To improve the efficiency of SQL query how to write the precedence of where statement conditions

We have to do not only write SQL, but also to write good performance of SQL statements.


(1) Select the most efficient table name order (valid only in the rule-based optimizer):

The Oracle parser processes the table names in the FROM clause in a right-to-left order, and the FROM clause is written in the final table (the underlying table, driving tables) will be processed first, and in the case where the FROM clause contains more than one table, you must select the table with the fewest 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 uses a bottom-up sequential parsing where clause, according to which the connection between tables must be written before other where conditions, and those that can filter out the maximum number of records must be written at the end of the WHERE clause. China. Webmaster Station

(3) Avoid using ' * ' in the SELECT clause:

During the parsing process, Oracle translates ' * ' into all column names, which is done by querying the data dictionary, which means more time is spent.

(4) Reduce the number of access to the database:

Oracle does a lot of work internally: Parsing SQL statements, estimating index utilization, binding variables, reading blocks, and so on.

(5) in Sql*plus, sql*forms and pro*c reset the ArraySize parameter, you can increase the amount of data retrieved per database access, the recommended value is 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) Integration of simple, unrelated database access:


If you have a few simple database query statements, you can integrate them into a single query (even if they are not related to each other).


(8) Delete duplicate records:

The most efficient method of deleting 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 (to be exact, before the delete command was executed) and when the truncate is applied, the rollback segment no longer holds any recoverable information. When the command is run, the data cannot be restored. So very few resources are invoked and the execution time is very short. (Truncate only applies if the whole table is removed, truncate is DDL is not DML). Chinaz_com

(10) Use commit as much as possible:

Whenever possible, commit is used as much in the program as possible, 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 the commit:

A. Information for recovering data on a rollback segment.

B. The lock obtained by the program statement.

C. Redo the space in the log buffer. [Email protected]

D. Oracle manages the internal costs of the above 3 resources.


(11) Replace the HAVING clause with a WHERE clause:

Avoid having a HAVING clause that filters the result set only after all records have been retrieved. 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 there.

(12) Reduce the query on the table:
In the SQL statement that contains the subquery, pay particular attention to reducing the query on the table. Example: Chinaz

SELECT Tab_name from TABLES where (tab_name,db_ver) = (Selecttab_name,db_ver from tab_columns where VERSION = 604)

(13) Improve SQL efficiency with intrinsic functions:

Complex SQL often sacrifices execution efficiency. The ability to master the above application function to solve the problem is very meaningful in practical 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 aliases on each column. In this way, you can reduce the parsing time and reduce the syntax errors caused by the column ambiguity. Chinaz_com

(15) Replace in with exists with not exists instead of in:


In 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 the table in the 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-d Isk_reads)/buffer_gets < 0.8 ORDER by 4 DESC;

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 for each record, update will pay 4, 5 more disk I/O for this insert,delete. 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 FRO M 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.

(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.

Inefficient: SELECT ... From DEPT WHERE SAL * > 25000; Efficient: SELECT ... From DEPT WHERE SAL > 25000/12;

(23) Replace with >=;:

Efficient: SELECT * from emp where DEPTNO >=4 inefficient: SELECT * from emp where DEPTNO >3

The difference between the two is that the former DBMS will jump directly to the first record that dept equals 4 and the latter will first navigate to 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 a 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 location where region = "Melbo Urne "

Inefficient: Select loc_id, Loc_desc, region from location WHERE loc_id = ten OR region = "MELBOURNE"

(25) Replace or with in:

This is a simple and easy-to-remember rule, but the actual execution will have to be tested, and under Oracle8i, the execution path seems to be the same:

Low efficiency:


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 and is not NULL on an indexed column:


To avoid using any nullable columns in the index, Oracle will not be able to use the index. For single-column indexes, this record will not exist in the index if the column contains null values. For composite indexes, if each column is empty, the same record does not exist in the index. 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 value of a, a and a record of (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.

Inefficient: (Index invalidation)

SELECT ... From DEPARTMENT WHERE dept_code are not NULL;

Efficient: (Index valid)

SELECT ... From DEPARTMENT WHERE Dept_code >=0; Webmaster. Station

(27) Always use the first column of an index:
If the index is built 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 that 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 ordered consumption: Www.Chinaz.com

Inefficient: Select Acct_num, Balance_amt from debit_transactions WHERE tran_date = ' 31-dec-95 ' UNION SELECT acct_num, Balance_amt From debit_transactions where tran_date = ' 31-dec-95 ' efficient: SELECT acct_num, Balance_amt from Debit_transactions WHERE tran_ DATE = ' 31-dec-95 ' UNION all SELECT acct_num, Balance_amt from debit_transactions WHERE tran_date = ' 31-dec-95 '

(29) Where to replace order by:

The ORDER by clause uses 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 used)

SELECT Dept_code from DEPT ORDER by Dept_type

Efficient: (using 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 '. In fact, after the Oracle type conversion, the statement translates to: SELECT ... From EMP WHERE EMPNO = to_number (' 123 ').

Fortunately, the type conversion did not occur on the index column, and the purpose 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 was 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) The WHERE clause to be careful:

The WHERE clause in some SELECT statements does not use an index. Here are some examples:

(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.

(b) A. If you retrieve the number of records in a table that has more than 30% data, there is no significant efficiency increase in using the index.

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 using 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 the sort_area_size of your database is well-provisioned. With union, minus, intersect can also be considered, after all, their readability is very strong.

(34) Optimize GROUP by:

The efficiency of the group BY statement can be improved by filtering out unwanted records before group by. The following two queries return the same results 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

To improve the efficiency of SQL query how to write the precedence of where statement conditions