13 Managing Space for Schema Objects

Specifying the PCTFREE Parameter

The PCTFREE parameter is used to set the percentage of a block to be reserved for possible updates to rows that already are contained in that block. For example, assume that you specify the following parameter within a CREATE TABLE statement:

PCTFREE 20 

This indicates that 20% of each data block used for the data segment of this table will be kept free and available for possible updates to the existing rows already within each block. Figure 13-1 illustrates PCTFREE.

Figure 13-1 PCTFREE

Description of the illustration admin018.gif

Notice that before the block reaches PCTFREE, the free space of the data block is filled by both the insertion of new rows and by the growth of the data block header.

Ensure that you understand the nature of table or index data before setting PCTFREE. Updates can cause rows to grow. New values might not be the same size as values they replace. If there are many updates in which data values get larger, PCTFREE should be increased. If updates to rows do not affect the total row width, PCTFREE can be low. Your goal is to find a satisfactory trade-off between densely packed data and good update performance.

The default for PCTFREE is 10 percent. You can use any integer between 0 and 99, inclusive, as long as the sum of PCTFREE and PCTUSED does not exceed 100.

Specifying the PCTUSED Parameter

After a data block becomes full as determined by PCTFREE, the database does not consider the block for the insertion of new rows until the percentage of the block being used falls below the parameter PCTUSED. Before this value is achieved, the database uses the free space of the data block only for updates to rows already contained in the data block. For example, assume that you specify the following parameter within a CREATE TABLE statement:

PCTUSED 40 

In this case, a data block used for the data segment of this table is not considered for the insertion of any new rows until the amount of used space in the block falls to 39% or less (assuming that the used space in the block has previously reached PCTFREE). Figure 13-2 illustrates this.

Figure 13-2 PCTUSED

Description of the illustration admin019.gif

The default value for PCTUSED is 40 percent. After the free space in a data block reaches PCTFREE, no new rows are inserted in that block until the percentage of space used falls below PCTUSED. The percent value is for the block space available for data after overhead is subtracted from total space.

You can specify any integer between 0 and 99 (inclusive) for PCTUSED, as long as the sum of PCTUSED and PCTFREE does not exceed 100.

Selecting Associated PCTUSED and PCTFREE Values

If you decide not to use the default values for PCTFREE or PCTUSED, keep the following guidelines in mind:

• The sum of PCTFREE and PCTUSED must be equal to or less than 100.

• If the sum equals 100, then the database attempts to keep no more than PCTFREE free space, and processing costs are highest.

• The smaller the difference between 100 and the sum of PCTFREE and PCTUSED (as in PCTUSED of 75, PCTFREE of 20), the more efficient space usage is, at some performance cost.

The following table contains examples that show how and why specific values for PCTFREE and PCTUSED are specified for tables.

Specifying the INITRANS Parameter

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.

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.

Identifying the Storage Parameters

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.

Space allocation is less complex in locally managed tablespaces than in dictionary-managed tablespaces. Storage parameters for objects in locally managed tablespaces are supported mainly for backward compatibility, and they are interpreted differently or are ignored.

For locally managed tablespaces, the Oracle Database server manages extents for you. If you specified the UNIFORM clause when the tablespace was created, then you told the database to create 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 13-1 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.

Setting Default Storage Parameters for Objects Created in a Tablespace

When you create a dictionary-managed tablespace you can specify default storage parameters. These values override the system defaults to become the defaults for objects created in that tablespace only. You specify the default storage values in the DEFAULT STORAGE clause of a CREATE or ALTER TABLESPACE statement. For a tablespace which you specifically create as locally managed, the DEFAULT STORAGE clause is not allowed.

Specifying Storage Parameters at Object Creation

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. The following example illustrates specifying storage parameters when a table is being created in a dictionary-managed tablespace:

CREATE TABLE  players
        (code  NUMBER(10) PRIMARY KEY,
         lastname  VARCHAR(20),
         firstname VARCHAR(15),
         position  VARCHAR2(20), 
         team VARCHAR2(20))
      PCTFREE 10 
      PCTUSED 40
      STORAGE 
         (INITIAL 25K
          NEXT 10K
          MAXEXTENTS 10
          MINEXTENTS 3);

For an object created in a locally managed tablespace, only the INITIAL parameter has meaning.

Setting Storage Parameters for Clusters

You set the storage parameters for nonclustered tables using the STORAGE clause of the CREATE TABLE or ALTER TABLE statement.

In contrast, you 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.

Setting Storage Parameters for Partitioned Tables

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.

Setting Storage Parameters for Index Segments

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

Setting Storage Parameters for LOBs, Varrays, and Nested Tables

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.

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):

1. Oracle Database default values

2. DEFAULT STORAGE clause of CREATE TABLESPACE statement

3. DEFAULT STORAGE clause of ALTER TABLESPACE statement

4. STORAGE clause of CREATE [TABLE | CLUSTER | MATERIALIZED VIEW | MATERIALIZED VIEW LOG | INDEX] statement

5. STORAGE clause of ALTER [TABLE | CLUSTER | MATERIALIZED VIEW | MATERIALIZED VIEW LOG | INDEX] statement

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.

Example of How Storage Parameters Effect Space Allocation

This discussion applies only to objects created in dictionary-managed tablespaces. For objects created in locally managed tablespaces, extents will either be system managed, or will all be of the same size.

Assume the following statement has been executed:

CREATE TABLE test_storage
   ( . . . )
   STORAGE (INITIAL 100K   NEXT 100K
      MINEXTENTS 2   MAXEXTENTS 5
      PCTINCREASE 50);

Also assume that the initialization parameter DB_BLOCK_SIZE is set to 2K. The following table shows how extents are allocated for the test_storage table. Also shown is the value for the incremental extent, as can be seen in the NEXT column of the USER_SEGMENTS or DBA_SEGMENTS data dictionary views:

If you change the NEXT or PCTINCREASE storage parameters with an ALTER statement (such as ALTER TABLE), the specified value replaces the current value stored in the data dictionary. For example, the following statement modifies the NEXT storage parameter of the test_storage table before the third extent is allocated for the table:

ALTER TABLE test_storage STORAGE (NEXT 500K);

As a result, the third extent is 500K when allocated, the fourth is (500K*1.5)=750K, and so forth.

Resumable Space Allocation Overview

This section provides an overview of resumable space allocation. It describes how resumable space allocation works, and specifically defines qualifying statements and error conditions.

Enabling and Disabling Resumable Space Allocation

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.

RESUMABLE_TIMEOUT  = 3600

ALTER SYSTEM SET RESUMABLE_TIMEOUT=0;

ALTER SESSION ENABLE RESUMABLE;

ALTER SESSION DISABLE RESUMABLE;

ALTER SESSION ENABLE RESUMABLE TIMEOUT 3600;

ALTER SESSION ENABLE RESUMABLE TIMEOUT 3600 NAME 'insert into table';

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.

Detecting Suspended Statements

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.

CREATE OR REPLACE TRIGGER resumable_default_timeout
AFTER SUSPEND
ON DATABASE
BEGIN
   DBMS_RESUMABLE.SET_TIMEOUT(10800);
END;

Operation-Suspended Alert

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 Space in Tablespaces " 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:

• If an undo segment has reached its space limit, then a message is sent to the DBA and the statement is aborted.

• If any other recoverable error has occurred, the timeout interval is reset to 8 hours.

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;

Segment Advisor

Oracle Database provides a Segment Advisor that helps you determine whether an object has space available for reclamation based on the level of space fragmentation within the object. The Segment Advisor can generate advice at three levels:

• At the object level, advice is generated for the entire object, such as a table. If the object is partitioned, then the advice is generated on all the partitions of the object. However, advice does not cascade to dependent objects such as indexes, LOB segments, and so forth.

• At the segment level, the advice is generated for a single segment, such as unpartitioned table, a partition or subpartition of a partitioned table, or an index or LOB column.

• At the tablespace level, advice is generated for every segment in the tablespace.

The best way to invoke the Segment Advisor is through Enterprise Manager. Please refer to Oracle Database 2 Day DBA for more information on how to use Enterprise Manager to invoke the Segment Advisor. In addition, you activate the Segment Advisor using procedures of the DBMS_ADVISOR package. Please refer to PL/SQL Packages and Types Reference for details on these parameters. The following procedures are relevant for the Segment Advisor:

Use this procedure to create the Segment Advisor Task. Specify 'Segment Advisor' as the value of the ADVISOR_NAME parameter.

Use this procedure to identify the target object for segment space advice. The parameter values of this procedure depend upon the object type. Table 13-3 lists the parameter values for each type of object.

Use this procedure to describe the segment advice you need. Table 13-4 shows the relevant input parameters of this procedure. Parameters not listed here are not used by the Segment Advisor.

The example that follows shows how to use the DBMS_ADVISOR procedures to activate the Segment Advisor for the sample table hr.employees. The user executing these procedures must have the EXECUTE object privilege on the package or the ADVISOR system privilege:

variable id number;
begin
  declare
  name varchar2(100);
  descr varchar2(500);
  obj_id number;
  begin
  name:='';
  descr:='Segment Advisor Example';
  dbms_advisor.create_task('Segment Advisor', :id, name, descr, NULL);
  dbms_advisor.create_object
     (name, 'TABLE', 'HR', 'EMPLOYEES', NULL, NULL, obj_id);
 dbms_advisor.set_task_parameter(name, 'RECOMMEND_ALL', 'TRUE');
 dbms_advisor.execute_task(name);
 end;
 end; 
 /

The Segment Advisor creates several types of results:

If you have specified TRUE for RECOMMEND_ALL in the SET_TASK_PARAMETER procedure, then the advisor generates a finding for each segment that qualifies for analysis. You can retrieve the findings by querying the DBA_ADVISOR_FINDINGS data dictionary view.

If segment shrink would result in benefit, then the advisor generates a recommendation for the segment. You can retrieve the recommendations by querying the DBA_ADVISOR_RECOMMENDATIONS dictionary view.

Every advisory recommendation is associated with a suggested action to perform segment shrink. You can retrieve the action by querying the DBA_ADVISOR_ACTIONS data dictionary review. This view provides the SQL you need to perform segment shrink.

All findings, recommendations, and actions are associated with an object. If the input to the advisor results in analysis of more than one segment, as with a tablespace or partitioned table, then one entry is created for each segment in the DBA_ADVISOR_OBJECTS dictionary view. You can correlate the object in this view with the object in the findings, recommendations, and actions views.

Please refer to Oracle Database Reference for a description of these views.

Table 13-5 shows which dictionary columns contain output from the Segment Advisor and summarizes the possible advisor outcomes.

Shrinking Database Segments

Oracle Database lets you 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. Shrink operations can be performed only on segments in tablespaces with automatic segment-space management. 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, described below. Segment shrink is available through the Enterprise Manager interface, which guides you through the required steps. The remainder of this section discusses manually implementing segment shrink.

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. Two optional clauses let you control how the shrink operation proceeds:

• The COMPACT clause lets you divide the shrink segment operation into two phases. When you specify COMPACT, Oracle Database defragments the segment space and compacts the table rows but postpones the resetting of the high water mark and the reallocation of the space until a future time. This option is useful if you have long-running queries that might span the operation and attempt to read from blocks that have been reclaimed. The defragmentation and compaction results are saved to disk, so the data movement does not have to be redone during the second phase. You can reissue the SHRINK SPACE clause without the COMPACT clause during off-peak hours to complete the second phase.

• The CASCADE clause extends the segment shrink operation to all dependent segments of the object. For example, if you specify CASCADE when shrinking a table segment, all indexes of the table will also be shrunk. (You need not specify CASCADE to shrink the partitions of a partitioned table.) To see a list of dependent segments of a given object, you can run the OBJECT_DEPENDENT_SEGMENTS procedure of the DBMS_SPACE package.

Segment shrink is an online, in-place operation. Unlike other space reorganization methods, segment shrink does not require extra disk space to be allocated. Indexes are maintained during the shrink operation and remain usable after the operation is complete. 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.

Additional benefits of shrink operations are these:

• Compaction of data leads to better cache utilization, which in turn leads to better online transaction processing (OLTP) performance.

• The compacted data requires fewer blocks to be scanned in full table scans, which in turns leads to better decision support system (DSS) performance.

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. Segment shrink is not supported for LOB segments or for tables with function-based indexes. Please refer to Oracle Database SQL Reference for the syntax and additional information on shrinking a database object, including restrictions.

Deallocating Unused Space

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 following statements deallocate unused space in a segment (table, index or cluster):

ALTER TABLE table DEALLOCATE UNUSED KEEP integer;
ALTER INDEX index DEALLOCATE UNUSED KEEP integer;
ALTER CLUSTER cluster DEALLOCATE UNUSED KEEP integer;

The KEEP clause is optional and lets you specify the amount of space retained in the segment. You can verify the deallocated space is freed by examining the DBA_FREE_SPACE view.

Using PL/SQL Packages to Display Information About Schema Object Space Usage

These Oracle-supplied PL/SQL packages provide information about schema objects:

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

Using Views to Display Information About Space Usage in Schema Objects

These views display information about space usage in schema objects:

The following sections contain examples of using some of these views.

SELECT SEGMENT_NAME, TABLESPACE_NAME, BYTES, BLOCKS, EXTENTS
    FROM DBA_SEGMENTS
    WHERE SEGMENT_TYPE = 'INDEX'
    AND OWNER='HR'
    ORDER BY SEGMENT_NAME;

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.

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;

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.

SELECT TABLESPACE_NAME, FILE_ID, BYTES, BLOCKS
    FROM DBA_FREE_SPACE
    WHERE TABLESPACE_NAME='SMUNDO';

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.

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' ;

Estimating the Space Use of a Table

The size of a database table can vary greatly depending on tablespace storage attributes, tablespace block size, and many other factors. The CREATE_TABLE_COST procedure of the DBMS_SPACE package lets you estimate the space use cost of creating a table. Please refer to [ARPLS] 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:

• TABLESPACE_NAME: The tablespace in which the object will be created. The default is the SYSTEM tablespace.

• ROW_COUNT: The anticipated number of rows in the table.

• PCT_FREE: The percentage of free space you want to reserve in each block for future expansion of existing rows due to updates.

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:

• USED_BYTES: The actual bytes used by the data, including overhead for block metadata, PCT_FREE space, and so forth.

• ALLOC_BYTES: The amount of space anticipated to be allocated for the object taking into account the tablespace extent characteristics.

Estimating the Space Use of an Index

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:

• DDL: The CREATE INDEX statement that would create the index. The table specified in this DDL statement must be an existing table.

• [Optional] PLAN_TABLE: The name of the plan table to use. The default is NULL.

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:

• USED_BYTES: The number of bytes representing the actual index data.

• ALLOC_BYTES: The amount of space allocated for the index in the tablespace.

Obtaining Object Growth Trends

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:

• OBJECT_OWNER: The owner of the object.

• OBJECT_NAME: The name of the object.

• PARTITION_NAME: The name of the table or index partition, is relevant. Specify NULL otherwise.

• OBJECT_TYPE: The type of the object.

• START_TIME: A TIMESTAMP value indicating the beginning of the growth trend analysis.

• END_TIME: A TIMESTAMP value indicating the end of the growth trend analysis. The default is "NOW".

• INTERVAL: The length in minutes of the reporting interval during which the procedure should retrieve space use information.

• SKIP_INTERPOLATED: Determines whether the procedure should omit values based on recorded statistics before and after the INTERVAL ('YES') or not ('NO'). This setting is useful when the result table will be displayed as a table rather than a chart, because you can see more clearly how the actual recording interval relates to the requested reporting interval.

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:

• TIMEPOINT: A TIMESTAMP value indicating the time of the reporting interval.

  Records are not produced for values of TIME that precede the oldest recorded statistics for the object.

• SPACE_USAGE: The number of bytes actually being used by the object data.

• SPACE_ALLOC: The number of bytes allocated to the object in the tablespace at that time.

• QUALITY: A value indicating how well the requested reporting interval matches the actual recording of statistics. This information is useful because there is no guaranteed reporting interval for object size use statistics, and the actual reporting interval varies over time and from object to object.

  The values of the QUALITY column are:

• • GOOD: The value whenever the value of TIME is based on recorded statistics with a recorded timestamp within 10% of the INTERVAL specified in the input parameters.

  • INTERPOLATED: The value did not meet the criteria for GOOD, but was based on recorded statistics before and after the value of TIME. Current in-memory statistics can be collected across all instances in a cluster and treated as the "recorded" value for the present time.

  • PROJECTION: The value of TIME is in the future as of the time the table was produced. In a Real Application Clusters environment, the rules for recording statistics allow each instance to choose independently which objects will be selected.

  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.