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Oracle Database provides proactive help in managing disk space for tablespaces by alerting you when available space is running low. Two alert thresholds are defined by default: warning and critical. The warning threshold is the limit at which space is beginning to run low. The critical threshold is a serious limit that warrants your immediate attention. The database issues alerts at both thresholds.
There are two ways to specify alert thresholds for both locally managed and dictionary managed tablespaces:
By percent full
For both warning and critical thresholds, when space used becomes greater than or equal to a percent of total space, an alert is issued.
By free space remaining (in kilobytes (KB))
For both warning and critical thresholds, when remaining space falls below an amount in KB, an alert is issued. Free-space-remaining thresholds are more useful for very large tablespaces.
Alerts for locally managed tablespaces are server-generated. For dictionary managed tablespaces, Enterprise Manager provides this functionality. See "Server-Generated Alerts" for more information.
New tablespaces are assigned alert thresholds as follows:
Locally managed tablespace—When you create a new locally managed tablespace, it is assigned the default threshold values defined for the database. A newly created database has a default of 85% full for the warning threshold and 97% full for the critical threshold. Defaults for free space remaining thresholds for a new database are both zero (disabled). You can change these database defaults, as described later in this section.
Dictionary managed tablespace—When you create a new dictionary managed tablespace, it is assigned the threshold values that Enterprise Manager lists for "All others" in the metrics categories "Tablespace Free Space (MB) (dictionary managed)" and "Tablespace Space Used (%) (dictionary managed)." You change these values on the Metric and Policy Settings page.
INITRANS specifies the number of update transaction entries for which space is initially reserved in the data block header. Space is reserved in the headers of all data blocks in the associated segment.
As multiple transactions concurrently access the rows of the same data block, space is allocated for each update transaction entry in the block. Once the space reserved by INITRANS is depleted, space for additional transaction entries is allocated out of the free space in a block, if available. Once allocated, this space effectively becomes a permanent part of the block header.
In earlier releases of Oracle Database, the MAXTRANS parameter limited the number of transaction entries that could concurrently use data in a data block. This parameter has been deprecated. Oracle Database now automatically allows up to 255 concurrent update transactions for any data block, depending on the available space in the block.The database ignores MAXTRANS when specified by users only for new objects created when the COMPATIBLE initialization parameter is set to 10.0 or greater.
You should consider the following when setting the INITRANS parameter for a schema object:
The space you would like to reserve for transaction entries compared to the space you would reserve for database data
The number of concurrent transactions that are likely to touch the same data blocks at any given time
For example, if a table is very large and only a small number of users simultaneously access the table, the chances of multiple concurrent transactions requiring access to the same data block is low. Therefore, INITRANS can be set low, especially if space is at a premium in the database.
Alternatively, assume that a table is usually accessed by many users at the same time. In this case, you might consider preallocating transaction entry space by using a high INITRANS. This eliminates the overhead of having to allocate transaction entry space, as required when the object is in use.
In general, Oracle recommends that you not change the value of INITRANS from its default.
This section describes the storage parameters that you can specify for schema object segments to tell the database how to store the object in the database. Schema objects include tables, indexes, partitions, clusters, materialized views, and materialized view logs
The following topics are contained in this section:
Storage parameters determine space allocation for objects when their segments are created in a tablespace. Not all storage parameters can be specified for every type of database object, and not all storage parameters can be specified in both the CREATE and ALTER statements. Storage parameters for objects in locally managed tablespaces are supported mainly for backward compatibility.
The Oracle Database server manages extents for locally managed tablespaces. If you specified the UNIFORM clause when the tablespace was created, then the database creates all extents of a uniform size that you specified (or a default size) for any objects created in the tablespace. If you specified the AUTOALLOCATE clause, then the database determines the extent sizing policy for the tablespace. So, for example, if you specific the INITIAL clause when you create an object in a locally managed tablespace you are telling the database to preallocate at least that much space. The database then determines the appropriate number of extents needed to allocate that much space.
Table contains a brief description of each storage parameter. For a complete description of these parameters, including their default, minimum, and maximum settings, see the Oracle Database SQL Reference.
In a tablespace that is specified as EXTENT MANAGEMENT LOCAL, the database uses the value of INITIAL with the extent size for the tablespace to determine the initial amount of space to reserve for the object. For example, in a uniform locally managed tablespace with 5M extents, if you specify an INITIAL value of 1M, then the database must allocate one 5M extent. If the extent size of the tablespace is smaller than the value of INITIAL, then the initial amount of space allocated will in fact be more than one extent.
In a tablespace that is specified as EXTENT MANAGEMENT LOCAL, MINEXTENTS is used to compute the initial amount of space that is allocated. The initial amount of space that is allocated is equal to INITIAL * MINEXTENTS.Thereafter it is set to 1 (as seen in the DBA_SEGMENTS view).
Defines a default buffer pool (cache) for a schema object. For information on the use of this parameter, see Oracle Database Performance Tuning Guide.
At object creation, you can specify storage parameters for each individual schema object. These parameter settings override any default storage settings. Use the STORAGE clause of the CREATE or ALTER statement for specifying storage parameters for the individual object.
Use the STORAGE clause of the CREATE TABLE or ALTER TABLE statement to set the storage parameters for non-clustered tables.
In contrast, set the storage parameters for the data segments of a cluster using the STORAGE clause of the CREATE CLUSTER or ALTER CLUSTER statement, rather than the individual CREATE or ALTER statements that put tables into the cluster. Storage parameters specified when creating or altering a clustered table are ignored. The storage parameters set for the cluster override the table storage parameters.
With partitioned tables, you can set default storage parameters at the table level. When creating a new partition of the table, the default storage parameters are inherited from the table level (unless you specify them for the individual partition). If no storage parameters are specified at the table level, then they are inherited from the tablespace.
Storage parameters for an index segment created for a table index can be set using the STORAGE clause of the CREATE INDEX or ALTER INDEX statement.
Storage parameters of an index segment created for the index used to enforce a primary key or unique key constraint can be set in either of the following ways:
In the ENABLE ... USING INDEX clause of the CREATE TABLE or ALTER TABLE statement
In the STORAGE clause of the ALTER INDEX statement
A table or materialized view can contain LOB, varray, or nested table column types. These entities can be stored in their own segments. LOBs and varrays are stored in LOB segments, while a nested table is stored in a storage table. You can specify a STORAGE clause for these segments that will override storage parameters specified at the table level.
Oracle Database Application Developer's Guide - Object-Relational Features for more information about varrays and nested tables
Changing Values of Storage Parameters
You can alter default storage parameters for tablespaces and specific storage parameters for individual objects if you so choose. Default storage parameters can be reset for a tablespace; however, changes affect only new objects created in the tablespace or new extents allocated for a segment. As discussed previously, you cannot specify default storage parameters for locally managed tablespaces, so this discussion does not apply.
The INITIAL and MINEXTENTS storage parameters cannot be altered for an existing table, cluster, index. If only NEXT is altered for a segment, the next incremental extent is the size of the new NEXT, and subsequent extents can grow by PCTINCREASE as usual.
If both NEXT and PCTINCREASE are altered for a segment, the next extent is the new value of NEXT, and from that point forward, NEXT is calculated using PCTINCREASE as usual.
Understanding Precedence in Storage Parameters
Starting with default values, the storage parameters in effect for a database object at a given time are determined by the following, listed in order of precedence (where higher numbers take precedence over lower numbers):
Oracle Database default values
Any storage parameter specified at the object level overrides the corresponding option set at the tablespace level. When storage parameters are not explicitly set at the object level, they default to those at the tablespace level. When storage parameters are not set at the tablespace level, Oracle Database system defaults apply. If storage parameters are altered, the new options apply only to the extents not yet allocated.
The storage parameters for temporary segments always use the default storage parameters set for the associated tablespace.
Oracle Database provides a means for suspending, and later resuming, the execution of large database operations in the event of space allocation failures. This enables you to take corrective action instead of the Oracle Database server returning an error to the user. After the error condition is corrected, the suspended operation automatically resumes. This feature is called resumable space allocation. The statements that are affected are called resumable statements.
The following is an overview of how resumable space allocation works. Details are contained in later sections.
The following operations are resumable:
SELECT statements that run out of temporary space (for sort areas) are candidates for resumable execution. When using OCI, the calls OCIStmtExecute() and OCIStmtFetch() are candidates.
INSERT, UPDATE, and DELETE statements are candidates. The interface used to execute them does not matter; it can be OCI, SQLJ, PL/SQL, or another interface. Also, INSERT INTO...SELECT from external tables can be resumable.
As for SQL*Loader, a command line parameter controls whether statements are resumable after recoverable errors.
The following statements are candidates for resumable execution:
The operation cannot acquire any more extents for a table/index/temporary segment/undo segment/cluster/LOB/table partition/index partition in a tablespace. For example, the following errors fall in this category:
The number of extents in a table/index/temporary segment/undo segment/cluster/LOB/table partition/index partition equals the maximum extents defined on the object. For example, the following errors fall in this category:
The user has exceeded his assigned space quota in the tablespace. Specifically, this is noted by the following error:
In a distributed environment, if a user enables or disables resumable space allocation, or if you, as a DBA, alter the RESUMABLE_TIMEOUT initialization parameter, only the local instance is affected. In a distributed transaction, sessions or remote instances are suspended only if RESUMABLE has been enabled in the remote instance.
In parallel execution, if one of the parallel execution server processes encounters a correctable error, that server process suspends its execution. Other parallel execution server processes will continue executing their respective tasks, until either they encounter an error or are blocked (directly or indirectly) by the suspended server process. When the correctable error is resolved, the suspended process resumes execution and the parallel operation continues execution. If the suspended operation is terminated, the parallel operation aborts, throwing the error to the user.
Different parallel execution server processes may encounter one or more correctable errors. This may result in firing an AFTER SUSPEND trigger multiple times, in parallel. Also, if a parallel execution server process encounters a non-correctable error while another parallel execution server process is suspended, the suspended statement is immediately aborted.
For parallel execution, every parallel execution coordinator and server process has its own entry in the DBA_ or USER_RESUMABLE view.
Resumable space allocation is only possible when statements are executed within a session that has resumable mode enabled. There are two means of enabling and disabling resumable space allocation. You can control it at the system level with the RESUMABLE_TIMEOUT initialization parameter, or users can enable it at the session level using clauses of the ALTER SESSION statement.
You can enable resumable space allocation system wide and specify a timeout interval by setting the RESUMABLE_TIMEOUT initialization parameter. For example, the following setting of the RESUMABLE_TIMEOUT parameter in the initialization parameter file causes all sessions to initially be enabled for resumable space allocation and sets the timeout period to 1 hour:
If this parameter is set to 0, then resumable space allocation is disabled initially for all sessions. This is the default.
You can use the ALTER SYSTEM SET statement to change the value of this parameter at the system level. For example, the following statement will disable resumable space allocation for all sessions:
Within a session, a user can issue the ALTER SESSION SET statement to set the RESUMABLE_TIMEOUT initialization parameter and enable resumable space allocation, change a timeout value, or to disable resumable mode.
A user can enable resumable mode for a session, using the following SQL statement:
To disable resumable mode, a user issues the following statement:
The default for a new session is resumable mode disabled, unless the RESUMABLE_TIMEOUT initialization parameter is set to a nonzero value.
The user can also specify a timeout interval, and can provide a name used to identify a resumable statement. These are discussed separately in following sections.
A timeout period, after which a suspended statement will error if no intervention has taken place, can be specified when resumable mode is enabled. The following statement specifies that resumable transactions will time out and error after 3600 seconds:
The value of TIMEOUT remains in effect until it is changed by another ALTER SESSION ENABLE RESUMABLE statement, it is changed by another means, or the session ends. The default timeout interval when using the ENABLE RESUMABLE TIMEOUT clause to enable resumable mode is 7200 seconds.
Resumable statements can be identified by name. The following statement assigns a name to resumable statements:
The NAME value remains in effect until it is changed by another ALTER SESSION ENABLE RESUMABLE statement, or the session ends. The default value for NAME is 'User username(userid), Session sessionid, Instance instanceid'.
The name of the statement is used to identify the resumable statement in the DBA_RESUMABLE and USER_RESUMABLE views.
Using a LOGON Trigger to Set Default Resumable Mode
Another method of setting default resumable mode, other than setting the RESUMABLE_TIMEOUT initialization parameter, is that you can register a database level LOGON trigger to alter a user's session to enable resumable and set a timeout interval.
When a resumable statement is suspended, the error is not raised to the client. In order for corrective action to be taken, Oracle Database provides alternative methods for notifying users of the error and for providing information about the circumstances.
Notifying Users: The AFTER SUSPEND System Event and Trigger
When a resumable statement encounter a correctable error, the system internally generates the AFTER SUSPEND system event. Users can register triggers for this event at both the database and schema level. If a user registers a trigger to handle this system event, the trigger is executed after a SQL statement has been suspended.
SQL statements executed within a AFTER SUSPEND trigger are always non-resumable and are always autonomous. Transactions started within the trigger use the SYSTEM rollback segment. These conditions are imposed to overcome deadlocks and reduce the chance of the trigger experiencing the same error condition as the statement.
Users can use the USER_RESUMABLE or DBA_RESUMABLE views, or the DBMS_RESUMABLE.SPACE_ERROR_INFO function, within triggers to get information about the resumable statements.
Triggers can also call the DBMS_RESUMABLE package to terminate suspended statements and modify resumable timeout values. In the following example, the default system timeout is changed by creating a system wide AFTER SUSPEND trigger that calls DBMS_RESUMABLE to set the timeout to 3 hours:
CREATE OR REPLACE TRIGGER resumable_default_timeout AFTER SUSPEND ON DATABASE BEGIN DBMS_RESUMABLE.SET_TIMEOUT(10800); END;
The following views can be queried to obtain information about the status of resumable statements
|DBA_RESUMABLE||These views contain rows for all currently executing or suspended resumable statements.|
|USER_RESUMABLE||They can be used by a DBA, AFTER SUSPEND trigger, or another session to monitor the progress of, or obtain specific information about, resumable statements|
|V$SESSION_WAIT||When a statement is suspended the session invoking the statement is put into a wait state. A row is inserted into this view for the session with the EVENT column containing "statement suspended, wait error to be cleared".|
The DBMS_RESUMABLE package helps control resumable space allocation. The following procedures can be invoked
This procedure aborts a suspended resumable statement. The parameter sessionID is the session ID in which the statement is executing. For parallel DML/DDL, sessionID is any session ID which participates in the parallel DML/DDL. Oracle Database guarantees that the ABORT operation always succeeds. It may be called either inside or outside of the AFTER SUSPEND trigger.
The caller of ABORT must be the owner of the session with sessionID, have ALTER SYSTEM privilege, or have DBA privileges.
|GET_SESSION_TIMEOUT(sessionID)||This function returns the current timeout value of resumable space allocation for the session with sessionID. This returned timeout is in seconds. If the session does not exist, this function returns -1.|
|SET_SESSION_TIMEOUT(sessionID, timeout)||This procedure sets the timeout interval of resumable space allocation for the session with sessionID. The parameter timeout is in seconds. The new timeout setting will applies to the session immediately. If the session does not exist, no action is taken.|
|GET_TIMEOUT()||This function returns the current timeout value of resumable space allocation for the current session. The returned value is in seconds.|
|SET_TIMEOUT(timeout)||This procedure sets a timeout value for resumable space allocation for the current session. The parameter timeout is in seconds. The new timeout setting applies to the session immediately.|
When a resumable session is suspended, an operation-suspended alert is issued on the object that needs allocation of resource for the operation to complete. Once the resource is allocated and the operation completes, the operation-suspended alert is cleared. Please refer to "Managing Tablespace Alerts" for more information on system-generated alerts.
Resumable Space Allocation Example: Registering an AFTER SUSPEND Trigger
In the following example, a system wide AFTER SUSPEND trigger is created and registered as user SYS at the database level. Whenever a resumable statement is suspended in any session, this trigger can have either of two effects:
Here are the statements for this example:
CREATE OR REPLACE TRIGGER resumable_default AFTER SUSPEND ON DATABASE DECLARE /* declare transaction in this trigger is autonomous */ /* this is not required because transactions within a trigger are always autonomous */ PRAGMA AUTONOMOUS_TRANSACTION; cur_sid NUMBER; cur_inst NUMBER; errno NUMBER; err_type VARCHAR2; object_owner VARCHAR2; object_type VARCHAR2; table_space_name VARCHAR2; object_name VARCHAR2; sub_object_name VARCHAR2; error_txt VARCHAR2; msg_body VARCHAR2; ret_value BOOLEAN; mail_conn UTL_SMTP.CONNECTION; BEGIN -- Get session ID SELECT DISTINCT(SID) INTO cur_SID FROM V$MYSTAT; -- Get instance number cur_inst := userenv('instance'); -- Get space error information ret_value := DBMS_RESUMABLE.SPACE_ERROR_INFO(err_type,object_type,object_owner, table_space_name,object_name, sub_object_name); /* -- If the error is related to undo segments, log error, send email -- to DBA, and abort the statement. Otherwise, set timeout to 8 hours. -- -- sys.rbs_error is a table which is to be -- created by a DBA manually and defined as -- (sql_text VARCHAR2(1000), error_msg VARCHAR2(4000), -- suspend_time DATE) */ IF OBJECT_TYPE = 'UNDO SEGMENT' THEN /* LOG ERROR */ INSERT INTO sys.rbs_error ( SELECT SQL_TEXT, ERROR_MSG, SUSPEND_TIME FROM DBMS_RESUMABLE WHERE SESSION_ID = cur_sid AND INSTANCE_ID = cur_inst ); SELECT ERROR_MSG INTO error_txt FROM DBMS_RESUMABLE WHERE SESSION_ID = cur_sid and INSTANCE_ID = cur_inst; -- Send email to receipient via UTL_SMTP package msg_body:='Subject: Space Error Occurred Space limit reached for undo segment ' || object_name || on ' || TO_CHAR(SYSDATE, 'Month dd, YYYY, HH:MIam') || '. Error message was ' || error_txt; mail_conn := UTL_SMTP.OPEN_CONNECTION('localhost', 25); UTL_SMTP.HELO(mail_conn, 'localhost'); UTL_SMTP.MAIL(mail_conn, 'sender@localhost'); UTL_SMTP.RCPT(mail_conn, 'recipient@localhost'); UTL_SMTP.DATA(mail_conn, msg_body); UTL_SMTP.QUIT(mail_conn); -- Abort the statement DBMS_RESUMABLE.ABORT(cur_sid); ELSE -- Set timeout to 8 hours DBMS_RESUMABLE.SET_TIMEOUT(28800); END IF; /* commit autonomous transaction */ COMMIT; END; /
This section explains how to reclaim wasted space, and also introduces the Segment Advisor, which is the Oracle Database component that identifies segments that have space available for reclamation.
In This Section
Over time, updates and deletes on objects within a tablespace can create pockets of empty space that individually are not large enough to be reused for new data. This type of empty space is referred to as fragmented free space.
Objects with fragmented free space can result in much wasted space, and can impact database performance. The preferred way to defragment and reclaim this space is to perform an online segment shrink. This process consolidates fragmented free space below the high water mark and compacts the segment. After compaction, the high water mark is moved, resulting in new free space above the high water mark. That space above the high water mark is then deallocated. The segment remains available for queries and DML during most of the operation, and no extra disk space need be allocated.
You use the Segment Advisor to identify segments that would benefit from online segment shrink. Only segments in locally managed tablespaces with automatic segment space management (ASSM) are eligible. Other restrictions on segment type exist
If a table with reclaimable space is not eligible for online segment shrink, or if you want to make changes to logical or physical attributes of the table while reclaiming space, you can use online table redefinition as an alternative to segment shrink. Online redefinition is also referred to as reorganization. Unlike online segment shrink, it requires extra disk space to be allocated.
The Segment Advisor identifies segments that have space available for reclamation. It performs its analysis by examining usage and growth statistics in the Automatic Workload Repository (AWR), and by sampling the data in the segment. It is configured to run automatically at regular intervals, and you can also run it on demand (manually). The regularly scheduled Segment Advisor run is known as the Automatic Segment Advisor.
The Segment Advisor generates the following types of advice:
If you receive a space management alert, or if you decide that you want to reclaim space, you should start with the Segment Advisor.
To use the Segment Advisor:
Check the results of the Automatic Segment Advisor.
To understand the Automatic Segment Advisor, see "Automatic Segment Advisor", later in this section. For details on how to view results.
The Automatic Segment Advisor is started by a Scheduler job that is configured to run during the default maintenance window. The default maintenance window is specified in the Scheduler, and is initially defined as follows:
The Automatic Segment Advisor does not analyze every database object. Instead, it examines database statistics, samples segment data, and then selects the following objects to analyze:
If an object is selected for analysis but the maintenance window expires before the Segment Advisor can process the object, the object is included in the next Automatic Segment Advisor run.
You cannot change the set of tablespaces and segments that the Automatic Segment Advisor selects for analysis. You can, however, enable or disable the Automatic Segment Advisor job, change the times during which the Automatic Segment Advisor is scheduled to run, or adjust Automatic Segment Advisor system resource utilization. See "Configuring the Automatic Segment Advisor Job" for more information.
You can manually run the Segment Advisor at any time with Enterprise Manager or with PL/SQL package procedure calls. Reasons to manually run the Segment Advisor include the following:
You can request advice from the Segment Advisor at three levels:
The OBJECT_TYPE column of Table 14-3 shows the types of objects for which you can request advice.
To run the Segment Advisor, you must have ADVISOR and CREATE JOB or CREATE ANY JOB privileges.
Running the Segment Advisor Manually with Enterprise Manager
There are two ways to run the Segment Advisor manually with Enterprise Manager:
Using the Segment Advisor Wizard
This method enables you to request advice at the tablespace level or object level. At the object level, you can request advice on tables, indexes, table partitions, and index partitions. Dependent objects such as LOB segments cannot be included in the analysis.
For example, if you display a list of tables on the Tables page (in the Administration section of Enterprise Manager Database Control), you can select a table and then select the Run Segment Advisor command from the Actions menu.
|Descrición de Table Page|
This method enables you to include the schema object's dependent objects in the Segment Advisor run. For example, if you select a table and select the Run Segment Advisor command, Enterprise Manager displays the table's dependent objects, such as partitions, index segments, LOB segments, and so on. You can then select dependent objects to include in the run.
In both cases, Enterprise Manager creates the Segment Advisor task as an Oracle Database Scheduler job. You can schedule the job to run immediately, or can take advantage of advanced scheduling features offered by the Scheduler.
To run the Segment Advisor manually with the Segment Advisor Wizard:
From the database Home page, under Related Links, click Advisor Central.
The Advisor Central page appears. (See Figure 14-2.)
Under Advisors, click Segment Advisor.
The first page of the Segment Advisor wizard appears.
Follow the wizard steps to schedule the Segment Advisor job, and then click Submit on the final wizard page.
The Advisor Central page reappears, with the new Segment Advisor job at the top of the list under the Results heading. The job status is SCHEDULED. (If you don't see your job, use the search fields above the list to display it.)
Check the status of the job. If it is not COMPLETED, click the Refresh button at the top of the page repeatedly. (Do not use your browser's Refresh icon.)
When the job status changes to COMPLETED, select the job by clicking in the Select column, and then click View Result.
|Advisor Central Page|
You can also run the Segment Advisor with the DBMS_ADVISOR package. You use package procedures to create a Segment Advisor task, set task arguments, and then execute the task. Table 14-2 shows the procedures that are relevant for the Segment Advisor. Please refer to Oracle Database PL/SQL Packages and Types Reference for more details on these procedures.
The example that follows shows how to use the DBMS_ADVISOR procedures to run the Segment Advisor for the sample table hr.employees. The user executing these package procedures must have the EXECUTE object privilege on the package or the ADVISOR system privilege.
Note that passing an object type of TABLE to DBMS_ADVISOR.CREATE_OBJECT amounts to an object level request. If the table is not partitioned, the table segment is analyzed (without any dependent segments like index or LOB segments). If the table is partitioned, the Segment Advisor analyzes all table partitions and generates separate findings and recommendations for each.
variable id number; begin declare name varchar2(100); descr varchar2(500); obj_id number; begin name:='Manual_Employees'; descr:='Segment Advisor Example'; dbms_advisor.create_task ( advisor_name => 'Segment Advisor', task_id => :id, task_name => name, task_desc => descr); dbms_advisor.create_object ( task_name => name, object_type => 'TABLE', attr1 => 'HR', attr2 => 'EMPLOYEES', attr3 => NULL, attr4 => NULL, attr5 => NULL, object_id => obj_id); dbms_advisor.set_task_parameter( task_name => name, parameter => 'recommend_all', value => 'TRUE'); dbms_advisor.execute_task(name); end; end; /
The Segment Advisor creates several types of results: recommendations, findings, actions, and objects. You can view results in the following ways:
|Result Type||Associated View||Description|
|Recommendations||DBA_ADVISOR_RECOMMENDATIONS||If a segment would benefit from a segment shrink or reorganization, the Segment Advisor generates a recommendation for the segment. Table 14-6 shows examples of generated findings and recommendations.|
|Findings||DBA_ADVISOR_FINDINGS||Findings are a report of what the Segment Advisor observed in analyzed segments. Findings include space used and free space statistics for each analyzed segment. Not all findings result in a recommendation. (There may be only a few recommendations, but there could be many findings.) When running the Segment Advisor manually with PL/SQL, if you specify 'TRUE' for recommend_all in the SET_TASK_PARAMETER procedure, then the Segment Advisor generates a finding for each segment that qualifies for analysis, whether or not a recommendation is made for that segment. For row chaining advice, the Automatic Segment Advisor generates findings only, and not recommendations. If the Automatic Segment Advisor has no space reclamation recommendations to make, it does not generate findings.|
|Actions||DBA_ADVISOR_ACTIONS||Every recommendation is associated with a suggested action to perform: either segment shrink or online redefinition (reorganization). The DBA_ADVISOR_ACTIONS view provides either the SQL that you need to perform a segment shrink, or a suggestion to reorganize the object.|
|Objects||DBA_ADVISOR_OBJECTS||All findings, recommendations, and actions are associated with an object. If the Segment Advisor analyzes more than one segment, as with a tablespace or partitioned table, then one entry is created in the DBA_ADVISOR_OBJECTS view for each analyzed segment. Table 14-3 defines the columns in this view to query for information on the analyzed segments. You can correlate the objects in this view with the objects in the findings, recommendations, and actions views.|
With Enterprise Manager (EM), you can view Segment Advisor results for both Automatic Segment Advisor runs and manual Segment Advisor runs. You can view the following types of results:
You can also view a list of the segments that were analyzed by the last Automatic Segment Advisor run.
The Segment Advisor Recommendations page appears. Recommendations are organized by tablespace.
The Recommendation Details page appears. You can initiate the recommended activity from this page (shrink or reorganize).
To view Segment Advisor results with EM—Last Automatic Segment Advisor run:
The Segment Advisor Recommendations page appears. (See Figure 14-3.)
The Recommendation Details page appears. (See Figure 14-4.) You can initiate the recommended activity from this page (shrink or reorganize).
To view Segment Advisor results with EM—Specific run:
If you ran the Segment Advisor with the Enterprise Manager wizard, the Advisor Central page appears after you submit the Segment Advisor task. Otherwise, to get to this page, on the database Home page, under Related Links, click Advisor Central.
Your Segment Advisor task appears in the Results section.
The Segment Advisor Task page appears, with recommendations organized by tablespace.
The Recommendation Details page appears. (See Figure 14-4.) You can initiate the recommended activity from this page (shrink or reorganize).
To view row chaining findings
The Segment Advisor Recommendations page appears. (See Figure 14-3.)
The Chained Row Analysis page appears, showing all segments that have chained rows, with a chained rows percentage for each.
The Segment Advisor Recommendations page appears.
The Automatic Segment Advisor Job page appears.
The Segments Processed In Last Run page appears. Use the search fields above the list to limit the segments displayed.
The headings of Table 14-6 show the columns in the DBA_ADVISOR_* views that contain output from the Segment Advisor. Refer to Oracle Database Reference for a description of these views. The table contents summarize the possible outcomes. In addition, Table 14-3 defines the columns in the DBA_ADVISOR_OBJECTS view that contain information on the analyzed segments.
Before querying the DBA_ADVISOR_* views, you can check that the Segment Advisor task is complete by querying the STATUS column in DBA_ADVISOR_TASKS.
select task_name, status from dba_advisor_tasks where owner = 'STEVE' and advisor_name = 'Segment Advisor'; TASK_NAME STATUS ------------------------------ ----------- Manual Employees COMPLETED
The following example shows how to query the DBA_ADVISOR_* views to retrieve findings from all Segment Advisor runs submitted by user STEVE:
SELECT af.task_name, ao.attr2 segname, ao.attr3 partition, ao.type, af.message from dba_advisor_findings af, dba_advisor_objects ao where ao.task_id = af.task_id and ao.object_id = af.object_id and ao.owner = 'STEVE'; TASK_NAME SEGNAME PARTITION TYPE MESSAGE ------------------ ------------ --------------- ---------------- -------------------------- Manual_Employees EMPLOYEES TABLE The free space in the obje ct is less than 10MB. Manual_Salestable4 SALESTABLE4 SALESTABLE4_P1 TABLE PARTITION Perform shrink, estimated savings is 74444154 bytes. Manual_Salestable4 SALESTABLE4 SALESTABLE4_P2 TABLE PARTITION The free space in the obje ct is less than 10MB.
|Table 4. Segment Advisor Outcomes: Summary|
|MESSAGE column of DBA_ADVISOR_FINDINGS||MORE_INFO column of DBA_ADVISOR_FINDINGS||BENEFIT_TYPE column of DBA_ADVISOR_RECOMMENDATIONS||ATTR1 column of DBA_ADVISOR_ACTIONS|
|Insufficient information to make a recommendation.||None||None||None|
|The free space in the object is less than 10MB.||Allocated Space:xxx: Used Space:xxx: Reclaimable Space :xxx||None||None|
|The object has some free space but cannot be shrunk because...||Allocated Space:xxx: Used Space:xxx: Reclaimable Space :xxx||None||None|
|The free space in the object is less than the size of the last extent.||Allocated Space:xxx: Used Space:xxx: Reclaimable Space :xxx||None||None|
|Perform shrink, estimated savings is xxx bytes.||Allocated Space:xxx: Used Space:xxx: Reclaimable Space :xxx||Perform shrink, estimated savings is xxx bytes.||The command to execute. For example: ALTER object SHRINK SPACE;)|
|Enable row movement of the table schema.table and perform shrink, estimated savings is xxx bytes.||Allocated Space:xxx: Used Space:xxx: Reclaimable Space :xxx||Enable row movement of the table schema.table and perform shrink, estimated savings is xxx bytes||The command to execute. For example: ALTER object SHRINK SPACE;)|
|Perform re-org on the object object, estimated savings is xxx bytes.
(Note: This finding is for objects with reclaimable space that are not eligible for online segment shrink.)
|Allocated Space:xxx: Used Space:xxx: Reclaimable Space :xxx||Perform re-org on the object object, estimated savings is xxx bytes.||Perform reorg|
|The object has chained rows that can be removed by re-org.||xx percent chained rows can be removed by re-org.||None||None|
The ASA_RECOMMENDATIONS procedure in the DBMS_SPACE package returns a nested table object that contains findings or recommendations for Automatic Segment Advisor runs and, optionally, manual Segment Advisor runs. Calling this procedure may be easier than working with the DBA_ADVISOR_* views, because the procedure performs all the required joins for you and returns information in an easily consumable format.
The following query returns recommendations by the most recent run of the Auto Segment Advisor, with the suggested command to run to follow the recommendations:
SELECT tablespace_name, segment_name, segment_type, partition_name, recommendations, c1 FROM table(dbms_space.asa_recommendations('FALSE', 'FALSE', 'FALSE')); TABLESPACE_NAME SEGMENT_NAME SEGMENT_TYPE ------------------------------ ------------------------------ -------------- PARTITION_NAME ------------------------------ RECOMMENDATIONS ----------------------------------------------------------------------------- C1 ----------------------------------------------------------------------------- TVMDS_ASSM ORDERS1 TABLE PARTITION ORDERS1_P2 Perform shrink, estimated savings is 57666422 bytes. alter table "STEVE"."ORDERS1" modify partition "ORDERS1_P2" shrink space TVMDS_ASSM ORDERS1 TABLE PARTITION ORDERS1_P1 Perform shrink, estimated savings is 45083514 bytes. alter table "STEVE"."ORDERS1" modify partition "ORDERS1_P1" shrink space TVMDS_ASSM_NEW ORDERS_NEW TABLE Perform shrink, estimated savings is 155398992 bytes. alter table "STEVE"."ORDERS_NEW" shrink space TVMDS_ASSM_NEW ORDERS_NEW_INDEX INDEX
Perform shrink, estimated savings is 102759445 bytes. alter index "STEVE"."ORDERS_NEW_INDEX" shrink space
The Automatic Segment Advisor is run by a Scheduler job. As such, you can use Enterprise Manager or PL/SQL package procedure calls to modify job attributes to suit your needs. The following are examples of modifications that you can make:
You can call DBMS_SCHEDULER package procedures to make these changes, but the easier way to is to use Enterprise Manager.
To configure the Automatic Segment Advisor job with Enterprise Manager:
|Space summary_seg advisor|
The Segment Advisor Recommendations page appears.
The Automatic Segment Advisor Job page appears.
The Edit Job page appears. This is the generic Scheduler page that enables you to modify job attributes.
|Edit job seg advisor|
The following views display information specific to the Automatic Segment Advisor. For details, see Oracle Database Reference.000000000
|DBA_AUTO_SEGADV_SUMMARY||Each row of this view summarizes one Automatic Segment Advisor run. Fields include number of tablespaces and segments processed, and number of recommendations made.|
|DBA_AUTO_SEGADV_CTL||Contains control information that the Automatic Segment Advisor uses to select and process segments. Each row contains information on a single object (tablespace or segment), including whether the object has been processed, and if so, the task ID under which it was processed and the reason for selecting it.|
You use online segment shrink to reclaim fragmented free space below the high water mark in an Oracle Database segment. The benefits of segment shrink are these:
Segment shrink is an online, in-place operation. DML operations and queries can be issued during the data movement phase of segment shrink. Concurrent DML operation are blocked for a short time at the end of the shrink operation, when the space is deallocated. Indexes are maintained during the shrink operation and remain usable after the operation is complete. Segment shrink does not require extra disk space to be allocated.
Segment shrink reclaims unused space both above and below the high water mark. In contrast, space deallocation reclaims unused space only above the high water mark. In shrink operations, by default, the database compacts the segment, adjusts the high water mark, and releases the reclaimed space.
Segment shrink requires that rows be moved to new locations. Therefore, you must first enable row movement in the object you want to shrink and disable any rowid-based triggers defined on the object. You enable row movement in a table with the ALTER TABLE ... ENABLE ROW MOVEMENT command.
Shrink operations can be performed only on segments in locally managed tablespaces with automatic segment space management (ASSM). Within an ASSM tablespace, all segment types are eligible for online segment shrink except these:
Before invoking online segment shrink, view the findings and recommendations of the Segment Advisor.
You invoke online segment shrink with Enterprise Manager (EM) or with SQL commands in SQL*Plus. The remainder of this section discusses the command line method.
You can shrink space in a table, index-organized table, index, partition, subpartition, materialized view, or materialized view log. You do this using ALTER TABLE, ALTER INDEX, ALTER MATERIALIZED VIEW, or ALTER MATERIALIZED VIEW LOG statement with the SHRINK SPACE clause. Refer to Oracle Database SQL Reference for the syntax and additional information on shrinking a database object, including restrictions.
Two optional clauses let you control how the shrink operation proceeds:
As with other DDL operations, segment shrink causes subsequent SQL statements to be reparsed because of invalidation of cursors unless you specify the COMPACT clause.
Shrink a table and all of its dependent segments (including LOB segments):
Shrink a LOB segment only:
Shrink a single partition of a partitioned table:
Shrink an IOT index segment and the overflow segment:
Shrink an IOT overflow segment only:
When you deallocate unused space, the database frees the unused space at the unused (high water mark) end of the database segment and makes the space available for other segments in the tablespace.
Prior to deallocation, you can run the UNUSED_SPACE procedure of the DBMS_SPACE package, which returns information about the position of the high water mark and the amount of unused space in a segment. For segments in locally managed tablespaces with automatic segment space management, use the SPACE_USAGE procedure for more accurate information on unused space.
The KEEP clause is optional and lets you specify the amount of space retained in the segment. You can verify that the deallocated space is freed by examining the DBA_FREE_SPACE view.
When creating tables and other data structures, you need to know how much space they will require. Each datatype has different space requirements. The Oracle Database PL/SQL User's Guide and Reference and Oracle Database SQL Reference contain extensive descriptions of datatypes and their space requirements.
Oracle Database provides data dictionary views and PL/SQL packages that allow you to display information about the space usage of schema objects. Views and packages that are unique to a particular schema object are described in the chapter of this book associated with that object. This section describes views and packages that are generic in nature and apply to multiple schema objects.
These Oracle-supplied PL/SQL packages provide information about schema objects:
|Package and Procedure/Function||Description|
|DBMS_SPACE.UNUSED_SPACE||Returns information about unused space in an object (table, index, or cluster).|
|DBMS_SPACE.FREE_BLOCKS||Returns information about free data blocks in an object (table, index, or cluster) whose segment free space is managed by free lists (segment space management is MANUAL).|
|DBMS_SPACE.SPACE_USAGE||Returns information about free data blocks in an object (table, index, or cluster) whose segment space management is AUTO.|
The following SQL*Plus example uses the DBMS_SPACE package to obtain unused space information.
SQL> VARIABLE total_blocks NUMBER SQL> VARIABLE total_bytes NUMBER SQL> VARIABLE unused_blocks NUMBER SQL> VARIABLE unused_bytes NUMBER SQL> VARIABLE lastextf NUMBER SQL> VARIABLE last_extb NUMBER SQL> VARIABLE lastusedblock NUMBER SQL> exec DBMS_SPACE.UNUSED_SPACE('SCOTT', 'EMP', 'TABLE', :total_blocks, - > :total_bytes,:unused_blocks, :unused_bytes, :lastextf, - > :last_extb, :lastusedblock); PL/SQL procedure successfully completed. SQL> PRINT TOTAL_BLOCKS ------------ 5 TOTAL_BYTES ----------- 10240 ... LASTUSEDBLOCK ------------- 3
These views display information about space usage in schema objects:
|DBA view describes storage allocated for all database segments. User view describes storage allocated for segments for the current user.|
|DBA view describes extents comprising all segments in the database. User view describes extents comprising segments for the current user.
|DBA view lists free extents in all tablespaces. User view shows free space information for tablespaces for which the user has quota.|
The following sections contain examples of using some of these views.
Example 1: Displaying Segment Information
The following query returns the name and size of each index segment in schema hr:
SELECT SEGMENT_NAME, TABLESPACE_NAME, BYTES, BLOCKS, EXTENTS FROM DBA_SEGMENTS WHERE SEGMENT_TYPE = 'INDEX' AND OWNER='HR' ORDER BY SEGMENT_NAME;
The query output is:
SEGMENT_NAME TABLESPACE_NAME BYTES BLOCKS EXTENTS ------------------------- --------------- -------- ------ ------- COUNTRY_C_ID_PK EXAMPLE 65536 32 1 DEPT_ID_PK EXAMPLE 65536 32 1 DEPT_LOCATION_IX EXAMPLE 65536 32 1 EMP_DEPARTMENT_IX EXAMPLE 65536 32 1 EMP_EMAIL_UK EXAMPLE 65536 32 1 EMP_EMP_ID_PK EXAMPLE 65536 32 1 EMP_JOB_IX EXAMPLE 65536 32 1 EMP_MANAGER_IX EXAMPLE 65536 32 1 EMP_NAME_IX EXAMPLE 65536 32 1 JHIST_DEPARTMENT_IX EXAMPLE 65536 32 1 JHIST_EMPLOYEE_IX EXAMPLE 65536 32 1 JHIST_EMP_ID_ST_DATE_PK EXAMPLE 65536 32 1 JHIST_JOB_IX EXAMPLE 65536 32 1 JOB_ID_PK EXAMPLE 65536 32 1 LOC_CITY_IX EXAMPLE 65536 32 1 LOC_COUNTRY_IX EXAMPLE 65536 32 1 LOC_ID_PK EXAMPLE 65536 32 1 LOC_STATE_PROVINCE_IX EXAMPLE 65536 32 1 REG_ID_PK EXAMPLE 65536 32 1 19 rows selected.
Example 2: Displaying Extent Information
Information about the currently allocated extents in a database is stored in the DBA_EXTENTS data dictionary view. For example, the following query identifies the extents allocated to each index segment in the hr schema and the size of each of those extents:<
SELECT SEGMENT_NAME, SEGMENT_TYPE, TABLESPACE_NAME, EXTENT_ID, BYTES, BLOCKS FROM DBA_EXTENTS WHERE SEGMENT_TYPE = 'INDEX' AND OWNER='HR' ORDER BY SEGMENT_NAME;
The query output is:
SEGMENT_NAME SEGMENT_TYPE TABLESPACE_NAME EXTENT_ID BYTES BLOCKS ------------------------- ------------ --------------- --------- -------- ------ COUNTRY_C_ID_PK INDEX EXAMPLE 0 65536 32 DEPT_ID_PK INDEX EXAMPLE 0 65536 32 DEPT_LOCATION_IX INDEX EXAMPLE 0 65536 32 EMP_DEPARTMENT_IX INDEX EXAMPLE 0 65536 32 EMP_EMAIL_UK INDEX EXAMPLE 0 65536 32 EMP_EMP_ID_PK INDEX EXAMPLE 0 65536 32 EMP_JOB_IX INDEX EXAMPLE 0 65536 32 EMP_MANAGER_IX INDEX EXAMPLE 0 65536 32 EMP_NAME_IX INDEX EXAMPLE 0 65536 32 JHIST_DEPARTMENT_IX INDEX EXAMPLE 0 65536 32 JHIST_EMPLOYEE_IX INDEX EXAMPLE 0 65536 32 JHIST_EMP_ID_ST_DATE_PK INDEX EXAMPLE 0 65536 32 JHIST_JOB_IX INDEX EXAMPLE 0 65536 32 JOB_ID_PK INDEX EXAMPLE 0 65536 32 LOC_CITY_IX INDEX EXAMPLE 0 65536 32 LOC_COUNTRY_IX INDEX EXAMPLE 0 65536 32 LOC_ID_PK INDEX EXAMPLE 0 65536 32 LOC_STATE_PROVINCE_IX INDEX EXAMPLE 0 65536 32 REG_ID_PK INDEX EXAMPLE 0 65536 32 19 rows selected.
For the hr schema, no segment has more than one extent allocated to it.
Example 3: Displaying the Free Space (Extents) in a Tablespace
Information about the free extents (extents not allocated to any segment) in a database is stored in the DBA_FREE_SPACE data dictionary view. For example, the following query reveals the amount of free space available as free extents in the SMUNDO tablespace:
SELECT TABLESPACE_NAME, FILE_ID, BYTES, BLOCKS FROM DBA_FREE_SPACE WHERE TABLESPACE_NAME='SMUNDO'; The query output is: TABLESPACE_NAME FILE_ID BYTES BLOCKS --------------- -------- -------- ------ SMUNDO 3 65536 32 SMUNDO 3 65536 32 SMUNDO 3 65536 32 SMUNDO 3 65536 32 SMUNDO 3 65536 32 SMUNDO 3 65536 32 SMUNDO 3 131072 64 SMUNDO 3 131072 64 SMUNDO 3 65536 32 SMUNDO 3 3407872 1664 10 rows selected.
Example 4: Displaying Segments that Cannot Allocate Additional Extents
It is possible that a segment cannot be allocated to an extent for any of the following reasons:
The following query returns the names, owners, and tablespaces of all segments that satisfy any of these criteria:
SELECT a.SEGMENT_NAME, a.SEGMENT_TYPE, a.TABLESPACE_NAME, a.OWNER FROM DBA_SEGMENTS a WHERE a.NEXT_EXTENT >= (SELECT MAX(b.BYTES) FROM DBA_FREE_SPACE b WHERE b.TABLESPACE_NAME = a.TABLESPACE_NAME) OR a.EXTENTS = a.MAX_EXTENTS OR a.EXTENTS = 'data_block_size' ;
Oracle Database provides two ways to plan capacity for database objects:
This section discusses the PL/SQL method. Refer to Enterprise Manager online help and Oracle Database 2 Day DBA for details on capacity planning with Enterprise Manager.
Three procedures in the DBMS_SPACE package enable you to predict the size of new objects and monitor the size of existing database objects. This section discusses those procedures and contains the following sections:
The size of a database table can vary greatly depending on tablespace storage attributes, tablespace block size, and many other factors. The
CREATE_TABLE_COSTprocedure of the DBMS_SPACE package lets you estimate the space use cost of creating a table. Please refer to Oracle Database PL/SQL Packages and Types Reference for details on the parameters of this procedure.
The procedure has two variants. The first variant uses average row size to estimate size. The second variant uses column information to estimate table size. Both variants require as input the following values:
In addition, the first variant also requires as input a value for AVG_ROW_SIZE, which is the anticipated average row size in bytes.
The second variant also requires for each anticipated column values for COLINFOS, which is an object type comprising the attributes COL_TYPE (the datatype of the column) and COL_SIZE (the number of characters or bytes in the column).
The procedure returns two values:
The CREATE_INDEX_COST procedure of the DBMS_SPACE package lets you estimate the space use cost of creating an index on an existing table.
The procedure requires as input the following values:
The results returned by this procedure depend on statistics gathered on the segment. Therefore, be sure to obtain statistics shortly before executing this procedure. In the absence of recent statistics, the procedure does not issue an error, but it may return inappropriate results. The procedure returns the following values:
The OBJECT_GROWTH_TREND procedure of the DBMS_SPACE package produces a table of one or more rows, where each row describes the space use of the object at a specific time. The procedure retrieves the space use totals from the Automatic Workload Repository or computes current space use and combines it with historic space use changes retrieved from Automatic Workload Repository. Please refer to [ARPLS] for detailed information on the parameters of this procedure.
The procedure requires as input the following values:
The procedure returns a table, each of row of which provides space use information on the object for one interval. If the return table is very large, the results are pipelined so that another application can consume the information as it is being produced. The output table has the following columns:
Records are not produced for values of TIME that precede the oldest recorded statistics for the object.
The values of the QUALITY column are:
The output returned by this procedure is an aggregation of values recorded across all instances in a RAC environment. Each value can be computed from a combination of GOOD and INTERPOLATED values. The aggregate value returned is marked GOOD if at least 80% of that value was derived from GOOD instance values.