and is not NULL to avoid using any nullable columns in the index, and Oracle will not be able to use that index. For single-column indexes, this record will not exist in the index if the column contains null values.
For composite indexes, this record does not exist in the index if each column is empty. If at least one column is not empty, the record exists in the index. For example, if a uniqueness index is established on column A and column B of a table, and the table has a A and a record of a A and a (123,null), Oracle will not accept the next record (insert) with the same A, B value (123,null). However, if all the index columns are empty,　　Oracle will assume that the entire key value is empty and null is not equal to NULL. So you can insert 1000 records with the same key value, of course they are empty! Because null values do not exist in the index column, a null comparison of indexed columns in the WHERE clause causes Oracle to deactivate the index.
Low efficiency:
(Index invalidation) SELECT ... From DEPARTMENT WHERE dept_code are not NULL;
Efficient:
(Index valid) SELECT ... From DEPARTMENT WHERE Dept_code >=0;
(27) Always use the first column of the index: if the index is on more than one column, the optimizer chooses to use the index only if its first column (leading column) is referenced by a WHERE clause.
This is also a simple and important rule, when referencing only the second column of an index, the optimizer uses a full table scan and ignores the index
(28) Replace union with Union-all
(if possible): When the SQL statement requires a union of two query result sets, the two result sets are merged in a union-all manner and then sorted before the final result is output.
If you use UNION ALL instead of union, this sort is not necessary.
Efficiency will therefore be improved.
It is important to note that the UNION all will output the same record in the two result set repeatedly.
So you still have to analyze the feasibility of using union all from the business requirements.
The UNION will sort the result set, which will use the memory of the sort_area_size.
The optimization of this memory is also very important.
The following SQL can be used to query the ordering of consumption inefficiencies: Select Acct_num, Balance_amt from debit_transactions WHERE tran_date = ' 31-dec-95 ' UNION SELECT ACC T_num, Balance_amt from debit_transactions WHERE tran_date = ' 31-dec-95 ' efficient: SELECT acct_num, Balance_amt from Debit_trans ACTIONS where tran_date = ' 31-dec-95 ' UNION all SELECT acct_num, Balance_amt from debit_transactions WHERE tran_date = ' 31 -dec-95 ' (29) uses the where instead of the ORDER by:order by clause to use the index only under two strict conditions.
All columns in an order by must be in the same index and remain in the order in which they are arranged in the index.
All columns in the ORDER by must be defined as non-empty.
The index used in the WHERE clause and the index used in the ORDER BY clause cannot be tied.
For example: Table DEPT contains the following: Dept_code PK not NULL DEPT_DESC NOT NULL DEPT_TYPE null inefficient:
(index not being used) SELECT Dept_code from DEPT ORDER by Dept_type Efficient:
(Use index) SELECT Dept_code from DEPT WHERE dept_type > 0
(30) Avoid changing the type of indexed columns.
: Oracle automatically makes simple type conversions to columns when comparing data of different data types.
Suppose Empno is an indexed column of a numeric type.
SELECT ... From EMP WHERE EMPNO = ' 123 ' Actually, after the Oracle type conversion, the statement is converted to: SELECT ... From EMP where EMPNO = To_number (' 123 ') Fortunately, the type conversion did not occur on the index column, and the use of the index was not changed.
Now, suppose Emp_type is an indexed column of a character type.
SELECT ... From EMP WHERE Emp_type = 123 This statement is converted by Oracle to: SELECT ... From EMP Whereto_number (emp_type) =123 This index will not be used because of the type conversions that occur internally! To avoid the implicit type conversion of your SQL by Oracle, it is best to explicitly express the type conversions.
Note When comparing characters to numbers, Oracle takes precedence over numeric types to character types
(31) WHERE clause to beware: the WHERE clause in some SELECT statements does not use an index.
Here are some examples.
In the following example,
(1) '! = ' will not use the index.
Remember, the index can only tell you what exists in the table, not what does not exist in the table.
(2) ' | | ' is a character join function.
As with other functions, the index is deactivated.
(3) ' + ' is a mathematical function.
As with other mathematical functions, the index is deactivated.
(4) The same index columns cannot be compared to each other, which will enable full table scanning.
A. If the number of records in a table that has more than 30% data is retrieved. Using indexes will not be significantly more efficient.
B. In certain situations, using an index may be slower than a full table scan, but this is the same order of magnitude difference.
In general, the use of indexes than the full table scan to block several times or even thousands of times!
(33) Avoid the use of resource-intensive operations: SQL statements with Distinct,union,minus,intersect,order by will start the SQL engine to perform the resource-intensive sorting (sort) function.
Distinct requires a sort operation, while the others need to perform at least two sorting.
Typically, SQL statements with union, minus, and intersect can be overridden in other ways.
If your database is well-sort_area_size, using union, minus, intersect can also be considered, after all, they are very readable
(34) Optimize GROUP by: increase the efficiency of the group BY statement by filtering out unwanted records before group by. The following two queries return the same result but the second one is significantly faster.
Inefficient: Select Job, AVG (SAL) from EMP GROUP job has job = ' president ' OR job = ' MANAGER ' efficient: Select Job, AVG (SAL) from EM P WHERE job = ' president ' OR job = ' MANAGER ' GROUP job

Go to:

Https://www.ibm.com/developerworks/community/wikis/home?lang=zh#!/wiki/oracle/page/oracle%E6%9F%A5%E8%AF%A2%E4 %bc%98%e5%8c%96

Oracle Query optimization