Free Practice Questions for Oracle 1z0-084 Exam

A high buffer cache hit ratio typically indicates that the database is effectively using the buffer cache and does not often need to read data from disk. However, this metric alone is not a reliable indicator of the I/O performance of the database for several reasons:

Full table scans and fast full index scans (A) can bypass the buffer cache by design if the blocks are not deemed reusable shortly, which can impact the cache hit ratio.

A high cache hit ratio (B) can be misleading if the database performance is poor due to other factors, such as inefficient queries or contention issues.

The buffer cache advisory (C) is a more valuable tool for understanding the potential impact of different cache sizes on the database's I/O performance. It simulates scenarios with different cache sizes and provides a more targeted recommendation.

The RECYCLE and KEEP buffer caches (D) are specialized caches designed for certain scenarios. While high hit ratios can be beneficial, they are not universally required; some workloads might not be significantly impacted by lower hit ratios in these caches.

A lower cache hit ratio (E) does not necessarily mean poor I/O performance. In some cases, a system with a well-designed storage subsystem and efficient queries might perform well even with a lower cache hit ratio.

References

Oracle Database 19c Performance Tuning Guide - Buffer Cache Hit Ratio

Oracle Database 19c Performance Tuning Guide - v$db_cache_advice

The CONCATENATION step in an execution plan is introduced during the SQL Transformation phase. This phase is part of the optimizer's query transformations which can include various techniques to rewrite the query for more efficient execution. The CONCATENATION operation is used to combine the results of two separate SQL operations, typically when there is an OR condition in the WHERE clause, as seen in the provided query.

References:

Oracle Database SQL Tuning Guide, 19c

Oracle Database Concepts, 19c

Statspack is a performance diagnostic tool provided by Oracle prior to the introduction of the Automatic Workload Repository (AWR). At snap level 6, Statspack captures the following types of statistics:

A (Correct): Parent and child latches are captured. Latch statistics provide information about contention for latches, which are low-level serialization mechanisms used by Oracle.

E (Correct): Enqueue statistics, which provide information on the waits for locks that manage the concurrency between users.

F (Correct): Segment-level statistics, which provide detailed information on database segments such as tables, indexes, etc., to identify I/O and contention issues.

C (Incorrect): While optimizer execution plans are an essential aspect of performance tuning, detailed execution plan capture is not part of the Statspack report at level 6.

D (Incorrect): Plan usage data refers to how frequently a plan is being used, which is more associated with AWR and not typically captured in Statspack reports.

References:

Oracle Database Performance Tuning Guide: Using Statspack

To configure and enable Database Smart Flash Cache, you must set the DB_FLASH_CACHE_SIZE parameter to reflect the combined size of the flash devices you intend to use for the cache. In this scenario, two flash devices are configured: /dev/sdj with 128G and /dev/sdk with 64G.

Determine the combined size of the flash devices intended for the Database Smart Flash Cache. In this case, it's 128G + 64G = 192G.

However, Oracle documentation suggests setting DB_FLASH_CACHE_SIZE to the exact sizes of the individual devices, separated by a comma when multiple devices are used.

Modify the parameter in the database initialization file (init.ora or spfile.ora) or using an ALTER SYSTEM command. Here's the command for altering the system setting:

ALTER SYSTEM SET DB_FLASH_CACHE_SIZE='128G,64G' SCOPE=SPFILE;

Since this is a static parameter, a database restart is required for the changes to take effect.

Upon database startup, it will allocate the Database Smart Flash Cache using the provided sizes for the specified devices.

It is important to note that MEMORY_TARGET and MEMORY_MAX_TARGET parameters should be configured independently of DB_FLASH_CACHE_SIZE. They control the Oracle memory management for the SGA and PGA, and do not directly correlate with the flash cache configuration.

References

Oracle Database 19c Documentation on Database Smart Flash Cache

Oracle Support Articles and Community Discussions on DB_FLASH_CACHE_SIZE Configuration

The execution plan shows a full table access for the SALES table. To reduce the execution time, creating an index on SALES.TIME_ID would be beneficial as it would allow the database to quickly sort and retrieve the most recent sales without the need to perform a full table scan, which is I/O intensive and slower. By indexing TIME_ID, which is used in the ORDER BY clause, the optimizer can take advantage of the index to efficiently sort and limit the result set to the ten most recent sales.

B (Incorrect): Replacing FETCH FIRST with ROWNUM would not necessarily improve the performance unless there is an appropriate index that the optimizer can use to avoid sorting the entire result set.

C (Incorrect): There is no indication that the current statistics are inaccurate; hence, collecting new statistics may not lead to performance improvement.

D (Incorrect): While adaptive plans can provide performance benefits by allowing the optimizer to adapt the execution strategy, the main issue here is the lack of an index on the ORDER BY column.

E (Incorrect): Creating an index on SALES.CUST_ID could improve join performance but would not address the performance issue caused by the lack of an index on the ORDER BY column.

References:

Oracle Database SQL Tuning Guide: Managing Indexes

Oracle Database SQL Tuning Guide: Using Indexes and Clusters

When sessions concurrently insert data into a table with an LOB column and one session needs to allocate a new extent because it cannot find available space, the wait event associated with this contention is "enq: HW - contention". The HW stands for High Water Mark which is related to space allocation in the database segment. When a session needs to allocate a new extent, it may raise this wait event in other sessions that are also attempting to allocate space in the same LOB segment.

References

Oracle Database 19c Reference Guide - enq: HW - contention