(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) Integration of simple, unrelated database access: If you have several simple database query statements, you can integrate them into a single query (even if they are not) (8) Delete duplicate records: The most efficient way to delete duplicate records (because of the use of rowID) Example: Delete FR OM 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 it was before it was deleted (exactly before the delete command was executed) and when the truncate is applied, the rollback segment no longer holds any recoverable information. When the command runs, The data cannot be restored. So 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 little as possible:
(11) Replace the HAVING clause with a WHERE clause: Avoid using the HAVING clause, which only filters 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, in order to filter 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 SQL statements that contain subqueries, pay particular attention to reducing queries on tables. Example: Select Tab_name from TABLES WHERE (tab_name,db_ver) = (select Tab_name,db_ver from Tab_c Olumns WHERE VERSION = 604) (13) improves 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 practice (14) Use table aliases (alias): When you concatenate multiple tables in an SQL statement, use the alias of the table and prefix the alias to each column. In this way, You can reduce the time to parse and reduce 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 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, B Uffer_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 ORD ER by 4 DESC;
(17) Improve efficiency with indexes: 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 the uniqueness of the primary key (primary key) Validation: 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 of INSERT, DELETE, update will pay more than 4, 5 times the 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 SQL statements first, converts lowercase letters to uppercase and then executes (20) Use the connector "+" connection string sparingly in Java code! (21) Avoid using not on indexed columns generally, we want to avoid using not on indexed columns, not to have 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: Inefficient: (23) Replace with >= > Efficient: 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 record dept equals 4 and the latter will first navigate to the Deptno=3 record and scan forward to the first dept greater than 3. (24) Replace or with union (for indexed columns) Typically, 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 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) inTo replace or this is a simple and easy-to-remember rule, but the actual execution will have 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 an indexed column and is not NULL 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, 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; (27) Always use the first column of the 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, when referencing only the second column of the 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 the output is eventuallyBefore the results are sorted. 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 sort consumption inefficiency: (29) use where instead of order by: The order by clause uses the index only under two strict conditions. All columns in the order by must be contained in the same index and maintained in the order in which they are arranged in the index. All columns in order by must be defined as non-empty. The index used in the &NBSP;WHERE clause and the index in the ORDER BY clause cannot be tied. Example: Table DEPT contains the following: dept_code PK not NULL &NBSP;DEPT_DESC NOT NULL &NBSP;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 &nb SP; (30) Avoid changing the type of indexed columns.: When comparing data of different data types, Oracle automatically makes simple type conversions for columns. Assume that 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 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 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 numeric values, Oracle takes precedence in converting numeric types to character types (31) WHERE clauses to beware: WHERE clauses in some SELECT statements do not use indexes. 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 have no significant efficiency gains. b. In certain cases, 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 your database is well-sort_area_size, using union, minus, intersect can also be considered, after all, they are highly readable (34) Optimization 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 results but the second one is obviously fast. Many .
SQL statement optimization in ORACLE
