Oracle Exam Questions 1Z0-117 Oracle Database 11g Release 2: SQL Tuning Exam Version:Demo
1.You ran a high load SQL statement that used an index through the SQL Tuning Advisor and accepted its recommendation for SQL profile creation. Subsequently you noticed that there has been a 2% growth in number of rows in the tables used by the SQL statement and database statistics have also been refreshed. How does this impact the created SQL profile? A. It becomes invalid and no longer used the optimizer. B. It remains valid and ensures that the optimizer always use the execution plan that was created before the changes happened. C. It remains and allows the optimizer to pick a different plan required. D. It becomes invalid and a new SQL profile is created for the statement by the auto tuning task. Answer: C 2. What are three common reasons for SQL statements to perform poorly? A. Full table scans for queries with highly selective filters B. Stale or missing optimizer statistics C. Histograms not existing on columns with evenly distributed data D. High index clustering factor E. OPTIMIZER_MODE parameter set to ALL_ROWS for DSS workload Answer: A,B,D Explanation: D: The clustering_factor measures how synchronized an index is with the data in a table. A table with a high clustering factor is out-of-sequence with the rows and large index range scans will consume lots of I/O. Conversely, an index with a low clustering_factor is closely aligned with the table and related rows reside together of each data block, making indexes very desirable for optimal access. Note: * (Not C) Histograms are feature in CBO and it helps to optimizer to determine how data are skewed(distributed) with in the column. Histogram is good to create for the column which are included in the WHERE clause where the column is highly skewed. Histogram helps to optimizer to decide whether to use an index or full-table scan or help the optimizer determine the fastest table join order. * OPTIMIZER_MODE establishes the default behavior for choosing an optimization approach for the instance. all_rows The optimizer uses a cost-based approach for all SQL statements in the session and optimizes with a goal of best throughput (minimum resource use to complete the entire statement).
3. You instance has these parameter settings: Which three statements are true about these settings if no hints are used in a SQL statement? A. A statement estimated for more than 10 seconds always has its degree of parallelism computed automatically. B. A statement with a computed degree of parallelism greater than 8 will be queued for a maximum of 10 seconds. C. A statement that executes for more than 10 seconds always has its degree of parallelism computed automatically. D. A statement with a computed degree of parallelism greater than 8 will raise an error. E. A statement with any computed degree of parallelism will be queued if the number of busy parallel execution processes exceeds 64. F. A statement with a computed degree of parallelism of 20 will be queued if the number of available parallel execution processes is less 5. Answer: C,E,F Explanation: C (not A): PARALLEL_MIN_TIME_THRESHOLD specifies the minimum execution time a statement should have before the statement is considered for automatic degree of parallelism. By default, this is set to 30 seconds. Automatic degree of parallelism is only enabled if PARALLEL_DEGREE_POLICY is set to AUTO or LIMITED. PARALLEL_DEGREE_LIMIT integer A numeric value for this parameter specifies the maximum degree of parallelism the optimizer can choose for a SQL statement when automatic degree of parallelism is active. Automatic degree of parallelism is only enabled if PARALLEL_DEGREE_POLICY is set to AUTO or LIMITED. E: PARALLEL_SERVERS_TARGET specifies the number of parallel server processes allowed to run parallel statements before statement queuing will be used. When the parameter PARALLEL_DEGREE_POLICY is set to AUTO, Oracle will queue SQL statements that require parallel execution, if the necessary parallel server processes are not available. Statement queuing will begin once the number of parallel server processes active on the system is equal to or greater than PARALLEL_SERVER_TARGET. F: PARALELL_MIN_MINPERCENT PARALLEL_MIN_PERCENT operates in conjunction with PARALLEL_MAX_SERVERS and PARALLEL_MIN_SERVERS. It lets you specify the minimum percentage
of parallel execution processes (of the value of PARALLEL_MAX_SERVERS) required for parallel execution. Setting this parameter ensures that parallel operations will not execute sequentially unless adequate resources are available. The default value of 0 means that no minimum percentage of processes has been set. Consider the following settings: PARALLEL_MIN_PERCENT = 50 PARALLEL_MIN_SERVERS = 5 PARALLEL_MAX_SERVERS = 10 If 8 of the 10 parallel execution processes are busy, only 2 processes are available. If you then request a query with a degree of parallelism of 8, the minimum 50% will not be met. Note: With automatic degree of parallelism, Oracle automatically decides whether or not a statement should execute in parallel and what degree of parallelism the statement should use. The optimizer automatically determines the degree of parallelism for a statement based on the resource requirements of the statement. However, the optimizer will limit the degree of parallelism used to ensure parallel server processes do not flood the system. This limit is enforced by PARALLEL_DEGREE_LIMIT. Values: CPU IO integer A numeric value for this parameter specifies the maximum degree of parallelism the optimizer can choose for a SQL statement when automatic degree of parallelism is active. Automatic degree of parallelism is only enabled if PARALLEL_DEGREE_POLICY is set to AUTO or LIMITED. Reference: PARALLEL_MIN_TIME_THRESHOLD PARALLEL_DEGREE_LIMIT PARALELL_MIN_MINPERCENT PARALELL_SERVERS_TARGET 4. Examine the parallelism parameters for your instance:
What are true about the execution of the query? A. It will execute in parallel only if the LINEITEM table has a dictionary DOP defined. B. DOP for the statement is determined by the dictionary DOP of the accessed objects. C. It is generated to execute in parallel. D. It will execute in parallel only if the estimated execution time is 10 or more seconds. E. DOP for the statement is calculated automatically. F. It may execute serially. Answer: E,F Explanation: E: F (not C): It may execute serially. See note below. Incorrect: A, B: Dictionary DOP not used with PARALLEL (AUTO) hint. D: The default value of parallel_min_time_threshold is 30 (not 10) seconds. Note: * parallel_min_percent PARALLEL_MIN_PERCENT operates in conjunction with PARALLEL_MAX_SERVERS and PARALLEL_MIN_SERVERS. It lets you specify the minimum percentage of parallel execution processes (of the value of PARALLEL_MAX_SERVERS) required for parallel execution. Setting this parameter ensures that parallel operations will not execute sequentially unless adequate resources are available. The default value of 0 means that no minimum percentage of processes has been set. Consider the following settings: PARALLEL_MIN_PERCENT = 50 PARALLEL_MIN_SERVERS = 5 PARALLEL_MAX_SERVERS = 10 If 8 of the 10 parallel execution processes are busy, only 2 processes are available. If you then request a query with a degree of parallelism of 8, the minimum 50% will not be met. 5. You have created some materialized views to improve the performance of several queries. Which four must be true to enable sessions to benefit from improved response time made possible by these materialized views? A. Query rewrite must be enabled for the sessions. B. Bitmap indexes must exist on all the columns involved in the join operations for the defining query of the MVIEWs.
C. All or part of the query results must be obtainable from one or more MVIEWs. D. Bitmap join indexes must exist on all the columns involved in the join operations. E. Session users must have query rewrite privilege. F. The MVIEWs must be enabled for query rewrite. G. All or part of the query results must be obtainable from one MVIEW. Answer: A,B,C,F Explanation: A: For a given user's session, ALTER SESSION can be used to disable or enable query rewrite for that session only. B: Bitmap indexes on the join columns would improve performance. C (not G) : One of the major benefits of creating and maintaining materialized views is the ability to take advantage of query rewrite, which transforms a SQL statement expressed in terms of tables or views into a statement accessing one or more materialized views that are defined on the detail tables. F: * A materialized view is only eligible for query rewrite if the ENABLE QUERY REWRITE clause has been specified, either initially when the materialized view was first created or subsequently with an ALTER MATERIALIZED VIEW statement. * Enabling or disabling query rewrite: by the CREATE or ALTER statement for individual materialized views by the initialization parameter QUERY_REWRITE_ENABLED by the REWRITE and NOREWRITE hints in SQL statements Note: * A materialized view is a replica of a target master from a single point in time. The master can be either a master table at a master site or a master materialized view at a materialized view site. Whereas in multimaster replication tables are continuously updated by other master sites, materialized views are updated from one or more masters through individual batch updates, known as a refreshes, from a single master site or master materialized view site. 6. Which three statements are true about the usage of optimizer hints? A. Whenever a query uses table aliases, the hints in the query must use the aliases. B. The OPTIMIZER_FEATURES_ENABLE parameter must be set to a version supports the hints used. C. The optimizer uses the execution plan with lower cost even if a hint is specified. D. A schema name for the table must be used in the hint if the table us qualified in the FROM clause. E. Hints can be used to override the optimization approach specified with the OPTIMIZER_MODE parameter.
F. A statement block can have only one hint, and that hint must be immediately after SELECT, UPDATE, INSERT, MERGE, or DELETE keyword. Answer: A,B,E Explanation: * You must specify the table to be accessed exactly as it appears in the statement. If the statement uses an alias for the table, then use the alias rather than the table name in the hint. * OPTIMIZER_FEATURES_ENABLE acts as an umbrella parameter for enabling a series of optimizer features based on an Oracle release number. For example, if you upgrade your database from release 10.1 to release 11.1, but you want to keep the release 10.1 optimizer behavior, you can do so by setting this parameter to 10.1.0. At a later time, you can try the enhancements introduced in releases up to and including release 11.1 by setting the parameter to 11.1.0.6. * If a SQL statement has a hint specifying an optimization approach and goal, then the optimizer uses the specified approach regardless of the presence or absence of statistics, the value of the OPTIMIZER_MODE initialization parameter, and the OPTIMIZER_MODE parameter of the ALTER SESSION statement. 7. Examine the Exhibit.
Given two sets of parallel execution processes, SS1 and SS2, which is true? A. Each process SS1 reads some of the rows from the CUSTOMERS table and sends all the rows it reads to each process in SS2. B. Each process in SS1 reads all the rows from the CUSTOMERS table and distributes the rows evenly among the processes in SS2. C. Each process in SS1 reads some of the rows from the SALES table and sends all the rows it reads to each process in SS2. D. Each process in SS1 reads all the rows from the SALES table and distributes the rows evenly among the processes in SS2. E. Each process in SS1 reads some of the rows from the SALES table and distributes the rows evenly among the processes in SS2. F. Each process in the SS1 reads some of the rows from the CUSTOMERS table and distributes the rows evenly among the processes in SS2. Answer: D Explanation: Note: * The execution starts with line 16 (accessing the SALES table), followed by line 15.
* PX BLOCKITERATOR The PX BLOCK ITERATOR row source represents the splitting up of the table EMP2 into pieces so as to divide the scan workload between the parallel scan slaves. The PX SEND and PX RECEIVE row sources represent the pipe that connects the two slave sets as rows flow up from the parallel scan, get repartitioned through the HASHtable queue, and then read by and aggregated on the top slave set. 8. Examine the parallel parameters for your instance. All parallel execution servers are available and the session use default parallelism settings. In which two cases will the degree of parallelism be automatically calculated? A. Statements accessing tables whom dictionary DOP is 2 or more B. Statements accessing tables whose dictionary DOP is DEFAULT C. Statements that are estimated to execute for more than 10 seconds serially D. Statements accessing tables with any setting for Dictionary DOP E. Statements with parallel hints Answer: B,E Explanation: PARALLEL_DEGREE_POLICY specifies whether or not automatic degree of Parallelism, statement queuing, and in-memory parallel execution will be enabled.
SyntaxPARALLEL_DEGREE_POLICY = { MANUAL LIMITED AUTO } LIMITED is used here: Enables automatic degree of parallelism for some statements but statement queuing and in-memory Parallel Execution are disabled. Automatic degree of parallelism is only applied to those statements that access tables or indexes decorated explicitly with the DEFAULT degree of parallelism using the PARALLEL clause. Statements that do not access any tables or indexes decorated with the DEFAULT degree of parallelism will retain the MANUAL behavior. Note: * In earlier versions of the Oracle Database, we had to determine the DOP more or less manually, either with a parallel hint or by setting a parallel degree with alter table. There was an automatic computation of the DOP available for the objects with dictionary DOP of default, derived from the simple formula CPU_COUNT * PARALLEL_THREADS_PER_CPU. 9. Which two tasks are performed during the optimization stage of a SQL statement? A. Evaluating the expressions and conditions in the query B. Checking the syntax and analyzing the semantics of the statement C. Separating the clauses of the SQL statement into structures that can be processed D. Inspecting the integrity constraints and optimizing the query based on this metadata E. Gathering the statistics before creating the execution plan for the statement Answer: D,E Explanation: Note: * Oracle SQL is parsed before execution, and a hard parse includes these steps:. Loading into shared pool - The SQL source code is loaded into RAM for parsing. (the "hard" parse step). Syntax parse - Oracle parses the syntax to check for misspelled SQL keywords.. Semantic parse - Oracle verifies all table & column names from the dictionary and checks to see if you are authorized to see the data.. Query Transformation - If enabled (query_rewrite=true), Oracle will transform complex SQL into simpler, equivalent forms and replace aggregations with materialized views, as appropriate.. Optimization - Oracle then creates an execution plan, based on your schema statistics (or maybe with statistics from dynamic sampling in 10g).. Create executable - Oracle builds an executable file with native file calls to service the SQL query. * The parsing process performs two main functions: o Syntax Check: is the statement a valid one. Does it make sense given the SQL grammar documented in the SQL Reference Manual. Does it follow all of the rules for SQL. o Semantic Analysis: Going beyond the syntax? is the statement valid in light of the objects in the database (do the tables and columns referenced exist). Do you have access to the objects? are the proper privileges in place? Are there ambiguities in the statement? for example if there are two tables T1 and T2 and both have a
column X, the query?select X from T1, T2 where?? is ambiguous, we don?t know which table to get X from. And so on. So, you can think of parsing as basically a two step process, that of a syntax check to check the validity of the statement and that of a semantic check? to ensure the statement can execute properly. Reference: Oracle hard-parse vs. soft parse 10. An application supplied by a new vendor is being deployed and the SQL statements have plan baselines provided by the supplier. The plans have been loaded from a SQL tuning set. You require the optimizer to use these baselines, but allow better plans to used, should any be created. Which two tasks would you perform to achieve this? A. Set the OPTIMIZER_USE_SQL_PLAN_BASELINES initialization parameter to TRUE. B. Set the OPTIMIZER_CAPTURE_SQL_PLAN_BASELINES initialization parameter to TRUE. C. Use the DBMS_SPM.ALTER_SQL_PLAN_BASELINE function to fix the plans. D. Use the DBMS_SPM.EVOLVE_SQL_PLAN_BASELINE function to fix the new plans. E. Use the DBMS_SPM.ALTER_SQL_BASELINE function to accept new plans. Answer: A,B Explanation: OPTIMIZER_USE_SQL_PLAN_BASELINES enables or disables the use of SQL plan baselines stored in SQL Management Base. When enabled, the optimizer looks for a SQL plan baseline for the SQL statement being compiled. If one is found in SQL Management B ase, then the optimizer will cost each of the baseline plans and pick one with the lowest cost.
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