Oracle Scans Test

Full table scans on a database instance can degrade the performance of the database. This test monitors the extent of full table scans happening on the database.

Note:

This test will not report metrics for an Oracle 12c CDB server.

Target of the test : An Oracle server

Agent deploying the test : An internal agent

Outputs of the test : One set of results for every SID monitored.

Configurable parameters for the test
  1. TEST PERIOD - How often should the test be executed
  2. Host – The host for which the test is to be configured
  3. Port - The port on which the server is listening

  4. User – In order to monitor an Oracle database server, a special database user account has to be created in every Oracle database instance that requires monitoring. A Click here hyperlink is available in the test configuration page, using which a new oracle database user can be created. Alternatively, you can manually create the special database user. When doing so, ensure that this user is vested with the select_catalog_role and create session privileges.

    The sample script we recommend for user creation (in Oracle database server versions before 12c) for eG monitoring is:

    create user oraeg identified by oraeg

    create role oratest;

    grant create session to oratest;

    grant select_catalog_role to oratest;

    grant oratest to oraeg;

    The sample script we recommend for user creation (in Oracle database server 12c) for eG monitoring is:

    alter session set container=<Oracle_service_name>;

    create user <user_name>identified by <user_password> container=current default tablespace <name_of_default_tablespace> temporary tablespace <name_of_temporary_tablespace>;

    Grant create session to <user_name>;                                

    Grant select_catalog_role to <user_name>;

    The name of this user has to be specified here.

  5. Password – Password of the specified database user

    This login information is required to query Oracle’s internal dynamic views, so as to fetch the current status / health of the various database components.

  6. Confirm password – Confirm the password by retyping it here.
  7. ISPASSIVE – If the value chosen is yes, then the Oracle server under consideration is a passive server in an Oracle cluster. No alerts will be generated if the server is not running. Measures will be reported as “Not applicable" by the agent if the server is not up.
  8. SSL- By default, this flag is set to No, as the target Oracle database is not SSL-enabled by default. If the target database is SSL-enabled, then set this flag to Yes.
  9. SSL Cipher-This parameter is applicable only if the target Oracle database is SSL-enabled, if not, set this parameter to none. A cipher suite is a set of cryptographic algorithms that are used before a client application and server exchange information over an SSL/TLS connection. It consist of sets of instructions on how to secure a network through SSL (Secure Sockets Layer) or TLS (Transport Layer Security). In this text box, provide a comma-seperated list of cipher suites that are allowed for SSL/TLS connection to the target database. By default, this parameter is set to none.
  10. TRUSTSTORE FILE- This parameter is applicable only if the target Oracle database is SSL-enabled, if not, set this parameter to none. TrustStore is used to store certificates from Certified Authorities (CA) that verify and authenticate the certificate presented by the server in an SSL connection. Therefore, the eG agent should have access to the truststore where the certificates are stored to authenticate and connect with the target database and collect metrics. For this, first import the certificates into the following default location <eG_INSTALL_DIR>/lib/security/mytruststore.jks. To know how to import the certificate into the truststore, refer toPre-requisites for monitoring Oracle Cluster. Then, provide the truststore file name in this text box. For example: mytruststore.jks. By default, none is specified against this text box.
  11. TRUSTSTORE TYPE-This parameter is applicable only if the target Oracle database is SSL-enabled, if not, set this parameter to none.Specify the type of truststore that contains the certificates for server authentication in this text box. For eg.,JKS. By default, this parameter is set to the value none.

  12. TRUSSTORE PASSWORD-This parameter is applicable only if the target Oracle database is SSL-enabled, if not, set this parameter to none. If a Truststore File name is provided, then, in this text box, provide the password that is used to obtain the associated certificate details from the Truststore File. By default, this parameter is set to none.
Measurements made by the test
Measurement Description Measurement Unit Interpretation

Percent long table scans:

The percentage of long table scans happening in the database

Percent

Ideally, this value should be lower than 10%. If more than 20% of scans are happening on long tables, the database/accesses to the database may need to be tuned.

Full table scans may happen due to several reasons. For instance, the indexes of a table may not be used properly in queries. By tuning the queries, the full table scans can be reduced and the database performance significantly improved.

Long table scans:

The number of long table scans that happened on the database instance
during the last measurement period

Number

 

Short table scans:

The number of short table scans that happened on the database instance during the last measurement period

Number

 

Full table scans:

 

The number of full table scans that happened on the database instance during the last measurement period.

 

Number

 

This type of scan reads all rows from a table and filters out those that do not meet the selection criteria.

There are two types of full-table scans, those against small tables STR-FTS and large-tables LT-FTS.

The rule for evaluative and tuning LT-FTS is simple. We evaluate the query and see if index access would result in less physical reads than the existing full-table scan. This usually involves timing the execution speed for the query (with the set timing on command in SQL*Plus) and timing the query with different index access plans:

  • Creating a function-based index - One common technique is to match the WHERE clause of the query with a function-based index.
  • Using index hints - If the CBO does not have enough statistical information about an index, you can force the CBO (temporarily) to use the index by adding an index hint to the query.

Once the fastest execution plan is derived, the tuning professional will enforce the execution plan by creating schema statistics to ensure that the CBO will always use the best index access.

The problem with ST-FTS occurs when a popular table is referenced. Because the FTS data blocks are not touched (pinged to the MRU end of the buffer), ST-FTS rows age quickly from the buffer, requiring Oracle to re-read them, over and over again.

In Oracle9i and beyond hidden parameter called _adaptive_direct_read that ensures that small table scans are cached. However, it is still a good idea to identify these small tables yourself and cache them in your KEEP pool.

The KEEP pool is a wonderful resource for ensuring that an object always resides in the data buffer RAM, and this is one of the few ways to guarantee 10% caching.

Now that we see the benefit of caching frequently-referenced table and indexes, we see how the KEEP pool is most important to small objects that are read into the data buffers via full-table scans.

Also, remember that frequently-referenced data blocks accessed via an index will tend to remain in the data buffer without using the KEEP pool because they are pinged to the MRU end of the buffer every time they are referenced.