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Distributed Queries → databases Restrictions on Distributed Queries Distributed queries are currently subject to the … currently does not support distributed queries that select user-defined types or object REF datatypes on remote tables.
Hierarchical Queries → operator extends the functionality of the CONNECT BY [ PRIOR ] condition of hierarchical queries by … hierarchical queries as follows: A join, if present, is evaluated first, whether the join is
Using Views in Queries → To issue a query or an INSERT, UPDATE, or DELETE statement against a view, you must have the SELECT, INSERT, UPDATE, or DELETE object privilege for the view, respectively, either explicitly or through a role. Views can be queried in the same manner as tables. For example, to query the Division1_staff view, enter a valid SELECT statement that references the view: SELECT * FROM Division1_staff; ENAME
Tuning Distributed Queries → queries, which it then sends to the remote nodes for execution. The remote nodes execute the … queries and send the results back to the local node. The local node then performs any necessary post
Creating Simple Queries → The list of expressions that appears after the SELECT keyword and before the FROM clause is called the select list. Within the select list, you specify one or more columns in the set of rows you want Oracle Database to return from one or more tables, views, or materialized views. The number of columns, as well as their datatype and length, are determined by the elements of the select list. If two
About Queries and Subqueries → called a subquery. This section describes some types of queries and subqueries and how to use them. The
Hierarchical Query Examples → Figure 9-1 Hierarchical Queries Description of \"Figure 9-1 Hierarchical Queries\" To find the
9 SQL Queries and Subqueries → This chapter describes SQL queries and subqueries. This chapter contains these sections: About … Queries and Subqueries Creating Simple Queries Hierarchical Queries The UNION [ALL], INTERSECT, MINUS … Operators Sorting Query Results Joins Using Subqueries Unnesting of Nested Subqueries Selecting from the DUAL Table Distributed
Using Collocated Inline Views → The most effective way of optimizing distributed queries is to access the remote databases as … distributed queries to take advantage of the performance gains offered by collocated inline views.
How Does Cost-Based Optimization Work? → The main task of optimization is to rewrite a distributed query to use collocated inline views. This optimization is performed in three steps: All mergeable views are merged. Optimizer performs collocated query block test. Optimizer rewrites query using collocated inline views. After the query is rewritten, it is executed and the data set is returned to the user. While cost-based optimization is performed
Setting Up Cost-Based Optimization → distributed queries, the operation is transparent to the user. In other words, the optimization occurs
Using the NO_MERGE Hint → The NO_MERGE hint prevents the database from merging an inline view into a potentially non-collocated SQL statement (see \"Using Hints\" ). This hint is embedded in the SELECT statement and can appear either at the beginning of the SELECT statement with the inline view as an argument or in the query block that defines the inline view. /* with argument */ SELECT /*+NO_MERGE(v)*/ t1.x, v.avg_y FROM
Analyzing the Execution Plan → An important aspect to tuning distributed queries is analyzing the execution plan. The feedback
Preparing the Database to Store the Plan → Before you can view the execution plan for the distributed query, prepare the database to store the execution plan. You can perform this preparation by executing a script. Execute the following script to prepare your database to store an execution plan: SQL> @UTLXPLAN.SQL Note: The utlxplan.sql file can be found in the $ORACLE_HOME/rdbms/admin directory. After you execute utlxplan.sql, a table, PLAN_TABLE,
Using the DRIVING_SITE Hint → The DRIVING_SITE hint lets you specify the site where the query execution is performed. It is best to let cost-based optimization determine where the execution should be performed, but if you prefer to override the optimizer, you can specify the execution site manually. Following is an example of a SELECT statement with a DRIVING_SITE hint: SELECT /*+DRIVING_SITE(dept)*/ * FROM emp, firstname.lastname@example.org
Using Hints → , load, network and CPU limitations, distributed queries, and so forth), you can specify a hint to
Viewing the Execution Plan → | | | | | | ------------------------------------------------------------------------------- If you are manually optimizing distributed queries by
Generating the Execution Plan → After you have prepared the database to store the execution plan, you are ready to view the plan for a specified query. Instead of directly executing a SQL statement, append the statement to the EXPLAIN PLAN FOR clause. For example, you can execute the following: EXPLAIN PLAN FOR SELECT d.dname FROM dept d WHERE d.deptno IN (SELECT deptno FROM email@example.com GROUP BY deptno HAVING COUNT (deptno) >3
Using Cost-Based Optimization → In addition to rewriting your queries with collocated inline views, the cost-based optimization … method optimizes distributed queries according to the gathered statistics of the referenced tables and … statement. Creating a collocated inline view reduces the amount of queries performed at a remote site, thereby reducing costly network
Queries → data. All queries begin with the SQL keyword SELECT, as in the following example: SELECT ename, empno … OraDatabase interface to execute queries. This method returns an OraDynaset object that is then used to … provides a way to insert new rows. See \"OraDynaset Object\". Queries can also require the program to