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Question 61:
Which view displays information about invalid database objects?
A) DBA_INVALID_OBJECTS
B) ALL_INVALID_OBJECTS
C) DBA_OBJECTS WHERE STATUS=’INVALID’
D) V$INVALID_OBJECTS
Answer: C
Explanation:
The DBA_OBJECTS view with a WHERE STATUS=’INVALID’ clause displays information about invalid database objects. While there is no dedicated view specifically named DBA_INVALID_OBJECTS, the DBA_OBJECTS view contains all database objects with their status, enabling administrators to query for invalid objects by filtering on the STATUS column.
Database objects become invalid when their dependencies change in ways that affect their definitions. Common scenarios include dropping or modifying tables referenced by views, altering specifications of packages or procedures called by other code, recompiling procedures that other procedures depend on, and changing column definitions in tables used by triggers or views.
Invalid objects must be recompiled before they can be used successfully. Oracle attempts automatic recompilation when invalid objects are accessed, but explicit recompilation is often preferable for production systems. The query SELECT owner, object_name, object_type FROM dba_objects WHERE status = ‘INVALID’ identifies all invalid objects in the database.
Recompiling invalid objects can be accomplished in several ways. Individual objects can be recompiled using ALTER commands like ALTER VIEW view_name COMPILE for views or ALTER PROCEDURE proc_name COMPILE for procedures. The UTL_RECOMP package provides procedures for recompiling all invalid objects in a schema or the entire database, handling dependencies automatically.
The DBMS_UTILITY.COMPILE_SCHEMA procedure recompiles all invalid objects in a specified schema. For example, EXEC DBMS_UTILITY.COMPILE_SCHEMA(‘schema_name’) attempts to recompile all invalid objects owned by that schema. The UTL_RECOMP.RECOMP_PARALLEL procedure recompiles invalid objects in parallel, significantly faster for databases with many invalid objects.
Monitoring for invalid objects is important after database maintenance activities. Operations like upgrades, patch applications, or schema changes frequently invalidate dependent objects. Checking for and recompiling invalid objects should be part of post-maintenance verification procedures to ensure applications function correctly.
Some invalid objects may remain invalid after recompilation attempts because their underlying issues have not been resolved. These cases require investigation to identify the root cause, such as missing tables, incompatible data types, or privilege issues that prevent successful compilation.
DBA_INVALID_OBJECTS is not an actual Oracle view name, though it would be descriptive if it existed. Oracle uses the general DBA_OBJECTS view with status filtering.
ALL_INVALID_OBJECTS is similarly not a real view. The ALL_OBJECTS view could be queried with status filtering, but it shows only objects accessible to the current user.
V$INVALID_OBJECTS does not exist as Oracle does not provide a dynamic performance view specifically for invalid objects.
Question 62:
What is the purpose of the NOAUDIT command in Oracle Database?
A) To enable audit trails
B) To disable previously enabled audit options
C) To view audit records
D) To purge audit data
Answer: B
Explanation:
The NOAUDIT command disables previously enabled audit options, removing audit policies that were established through AUDIT commands. This command provides administrators with control over which database activities are monitored and logged, enabling fine-tuning of audit configurations to balance security requirements against performance and storage impacts.
Auditing in Oracle Database captures information about database activities such as user logons, privilege usage, object access, and SQL statement execution. While auditing is essential for security and compliance, excessive auditing can generate large volumes of audit data and impact performance. The NOAUDIT command enables administrators to disable auditing for specific operations that are no longer required to be monitored.
The NOAUDIT syntax mirrors the AUDIT syntax. To disable statement auditing, use NOAUDIT statement_option. To stop auditing specific operations on objects, use NOAUDIT operation ON object_name. To disable privilege auditing, use NOAUDIT privilege. The symmetry between AUDIT and NOAUDIT commands makes it straightforward to reverse auditing configurations.
Examples of NOAUDIT usage include NOAUDIT SELECT ON employees to stop auditing SELECT operations on the employees table, NOAUDIT CREATE TABLE to disable auditing of table creation statements, and NOAUDIT DELETE ANY TABLE to turn off auditing of the DELETE ANY TABLE system privilege usage.
Oracle supports different auditing models. Traditional auditing uses the AUDIT and NOAUDIT commands with audit records stored in the SYS.AUD$ table or operating system files. Unified auditing, introduced in Oracle 12c, provides a consolidated audit trail and uses different syntax through audit policies. The NOAUDIT command works with traditional auditing, while unified auditing uses different policy management commands.
Audit policy management requires careful planning. Before disabling auditing with NOAUDIT, administrators should verify that the auditing is no longer required for compliance or security monitoring. Some regulatory requirements mandate specific auditing configurations that cannot be disabled without violating compliance obligations.
Impact assessment is important when modifying audit configurations. Disabling auditing with NOAUDIT immediately stops capture of those activities. Existing audit records remain in the audit trail, but new occurrences are not logged. This behavior enables administrators to preserve historical audit data while stopping future collection.
Enabling audit trails is accomplished with the AUDIT command, the opposite of NOAUDIT.
Viewing audit records involves querying audit trail views like DBA_AUDIT_TRAIL or using audit analysis tools, not using NOAUDIT.
Purging audit data requires either truncating audit tables or using DBMS_AUDIT_MGMT procedures for managing audit data lifecycle, separate from NOAUDIT functionality.
Question 63:
Which background process performs instance recovery after a database crash?
A) PMON
B) SMON
C) LGWR
D) DBWn
Answer: B
Explanation:
SMON (System Monitor) performs instance recovery after a database crash by applying redo log entries to restore the database to a consistent state. When an instance terminates abnormally due to power failure, system crash, or shutdown abort, transactions that were committed but not fully written to data files must be recovered, and uncommitted transactions must be rolled back.
Instance recovery occurs automatically during the next database startup. When the instance mounts the database and prepares to open it, SMON checks whether the database requires recovery by examining control file information. If the database shutdown was not clean, SMON initiates instance recovery before allowing the database to open for user access.
The recovery process has two phases: rolling forward and rolling back. During the roll-forward phase, SMON applies all committed transactions from redo log files that were not yet written to data files, bringing data files forward to the point of failure. During the rollback phase, SMON uses undo data to reverse changes made by uncommitted transactions, ensuring that only committed work remains in the database.
The checkpoint mechanism determines how much recovery work is required. More frequent checkpoints reduce recovery time by ensuring that more committed changes are written to disk before a crash. The time required for instance recovery is influenced by initialization parameters like FAST_START_MTTR_TARGET, which sets a target time in seconds for instance recovery completion.
SMON also performs other essential background tasks beyond instance recovery. It cleans up temporary segments that are no longer needed, coalesces free space in dictionary-managed tablespaces to reduce fragmentation, recovers failed transactions imported from distributed databases, and maintains various internal structures for optimal database operation.
In Oracle Real Application Clusters (RAC) environments, SMON from surviving instances can perform instance recovery for failed nodes. This capability ensures high availability by allowing other cluster members to recover failed instances and make the database fully operational even when some nodes are down.
Monitoring instance recovery progress can be done through alert log messages and dynamic performance views. The V$INSTANCE_RECOVERY view provides estimates of recovery time and I/O requirements based on current system activity. This information helps administrators tune checkpoint frequency and buffer cache size for optimal recovery characteristics.
PMON (Process Monitor) cleans up after failed user processes by releasing locks and rolling back uncommitted transactions at the session level, but it does not perform instance-level recovery after crashes.
LGWR (Log Writer) writes redo entries to online redo log files during normal operation, creating the information that SMON uses during recovery, but LGWR does not perform the recovery process itself.
DBWn (Database Writer) writes modified buffers from memory to data files during normal operation but does not manage instance recovery operations.
Question 64:
What is the purpose of the MERGE statement in Oracle SQL?
A) To combine two tables into one
B) To conditionally insert or update rows based on whether they exist
C) To merge database instances
D) To consolidate tablespaces
Answer: B
Explanation:
The MERGE statement conditionally inserts or updates rows based on whether they exist, providing an efficient way to synchronize data between tables. This single SQL statement can perform both INSERT and UPDATE operations in one pass through the data, making it particularly useful for ETL processes, data warehouse loads, and maintaining dimension tables.
The MERGE statement evaluates a join condition to determine whether each row from a source exists in the target table. When a match is found, the specified UPDATE operation occurs. When no match exists, the specified INSERT operation executes. This conditional logic eliminates the need for separate existence checks and multiple DML statements.
Basic MERGE syntax follows this structure: MERGE INTO target_table USING source_table ON (join_condition) WHEN MATCHED THEN UPDATE SET columns WHEN NOT MATCHED THEN INSERT VALUES. The ON clause defines how to match rows between source and target tables, while the WHEN clauses specify actions for matched and unmatched rows.
Advanced MERGE capabilities include conditional clauses that further refine when operations execute. The WHERE clause in WHEN MATCHED blocks enables updates only for rows meeting additional criteria. The WHERE clause in WHEN NOT MATCHED blocks restricts which new rows are inserted. DELETE clauses within WHEN MATCHED blocks remove matched rows that meet specified conditions.
Performance benefits of MERGE are significant for large-scale data operations. A single MERGE statement can perform work that would otherwise require multiple SELECT, INSERT, and UPDATE statements. Oracle executes MERGE efficiently by reading source and target tables once, applying all logic during that single pass. This reduces I/O operations, minimizes parse overhead, and simplifies application code.
Common use cases include updating dimension tables in data warehouses where new records are inserted and existing records are updated based on incoming data, synchronizing staging tables with production tables during ETL processes, maintaining slowly changing dimensions that track historical changes, and implementing upsert operations where the application does not know whether records already exist.
Error handling in MERGE statements follows standard Oracle transaction semantics. The entire MERGE operation succeeds or fails as a unit. If any row operation violates a constraint or encounters an error, the entire statement rolls back unless the LOG ERRORS clause is used to capture errors without aborting the statement.
Combining two tables into one permanently involves CREATE TABLE AS SELECT or INSERT INTO statements combined with DROP TABLE, not MERGE functionality.
Merging database instances is not a supported Oracle operation. Instance management involves starting, stopping, and configuring separate instances.
Consolidating tablespaces requires ALTER TABLESPACE and data reorganization operations, unrelated to the MERGE SQL statement.
Question 65:
Which parameter controls the maximum size of the shared pool in the SGA?
A) SHARED_POOL_SIZE
B) SGA_TARGET
C) SHARED_POOL_MAX_SIZE
D) Both A and B
Answer: D
Explanation:
Both SHARED_POOL_SIZE and SGA_TARGET parameters control the maximum size of the shared pool, but they work differently depending on the memory management mode configured for the database instance.
When automatic shared memory management is not enabled, the SHARED_POOL_SIZE parameter directly specifies the size of the shared pool in bytes. Administrators set this parameter to allocate a fixed amount of memory to the shared pool, and Oracle maintains the shared pool at approximately this size throughout instance operation. This manual approach requires administrators to size the shared pool based on workload requirements and available memory.
When automatic shared memory management is enabled by setting SGA_TARGET, Oracle automatically manages the distribution of memory among SGA components including the shared pool. In this mode, SGA_TARGET specifies the total memory available for the entire SGA, and Oracle dynamically allocates memory to different components based on workload demands. The shared pool can grow or shrink within the total SGA_TARGET allocation as needed.
With automatic management, SHARED_POOL_SIZE can still be set to establish a minimum size for the shared pool, preventing it from shrinking below a specified threshold even when automatic tuning suggests otherwise. This capability enables administrators to guarantee minimum resources for critical components while still benefiting from automatic management for the remainder.
The shared pool contains several important subcomponents including the library cache for SQL statements and execution plans, the data dictionary cache for metadata about database objects, the result cache for query and function results, and various other memory structures for PL/SQL code, sessions, and control information.
Proper shared pool sizing is important for performance because undersized shared pools lead to frequent aging out of cached information, causing excessive parsing of SQL statements and repeated reads of data dictionary information. Oversized shared pools waste memory that could be allocated to other components like the buffer cache.
Monitoring shared pool usage through views like VSGASTAT,VSGASTAT, V SGASTAT,VSHARED_POOL_ADVICE, and V$SGAINFO helps administrators assess whether the shared pool is adequately sized. Statistics like library cache hit ratios and reload counts indicate whether SQL statements are being aged out too frequently.
SHARED_POOL_MAX_SIZE is not a valid Oracle initialization parameter. The maximum size is controlled by SGA_TARGET or SGA_MAX_SIZE depending on the memory management configuration.
Question 66:
What is the purpose of the EXPLAIN PLAN command in Oracle?
A) To execute a SQL statement
B) To display the execution plan that the optimizer would use for a SQL statement
C) To compile PL/SQL code
D) To analyze table statistics
Answer: B
Explanation:
The EXPLAIN PLAN command displays the execution plan that the optimizer would use for a SQL statement without actually executing the statement. This capability enables database administrators and developers to understand how Oracle will process queries, identify potential performance problems, and evaluate tuning alternatives before committing to specific approaches.
Execution plans show the series of operations Oracle performs to execute a SQL statement, including the order in which tables are accessed, the access methods for each table (full table scan, index scan, etc.), the join methods used to combine data from multiple tables, and the estimated cost and cardinality of each operation. Understanding these details is essential for performance tuning.
The EXPLAIN PLAN syntax is EXPLAIN PLAN FOR followed by the SQL statement to analyze. For example, EXPLAIN PLAN FOR SELECT * FROM employees WHERE department_id = 10 generates the execution plan for this query without executing it. The plan is stored in the PLAN_TABLE, a special table that holds execution plan information.
After running EXPLAIN PLAN, administrators query PLAN_TABLE to view the results. Oracle provides the DBMS_XPLAN package with functions that format plan output nicely. The query SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY()) shows the most recent execution plan in a readable format with indentation showing operation hierarchy and columns displaying operation names, object names, costs, and cardinalities.
Execution plans reveal critical information for tuning decisions. Full table scans on large tables might indicate missing indexes. Nested loop joins with high cardinalities might suggest hash joins would be more efficient. Cartesian products indicate missing or incorrect join conditions. High-cost operations identify where most resources are consumed.
The optimizer uses current statistics when generating execution plans through EXPLAIN PLAN. If statistics are stale or missing, the execution plan might not reflect what would actually occur during execution. Ensuring statistics are current is important for getting accurate execution plans.
EXPLAIN PLAN shows the plan the optimizer would choose at the moment the command executes, but the actual runtime plan might differ. Bind variable values, system load, and other factors can cause runtime optimization to select different plans. For actual execution plans, administrators can query V$SQL_PLAN for plans used during actual statement execution.
Alternatives to EXPLAIN PLAN include the AUTOTRACE feature in SQL*Plus, which shows execution plans and execution statistics together, and the SQL Monitoring feature in Oracle Database 11g and later, which provides real-time and historical execution plan information with actual runtime statistics.
Executing SQL statements involves running queries directly, not using EXPLAIN PLAN.
Compiling PL/SQL code uses ALTER PROCEDURE or ALTER PACKAGE commands, not EXPLAIN PLAN.
Analyzing table statistics uses DBMS_STATS procedures, unrelated to execution plan generation.
Question 67:
Which type of join returns all rows from both tables, matching rows where possible and including unmatched rows with NULL values?
A) Inner Join
B) Left Outer Join
C) Right Outer Join
D) Full Outer Join
Answer: D
Explanation:
Full Outer Join returns all rows from both tables, matching rows where the join condition is satisfied and including unmatched rows from both tables with NULL values for columns from the opposite table. This join type combines the results of both left outer join and right outer join, ensuring that no rows from either table are excluded from the result set.
The syntax for full outer join in Oracle can be written using either explicit FULL OUTER JOIN syntax or using the older (+) notation, though full outer joins are not supported with the (+) notation. The standard SQL syntax is SELECT columns FROM table1 FULL OUTER JOIN table2 ON join_condition. This returns all rows from table1, all rows from table2, and matches them where the join condition is true.
Full outer joins are useful when analyzing data from multiple sources where you want to see all records regardless of whether matches exist. Common use cases include comparing datasets to identify records that exist in one table but not another, generating reports that must include all entities from multiple related tables even when relationships are incomplete, and reconciling data between systems where some records may be missing in either system.
Understanding the result set structure is important. Rows that match based on the join condition appear with actual values from both tables. Rows from the left table that have no match in the right table appear with NULL values for all columns from the right table. Rows from the right table that have no match in the left table appear with NULL values for all columns from the left table.
Performance considerations exist with full outer joins. They typically require more resources than inner joins because Oracle must process all rows from both tables and identify unmatched rows. For very large tables, full outer joins can consume significant memory and CPU resources. Proper indexing on join columns and adequate statistics help optimize full outer join performance.
Full outer joins differ from other join types in completeness. An inner join returns only matched rows, excluding any rows without matches in either table. A left outer join returns all rows from the left table and matched rows from the right table, excluding unmatched right table rows. A right outer join returns all rows from the right table and matched rows from the left table, excluding unmatched left table rows. Only full outer join returns all rows from both tables.
Alternative approaches can sometimes replace full outer joins with better performance. UNION operations combining left and right outer joins can produce the same result and might execute faster in some scenarios. Application logic can also handle missing data by running separate queries and combining results programmatically.
Inner Join returns only rows where the join condition is satisfied, excluding unmatched rows entirely.
Left Outer Join returns all rows from the left table but only matched rows from the right table.
Right Outer Join returns all rows from the right table but only matched rows from the left table.
Question 68:
What is the purpose of the USER_TABLES view in Oracle Database?
A) To display all tables in the database
B) To show tables owned by the current user
C) To list table statistics for performance tuning
D) To display temporary tables
Answer: B
Explanation:
The USER_TABLES view shows tables owned by the current user, providing a personalized view of tables that belong to the currently connected schema. This view is part of the USER family of data dictionary views that show objects accessible to and owned by the current user without requiring elevated privileges.
USER_TABLES contains detailed information about each table including table name, tablespace name, number of rows (from statistics), number of blocks used, average row length, compression status, partitioning information, and various other attributes related to physical storage and logical structure. This information supports table management, capacity planning, and performance analysis.
The view is useful for developers and application administrators who need to manage their own schema objects without requiring broader database visibility. Common queries include listing all tables owned by the current user with SELECT table_name FROM user_tables, finding tables in specific tablespaces, identifying tables that need statistics gathering, and locating tables with specific characteristics like compression or partitioning.
Oracle provides three parallel families of data dictionary views for different visibility levels. USER views show objects owned by the current user. ALL views show objects the current user has privileges to access, including objects owned by other users. DBA views show all objects in the database regardless of ownership or access privileges, requiring elevated permissions to query.
For tables specifically, USER_TABLES shows tables owned by the current schema, ALL_TABLES includes tables the user can access through privileges or public synonyms, and DBA_TABLES displays every table in the database. Understanding which view to use depends on the required scope of information and available privileges.
The relationship between these views and actual system catalog tables is important. Dictionary views are built on top of base system tables and provide formatted, user-friendly interfaces to metadata. The views perform necessary joins and transformations to present data in accessible formats, abstracting the complexity of underlying catalog structures.
Information in USER_TABLES is particularly valuable for automated scripts and applications that need to discover schema structure dynamically. Applications can query USER_TABLES to generate DDL, validate object existence, or build dynamic SQL based on available tables.
Displaying all tables in the database requires DBA_TABLES or ALL_TABLES, not USER_TABLES which shows only tables owned by the current user.
Listing table statistics for performance tuning can involve USER_TABLES, but dedicated statistics views like USER_TAB_STATISTICS provide more comprehensive statistics information.
Displaying temporary tables involves querying USER_TABLES with filtering on the TEMPORARY column, but USER_TABLES is not exclusively for temporary tables.
Question 69:
Which command is used to remove all rows from a table and reset its storage?
A) DELETE FROM table_name;
B) DROP TABLE table_name;
C) TRUNCATE TABLE table_name;
D) CLEAR TABLE table_name;
Answer: C
Explanation:
The TRUNCATE TABLE command removes all rows from a table and resets its storage to minimal levels, providing the fastest method to empty a table. Unlike DELETE which removes rows individually as DML operations, TRUNCATE is a DDL operation that deallocates storage and resets the table’s high water mark.
TRUNCATE operates fundamentally differently from DELETE. It releases all storage except the initial extent or minimal extents specified in the table’s storage parameters, making the space available for other database objects. The operation executes very quickly regardless of table size because it does not generate undo data for individual rows, instead marking entire extents as free. However, TRUNCATE cannot be rolled back because it is a DDL command that auto-commits.
The syntax TRUNCATE TABLE table_name immediately removes all rows and resets storage. Optional clauses modify behavior: DROP STORAGE releases all storage back to the tablespace (default behavior), REUSE STORAGE keeps allocated extents for the table to use again, and CASCADE automatically truncates child tables in referential integrity relationships.
TRUNCATE has important restrictions compared to DELETE. It cannot be used with a WHERE clause to selectively remove rows; it always removes all rows. It cannot be rolled back as part of a transaction. It cannot be used on tables that are parents in enabled foreign key relationships unless the CASCADE option is specified. Triggers do not fire during TRUNCATE operations because it is not a DML operation.
Use cases for TRUNCATE include clearing staging tables in ETL processes where all data is replaced regularly, resetting tables during application testing and development, removing data from temporary work tables after batch processing, and purging historical data from tables when all rows are obsolete. In all these scenarios, the inability to roll back is acceptable and the performance benefits are valuable.
Performance advantages of TRUNCATE are substantial. For a table with millions of rows, DELETE might take hours while TRUNCATE completes in seconds. The difference stems from DELETE generating undo for every row, potentially generating redo logs, and firing triggers for each row if triggers exist. TRUNCATE bypasses all these mechanisms by operating at the storage level.
Replication and recovery considerations exist with TRUNCATE. Because it is DDL, TRUNCATE may be handled differently by replication solutions compared to DML operations. In standby databases, TRUNCATE applies immediately when redo arrives. Backup strategies must account for TRUNCATE being unrecoverable to specific points in time between backups.
DELETE FROM table_name removes all rows but operates as DML, generating undo, allowing rollback, being much slower, and not resetting storage.
DROP TABLE removes the entire table structure and data, not just rows, making it inappropriate when the table structure should be retained.
CLEAR TABLE is not valid Oracle syntax and would result in an error.
Question 70:
What is the purpose of the DBMS_SCHEDULER package in Oracle?
A) To manage database backups
B) To schedule and manage database jobs and tasks
C) To optimize SQL queries
D) To monitor database performance
Answer: B
Explanation:
The DBMS_SCHEDULER package schedules and manages database jobs and tasks, providing comprehensive job scheduling capabilities that replace the older DBMS_JOB package with enhanced functionality, flexibility, and manageability. DBMS_SCHEDULER enables administrators to automate routine database maintenance, run periodic reports, execute batch processes, and orchestrate complex workflows.
DBMS_SCHEDULER introduces several key concepts. Jobs define what should be executed, which can be PL/SQL blocks, stored procedures, external executables, or scripts. Schedules define when jobs should run, specifying frequencies, intervals, dates, and times. Programs encapsulate the actual work to be performed, separating what to execute from when to execute it. Chains enable complex workflows with dependencies between multiple steps.
Creating jobs with DBMS_SCHEDULER is flexible and powerful. The basic syntax uses DBMS_SCHEDULER.CREATE_JOB to define a job with parameters specifying the job name, job type, job action (what to execute), schedule or timing information, and whether the job should be enabled immediately. For example, creating a job to gather statistics nightly involves specifying a PL/SQL procedure and a daily schedule.
Schedules can be simple or complex. Simple schedules use the repeat_interval parameter with calendar syntax like FREQ=DAILY; BYHOUR=2; BYMINUTE=0 for daily execution at 2 AM. Complex schedules can specify multiple execution times, exclude specific dates, or use sophisticated calendar expressions supporting business day calculations, holidays, and recurring patterns.
Job chains enable workflow orchestration where multiple steps execute with conditional logic and dependencies. Each chain step is a separate job, and chain rules define the flow between steps based on outcomes. This capability supports ETL processes, where data extraction, transformation, and loading steps must execute in sequence with error handling.
DBMS_SCHEDULER provides comprehensive management capabilities. Jobs can be enabled, disabled, run immediately, stopped, modified, and monitored through package procedures. Views like DBA_SCHEDULER_JOBS, DBA_SCHEDULER_JOB_RUN_DETAILS, and DBA_SCHEDULER_RUNNING_JOBS provide information about job configuration, execution history, and current status.
Resource management integration allows jobs to consume resources according to predefined resource groups. Job classes associate jobs with resource consumer groups, enabling prioritization of critical jobs and limiting resource consumption of lower-priority work.
Advantages over DBMS_JOB include better logging and error handling, support for external executables, comprehensive scheduling with calendar expressions, job chains for complex workflows, integration with resource manager, and enhanced monitoring through data dictionary views.
Managing database backups primarily uses RMAN, though DBMS_SCHEDULER can schedule backup jobs.
Optimizing SQL queries involves SQL Tuning Advisor and related tools, not DBMS_SCHEDULER.
Monitoring database performance uses AWR, ADDM, and performance views, separate from job scheduling functionality.
Question 71:
Which SQL function returns the current database user name?
A) CURRENT_USER
B) USER
C) SESSION_USER
D) Both A and B
Answer: D
Explanation:
Both CURRENT_USER and USER functions return the current database user name, though they can behave differently in specific contexts involving definer rights and invoker rights procedures. Understanding when to use each function requires knowing Oracle’s security model for stored procedures and how privileges are applied.
The USER function returns the name of the currently logged-in database user. In most contexts, this is the schema name under which the session is operating. For example, SELECT USER FROM DUAL executed by a user logged in as SCOTT returns SCOTT. This function has existed since early Oracle versions and is widely used in applications for auditing, logging, and security checks.
The CURRENT_USER function also returns the current user name but behaves differently within stored procedures. In procedures defined with AUTHID CURRENT_USER (invoker rights), CURRENT_USER returns the name of the user executing the procedure. In procedures defined with AUTHID DEFINER (definer rights, the default), CURRENT_USER returns the name of the procedure owner while it executes, not the caller’s name.
This distinction is important for security-sensitive operations. Within a definer rights procedure owned by ADMIN_USER and executed by REGULAR_USER, the USER function returns REGULAR_USER while CURRENT_USER returns ADMIN_USER. This difference reflects that the procedure executes with ADMIN_USER’s privileges, not REGULAR_USER’s privileges.
For most application code and queries outside stored procedures, USER and CURRENT_USER return identical values. The difference manifests primarily within stored procedures where privilege context matters. When writing stored procedures that need to know who called them, USER is typically appropriate. When procedures need to know under whose authority they are operating for privilege checking, CURRENT_USER is correct.
Additional user-related functions complement USER and CURRENT_USER. The SESSION_USER function returns the session user name, which remains constant throughout the session and equals the user who established the connection. The SYS_CONTEXT(‘USERENV’, ‘CURRENT_USER’) approach provides similar information through the userenv context.
Question 72:
What is the purpose of a foreign key constraint in Oracle Database?
A) To ensure uniqueness of values in a column
B) To establish and enforce referential integrity between tables
C) To create indexes automatically
D) To optimize query performance
Answer: B
Explanation:
A foreign key constraint establishes and enforces referential integrity between tables by ensuring that values in the foreign key column(s) match values in the referenced primary key or unique key column(s) of another table. This constraint maintains data consistency across related tables and prevents orphaned records that reference non-existent parent records.
Foreign keys define parent-child relationships between tables. The child table contains the foreign key column(s) that reference the parent table’s primary key or unique key. Oracle enforces that every non-NULL value in the foreign key must exist in the parent table’s referenced key, preventing insertion of child records without corresponding parents and deletion of parent records that have dependent children.
Creating foreign key constraints uses the REFERENCES clause or the FOREIGN KEY clause in CREATE TABLE or ALTER TABLE statements. For example, ALTER TABLE orders ADD CONSTRAINT fk_customer FOREIGN KEY (customer_id) REFERENCES customers(customer_id) creates a foreign key on the customer_id column in orders that references the customer_id column in customers.
Referential integrity enforcement occurs during DML operations. When inserting or updating rows in the child table, Oracle verifies that foreign key values exist in the parent table. When deleting or updating referenced key values in the parent table, Oracle checks whether dependent child records exist and either prevents the operation or cascades changes based on the constraint definition.
Cascade options modify default foreign key behavior. ON DELETE CASCADE automatically deletes child records when the parent record is deleted, maintaining integrity by removing orphaned records. ON DELETE SET NULL sets foreign key columns to NULL when the parent is deleted, preserving child records but breaking the relationship. Without cascade options, Oracle prevents deletion of parent records with existing children.
Foreign keys do not automatically create indexes on the child table columns, though indexing foreign keys is strongly recommended. Without indexes, operations on parent tables can cause full table locks on child tables, creating contention and performance problems. Administrators should manually create indexes on foreign key columns to improve performance and reduce locking.
Performance implications of foreign keys include validation overhead during DML operations and potential locking issues. Every insert or update to child tables requires checking parent table existence. Every delete or update to parent tables requires checking for dependent children. These validations consume resources, though proper indexing minimizes the impact.
Question 73:
Which parameter specifies the maximum number of redo log files that can be created for each online redo log group?
A) MAX_LOG_MEMBERS
B) MAXLOGFILES
C) LOG_FILE_MEMBERS
D) MAXMEMBERS
Answer: A
Explanation:
The MAXLOGMEMBERS parameter specifies the maximum number of redo log files (members) that can be created for each online redo log group. This parameter is set during database creation in the CREATE DATABASE statement and controls how many identical copies of each redo log Oracle can maintain for redundancy purposes.
Redo log groups contain one or more members, which are identical copies of the redo log. Multiple members provide protection against media failure by storing copies on different physical disks or storage devices. If one member becomes unavailable or corrupted, Oracle continues operating using the remaining members without interruption.
MAXLOGMEMBERS sets the upper limit on multiplexing within each group but does not determine how many members actually exist. During database creation or when adding log files using ALTER DATABASE ADD LOGFILE MEMBER, administrators can create up to MAXLOGMEMBERS copies within each group. Typical values range from 2 to 5, with most production databases using 2 or 3 members for redundancy.
The parameter works in conjunction with MAXLOGFILES, which limits the total number of redo log groups that can exist. Together, these parameters establish boundaries for redo log configuration, though they can be changed only by recreating the control file, making initial sizing important.
Best practices recommend multiplexing redo logs with at least two members per group on separate physical devices. This configuration protects against single disk failures and ensures database availability even when one redo log copy is unavailable. Critical databases often use three members with strategic placement across different storage systems or locations.
Performance considerations exist with multiple log members. Oracle writes to all members simultaneously, so write performance depends on the slowest member. Placing members on fast storage and distributing I/O across multiple controllers helps maintain performance while providing redundancy. Asymmetric configurations where members reside on storage with different performance characteristics should be avoided.
Question 74:
What is the purpose of the GRANT command in Oracle Database?
A) To create new database users
B) To assign privileges or roles to users or roles
C) To modify table structures
D) To backup the database
Answer: B
Explanation:
The GRANT command assigns privileges or roles to users or roles, forming the foundation of Oracle’s security model by controlling what actions users can perform on database objects and what system-level operations they can execute. GRANT enables administrators to implement least-privilege principles, role-based access control, and fine-grained authorization.
Oracle distinguishes between system privileges and object privileges. System privileges control database-level operations like CREATE TABLE, CREATE USER, or ALTER SYSTEM. Object privileges control operations on specific schema objects like SELECT, INSERT, UPDATE, or DELETE on particular tables. The GRANT syntax varies slightly between these privilege types.
Granting system privileges uses the syntax GRANT privilege TO user_or_role. For example, GRANT CREATE TABLE TO user1 allows user1 to create tables in their own schema. GRANT CREATE ANY TABLE TO user1 allows creating tables in any schema. The ANY keyword extends privileges beyond the user’s own schema, requiring careful consideration before granting.
Granting object privileges requires specifying both the privilege and the object. The syntax GRANT privilege ON object TO user_or_role assigns specific operations on named objects. For example, GRANT SELECT, INSERT ON employees TO user1 allows user1 to query and insert rows in the employees table but not update or delete. Column-level privileges can restrict access further.
The WITH GRANT OPTION clause enables privilege propagation. When granted system or object privileges with this option, recipients can grant the same privileges to other users. This cascading grant capability enables delegated administration but requires careful management to prevent unintended privilege proliferation.
The WITH ADMIN OPTION for system privileges serves a similar purpose but behaves differently. Users granted system privileges with ADMIN OPTION can grant those privileges to others and can also revoke them, even if they were not the original grantor. This administrative flexibility differs from object privileges where only the grantor can revoke.
Roles simplify privilege management by grouping related privileges. Granting a role to users with GRANT role_name TO user assigns all privileges contained in that role. Role-based access control reduces administrative overhead and improves consistency by managing privileges through roles rather than individual grants to users.
Question 75:
Which view displays information about database locks?
A) DBA_LOCKS
B) V$LOCK
C) V$LOCKED_OBJECT
D) Both B and C
Answer: D
Explanation:
Both VLOCKandVLOCK and V LOCKandVLOCKED_OBJECT display information about database locks, but from different perspectives and with different levels of detail. Understanding both views is important for diagnosing locking issues, identifying blocking sessions, and resolving contention problems that impact application performance.
The V$LOCK view provides comprehensive information about all locks currently held or requested in the database. It includes columns for session identifier, lock type, lock mode, block status indicating whether the lock is blocking others, and request mode showing the level of lock being requested. This view shows all types of locks including table locks, row locks, DML locks, DDL locks, and system locks.
V$LOCKED_OBJECT focuses specifically on objects that are currently locked, providing a more object-centric view. It shows which database objects are locked, which sessions hold those locks, and what modes the locks are in. This view joins lock information with object information, making it easier to identify which tables or other objects are involved in locking conflicts.
Lock types in V$LOCK use two-letter codes. TM represents table locks (DML locks on tables). TX represents transaction locks (row-level locks). UL represents user-defined locks. Each type serves different purposes in maintaining data consistency and coordinating concurrent access.
Lock modes indicate the level of restriction. Mode 2 (Row Share) allows concurrent access but prevents exclusive locking. Mode 3 (Row Exclusive) allows concurrent access but prevents share locking. Mode 4 (Share) allows concurrent queries but prevents updates. Mode 5 (Share Row Exclusive) allows row-level exclusive access. Mode 6 (Exclusive) prevents all concurrent access.
Question 76:
What is the purpose of the NVL function in Oracle SQL?
A) To concatenate strings
B) To replace NULL values with a specified alternative value
C) To convert data types
D) To calculate averages
Answer: B
Explanation:
The NVL function replaces NULL values with a specified alternative value, enabling queries to handle missing or undefined data gracefully by substituting meaningful defaults. This function is essential for ensuring that calculations, concatenations, and comparisons work correctly even when columns contain NULL values.
The syntax is NVL(expression, replacement_value). If expression evaluates to NULL, the function returns replacement_value. If expression is not NULL, it returns the original expression value unchanged. Both arguments must be compatible data types, or Oracle performs implicit conversion when possible.
Common use cases include replacing NULL numeric values with zeros for calculations like NVL(commission_pct, 0) * salary to ensure commission calculations work even for employees without commission percentages, substituting NULL dates with default dates in reports, replacing NULL strings with placeholder text like NVL(middle_name, ‘N/A’) for display purposes, and ensuring concatenations work correctly since NULL concatenated with anything yields NULL.
Understanding NULL behavior in Oracle is crucial for using NVL effectively. NULL represents unknown or missing information, not zero or empty string. Comparisons with NULL using standard operators like = or <> always return NULL, never TRUE or FALSE. This behavior necessitates using IS NULL or IS NOT NULL predicates for NULL checks and NVL for providing alternatives.
NVL handles different data types appropriately. For numeric columns, NVL(numeric_column, 0) replaces NULL with zero. For character columns, NVL(char_column, ‘default’) substitutes default text. For date columns, NVL(date_column, SYSDATE) can provide the current date as a fallback. The replacement value must match the expression’s data type or be convertible to it.
Performance considerations exist with NVL in large queries. While individual NVL calls are efficient, using NVL in WHERE clauses can prevent index usage if applied to indexed columns. For example, WHERE NVL(column, 0) = 0 cannot use an index on column, while WHERE column IS NULL OR column = 0 might use the index more effectively.
Related functions extend NVL capabilities. NVL2(expression, value_if_not_null, value_if_null) provides different return values based on whether the expression is NULL or not NULL. COALESCE(expr1, expr2, expr3, …) returns the first non-NULL expression from a list, providing more flexibility than NVL when multiple fallback values exist.
Best practices recommend using NVL judiciously. Not all NULL values should be replaced; sometimes NULL conveys meaningful information that “no value exists.” Overusing NVL can mask data quality issues that should be addressed at the source rather than in queries.
Concatenating strings uses the CONCAT function or concatenation operator ||, not NVL, though NVL is often used within concatenations to handle NULLs.
Converting data types uses functions like TO_CHAR, TO_DATE, and TO_NUMBER, not NVL.
Calculating averages uses the AVG aggregate function, though NVL might be used to exclude or include NULL values in average calculations.
Question 77:
Which command creates a restore point in Oracle Database?
A) CREATE RESTORE POINT restore_point_name
B) SET RESTORE POINT restore_point_name
C) DEFINE RESTORE POINT restore_point_name
D) MARK RESTORE POINT restore_point_name
Answer: A
Explanation:
The CREATE RESTORE POINT command creates a restore point in Oracle Database, establishing a named point in time to which the database can be restored or flashed back. Restore points provide administrators with convenient recovery targets for database flashback operations or point-in-time recovery scenarios.
Restore points capture the current system change number (SCN) and associate it with a user-defined name. This named SCN serves as a recovery target, eliminating the need to determine specific timestamps or SCN values when restoring the database. Restore points simplify recovery operations and reduce the likelihood of errors in specifying recovery targets.
Oracle supports two types of restore points: normal and guaranteed. Normal restore points are lightweight markers that exist only as long as the flashback database logs retain information about that point in time. They are automatically aged out when flashback logs are recycled. Guaranteed restore points ensure that all necessary flashback logs are retained regardless of retention policies, guaranteeing the ability to flashback to that point.
Creating a normal restore point uses the syntax CREATE RESTORE POINT restore_point_name. For example, CREATE RESTORE POINT before_upgrade creates a restore point named before_upgrade at the current SCN. This restore point can be used subsequently for flashback database operations or as a recovery target.
Guaranteed restore points require additional syntax: CREATE RESTORE POINT restore_point_name GUARANTEE FLASHBACK DATABASE. Guaranteed restore points prevent flashback log recycling, ensuring recovery capability at the cost of increased storage consumption. They are useful before major changes like application upgrades where fallback capability must be absolutely certain.
Use cases for restore points include marking points before major application deployments for rollback capability, creating recovery targets before risky operations like data purges or schema changes, establishing test environment reset points for repeatable testing scenarios, and providing named recovery targets for disaster recovery procedures.
Managing restore points involves monitoring storage consumption for guaranteed restore points and dropping restore points when no longer needed using DROP RESTORE POINT restore_point_name. The DBA_RESTORE_POINTS view shows existing restore points with their SCNs, creation times, types, and for guaranteed restore points, the storage consumed.
Flashback database operations use restore points as targets: FLASHBACK DATABASE TO RESTORE POINT restore_point_name returns the database to the state it was in when the restore point was created. This operation is much faster than traditional point-in-time recovery because it uses flashback logs rather than applying redo.
Question 78:
What is the purpose of index-organized tables (IOTs) in Oracle Database?
A) To create faster indexes
B) To store table data in index structure for improved access performance
C) To automatically partition tables
D) To compress table data
Answer: B
Explanation:
Index-organized tables (IOTs) store table data in index structure for improved access performance by maintaining table rows in primary key order within a B-tree index structure. Unlike heap-organized tables where data is stored separately from indexes, IOTs store all row data directly in the primary key index, eliminating the need for separate table storage and index storage.
IOTs are particularly beneficial for tables where most access is through the primary key. Since the entire row is stored in the index structure ordered by primary key, queries that access data by primary key require only a single I/O operation to retrieve the complete row. In contrast, heap-organized tables require an index lookup followed by a separate table access, doubling the I/O.
The structure of IOTs differs fundamentally from traditional tables. The primary key is mandatory for IOTs because it determines the physical organization of data. Rows are stored in primary key order, making range scans on the primary key extremely efficient. All non-key columns are stored with their corresponding key in the index leaf blocks, making the entire row available without additional access.
Creating IOTs uses modified CREATE TABLE syntax including the ORGANIZATION INDEX clause. For example, CREATE TABLE orders (order_id NUMBER PRIMARY KEY, order_date DATE, amount NUMBER) ORGANIZATION INDEX creates an IOT where rows are stored in order_id order within an index structure.
Overflow segments handle rows that are too large to fit efficiently in index blocks. When row sizes exceed a threshold, non-key columns can be stored in a separate overflow segment while key columns and a pointer remain in the index structure. The OVERFLOW clause in the CREATE TABLE statement specifies this behavior, balancing performance against storage efficiency.
Use cases for IOTs include code lookup tables where primary key access dominates, queue tables where rows are accessed in order, audit tables with time-based primary keys where recent entries are queried most frequently, and any table where primary key access patterns predominate and range scans are common.
Performance characteristics of IOTs show advantages for primary key access and range scans but potential disadvantages for full table scans and secondary index maintenance. Full table scans of IOTs traverse the primary key index, which can be less efficient than scanning heap storage. Secondary indexes on IOTs store logical ROWIDs rather than physical ROWIDs, adding indirection that slightly increases access cost.
Space efficiency is generally better with IOTs because there is no duplication between table and index storage for primary key columns. However, secondary indexes on IOTs consume more space due to storing logical ROWIDs. Overall space savings depend on the ratio of primary key columns to total columns.
Question 79:
Which parameter controls the number of archived redo log files that must be retained for recovery?
A) LOG_ARCHIVE_RETENTION
B) UNDO_RETENTION
C) There is no single parameter; retention is managed through RMAN retention policies
D) ARCHIVE_LOG_RETENTION
Answer: C
Explanation:
There is no single parameter that controls archived redo log file retention; instead, retention is managed through RMAN retention policies and backup strategies. Oracle provides flexible mechanisms for determining how long archived logs should be retained based on recovery requirements, backup status, and storage availability rather than a simple parameter-based approach.
RMAN retention policies determine when backups and archived logs become obsolete and eligible for deletion. The CONFIGURE RETENTION POLICY command establishes rules for how long backups must be kept. Common retention policies include REDUNDANCY n, which keeps n backup copies of each file, and RECOVERY WINDOW OF n DAYS, which ensures recoverability to any point within the past n days.
Archived redo logs are retained as long as they are needed to meet the retention policy. For a recovery window policy, archived logs spanning the window must be retained. For a redundancy policy, archived logs needed to recover the required number of backup copies must be kept. RMAN automatically determines which archived logs are obsolete based on these policies and existing backups.
The DELETE OBSOLETE command in RMAN removes backup files and archived logs that are no longer needed according to the retention policy. This command evaluates which files are required for recovery given current backups and retention settings, deleting only files that are no longer necessary. Automatic deletion can also be configured to occur during backup operations.
Additional factors influence archived log retention beyond basic retention policies. In Data Guard environments, archived logs must be retained until applied to all standby databases. The LOG_ARCHIVE_DEST_n parameters can include mandatory and alternate settings that affect log availability requirements. Disk space limitations may necessitate more aggressive log deletion to prevent file system full conditions.
The DB_RECOVERY_FILE_DEST and DB_RECOVERY_FILE_DEST_SIZE parameters define the Fast Recovery Area (FRA) where Oracle can automatically manage archived logs, backups, and other recovery files. When the FRA is configured, Oracle can automatically delete obsolete archived logs when space is needed, respecting retention policies while preventing space exhaustion.
Best practices recommend aligning archived log retention with backup frequency and recovery requirements. If daily backups occur and one-week point-in-time recovery is required, archived logs for at least the past week plus logs needed to recover the most recent backup must be retained. Documenting retention policies and testing recovery procedures ensures that log retention is adequate.
Question 80:
What is the purpose of the ROLLBACK command in Oracle Database?
A) To save changes permanently to the database
B) To undo changes made in the current transaction
C) To switch to a previous database version
D) To restore from backup
Answer: B
Explanation:
The ROLLBACK command undoes changes made in the current transaction, reversing all modifications since the last COMMIT or since the transaction began. This command is essential for maintaining data consistency by allowing applications and users to abandon uncommitted work when errors occur or when changes should not be made permanent.
ROLLBACK is one of Oracle’s transaction control commands, working alongside COMMIT and SAVEPOINT to manage transaction boundaries and data consistency. When a transaction executes DML statements like INSERT, UPDATE, or DELETE, those changes remain tentative until committed. ROLLBACK cancels all these uncommitted changes, restoring the database to its state at the last commit point.
The mechanism behind ROLLBACK relies on undo data stored in the undo tablespace. Before modifying data, Oracle saves before-images of changed rows in undo segments. When ROLLBACK executes, Oracle applies these before-images to restore data to its prior state. This same undo data also provides read consistency for queries that need to see data as it existed before uncommitted changes.
ROLLBACK can be complete or partial. A complete rollback using the simple ROLLBACK command undoes all changes in the current transaction. A partial rollback using ROLLBACK TO SAVEPOINT savepoint_name undoes only changes made after the specified savepoint, preserving earlier transaction work.
Savepoints enable fine-grained transaction control. The SAVEPOINT savepoint_name command creates a named point within a transaction to which later rollback can occur. This capability is valuable in complex transactions where part of the work should be retained while other parts are abandoned, or in error handling scenarios where only failed portions of larger transactions need reversal.
Automatic rollback occurs in several situations. When a session terminates abnormally due to process failure or network disruption, Oracle automatically rolls back any uncommitted transaction for that session. When deadlocks occur, Oracle automatically rolls back one of the participating transactions to break the deadlock. Application code can also implement exception handling that rolls back transactions when errors occur.
Transaction behavior and isolation levels affect when ROLLBACK is necessary. In Oracle’s default READ COMMITTED isolation level, each statement sees only committed data, and rolled-back changes are never visible to other sessions. However, the session performing the rollback sees its own uncommitted changes until ROLLBACK executes, after which it too sees the original committed data.
Performance considerations with ROLLBACK include the cost of applying undo to reverse changes. Large transactions that modify many rows require correspondingly extensive work to roll back. For this reason, breaking large operations into smaller committed transactions or using partial rollbacks to savepoints can reduce the cost of recovery from errors while maintaining appropriate transaction boundaries.
Best practices recommend explicit transaction control in applications. Rather than relying on implicit commits or leaving transactions open indefinitely, applications should commit successful work promptly and roll back when errors occur. Clear transaction boundaries improve concurrency, reduce undo consumption, and make application behavior more predictable.
