Syntax

TYPE COMPREC IS RECORD(
  ownname           varchar2(255),
  objname           varchar2(255),
  blkcnt_cmp        PLS_INTEGER,
  blkcnt_uncmp      PLS_INTEGER,
  row_cmp           PLS_INTEGER,
  row_uncmp         PLS_INTEGER,
  cmp_ratio         NUMBER,
  objtype           PLS_INTEGER);

Fields

Syntax

TYPE compreclist IS TABLE OF comprec;

In Oracle Database 23ai, this procedure has been enhanced to estimate the compression ratio faster for LOBs while using less space. Now you can also estimate the compression ratio for BasicFile LOBs. This helps you decide upfront whether you want to compress BasicFile LOBs, before migrating BasicFile LOBs to SecureFile LOBs. You can also estimate the compression ratio at the LOB byte level and the time taken, in hours, to compress the LOB data in the table.

The compression ratio is estimated for the number of rows in the LOB column that you specify. For example, let's consider that the compression ratio is 2.33. It indicates that after you enable the compression feature, you can save around half of the space for the sampled rows in the LOB column.

Disclaimer: The compression ratio is an approximate value, which is calculated based on the sampled rows in the LOB column. The actual space that you save when you enable compression for the complete table may be different.

Syntax

The syntax to get the compression ratio differs for objects, LOBs, IOTs, and indexes on a table.

• Syntax to get the compression ratio for an object (table or index, default is table).

  DBMS_COMPRESSION.GET_COMPRESSION_RATIO (
     scratchtbsname        IN     VARCHAR2, 
     ownname               IN     VARCHAR2, 
     objname               IN     VARCHAR2,
     subobjname            IN     VARCHAR2,
     comptype              IN     NUMBER,
     blkcnt_cmp            OUT    PLS_INTEGER,
     blkcnt_uncmp          OUT    PLS_INTEGER,
     row_cmp               OUT    PLS_INTEGER,
     row_uncmp             OUT    PLS_INTEGER,
     cmp_ratio             OUT    NUMBER,
     comptype_str          OUT    VARCHAR2,
     block_compr_ratio     OUT    PLS_INTEGER,
     byte_comp_ratio       OUT    NUMBER,
     subset_numrows        IN     NUMBER  DEFAULT COMP_RATIO_MINROWS,
     objtype               IN     PLS_INTEGER DEFAULT OBJTYPE_TABLE); 
  

• Syntax to get compression ratio for BasicFile and SecureFile LOBs:

  DBMS_COMPRESSION.GET_COMPRESSION_RATIO (
     scratchtbsname        IN     VARCHAR2,
     tabowner              IN     VARCHAR2,
     tabname               IN     VARCHAR2,
     lobname               IN     VARCHAR2,
     partname              IN     VARCHAR2,
     comptype              IN     NUMBER,
     blkcnt_cmp            OUT    PLS_INTEGER,
     blkcnt_uncmp          OUT    PLS_INTEGER,
     lobcnt                OUT    PLS_INTEGER,
     cmp_ratio             OUT    NUMBER,
     comptype_str          OUT    VARCHAR2,
     byte_comp_ratio       OUT    NUMBER,
     total_time            OUT    NUMBER
     subset_numrows        IN     NUMBER DEFAULT COMP_RATIO_LOB_MAXROWS);
  

• Syntax to get the compression ratio for all indexes on a table. The compression ratios are returned as a collection.

  DBMS_COMPRESSION.GET_COMPRESSION_RATIO (
    scratchtbsname        IN     VARCHAR2,
    ownname               IN     VARCHAR2,
    tabname               IN     VARCHAR2,
    comptype              IN     NUMBER,
    index_cr              OUT    DBMS_COMPRESSION.COMPRECLIST,
    comptype_str          OUT    VARCHAR2,
    subset_numrows        IN     NUMBER DEFAULT COMP_RATIO_INDEX_MINROWS);

• Syntax to get the compression ratio for IOTs.

  DBMS_COMPRESSION.GET_COMPRESSION_RATIO (
       scratchtbsname       IN     VARCHAR2,
       ownname              IN     VARCHAR2,
       objname              IN     VARCHAR2,
       subobjname           IN     VARCHAR2,
       comptype             IN     NUMBER,
       iotcomp_cr           OUT    DBMS_COMPRESSION.COMPRECLIST,
       comptype_str         OUT    VARCHAR2,
       subset_numrows       IN     NUMBER DEFAULT COMP_RATIO_INDEX_MINROWS);  

Parameters

Example: Estimate the compression ratio for inline and out-of-line LOBs

The following example shows how to estimate the compression ratio for LOBs.

SET SERVEROUTPUT ON
DECLARE
    bcmp                    INTEGER;
    buncmp                  INTEGER;
    lobcmp                  INTEGER;
    cr                      NUMBER;
    byte_cr                 NUMBER;
    cstr                    VARCHAR2(2000);
    total_time              NUMBER;
    l_segment_name          VARCHAR2(30); 
    l_segment_size_blocks   NUMBER; 
    l_segment_size_bytes    NUMBER; 
    l_used_blocks           NUMBER;  
    l_used_bytes            NUMBER;  
    l_expired_blocks        NUMBER;  
    l_expired_bytes         NUMBER;  
    l_unexpired_blocks      NUMBER;  
    l_unexpired_bytes       NUMBER;
BEGIN
   DBMS_COMPRESSION.GET_COMPRESSION_RATIO (
     scratchtbsname        => 'LOBTBSP',
     tabowner              => 'CMPADV',
     tabname               => p_tablename,
     lobname               => 'C',
     partname              => NULL,
     comptype              => 256,
     blkcnt_cmp            => bcmp,
     blkcnt_uncmp          => buncmp,
     lobcnt                => lobcmp,
     cmp_ratio             => cr,
     comptype_str          => cstr,
     subset_numrows        => 1000,
     byte_comp_ratio       => byte_cr,
     total_time            => total_time
);
DBMS_OUTPUT.put_line('Estimated ratio of blocks used by the uncompressed data to the compressed data :  ' || cr);
DBMS_OUTPUT.put_line('Estimated ratio of bytes used by the uncompressed data to the compressed data  :  ' || byte_cr);
END;
/

To understand the output of this procedure, let's consider tab_inline, an inline table, and tab_outofline, an out-of-line table as shown in the following example.

CREATE TABLE tab_inline
(
    a NUMBER,
    c CLOB
)
LOB(c) STORE AS SECUREFILE  (ENABLE STORAGE IN ROW CACHE LOGGING);

CREATE TABLE tab_outofline
(
    a NUMBER,
    c CLOB
)
LOB(c) STORE AS SECUREFILE  (DISABLE STORAGE IN ROW CACHE LOGGING);

Data is stored in different ways in tab_inline and tab_outofline. In the tab_inline table, if the LOB is less than 4K, then data is stored in the table segment; otherwise, it is stored in the LOB segment. For the tab_outofline table, data of all sizes is stored in the LOB segment.

Let's consider that you have inserted 1000 LOBs of 3K each in both the tables, and then calculate the compression ratios. You can use the dbms_space.space_usage procedure to calculate the space used by the data that is stored in the LOB segments.

Sample output of compression ratio for inline LOBs.

Estimated block compression ratio : 1
Estimated byte compression ratio           : 57.6
Space used(in bytes)                       : 0
space used(in blocks)                      : 0

Sample output of compression ratio for out-of-line LOBs.

Estimated block compression ratio : 1
Estimated byte compression ratio           : 56.1
Space used(in bytes)                       : 8 MB
space used(in blocks)                      : 1000

In this example, even though the estimated byte and block compression ratios are almost the same for inline and out-of-line LOBs, the space that is used is different. In the case of tab_inline, LOB segment is not used so the space used is 0. In both cases, the data is approximately 3KB, which is small. Therefore, the data before and after compression uses the same number of blocks (that is 1 block), so the block compression ratio is 1. However, the byte level compression ratio, byte_comp_ratio, which compares the actual number of bytes used by the LOBs before and after compression is 57.6 or 56.1.

Example: Estimate the compression ratio for indexes on a table with low compression type

The following example shows how to estimate the compression ratio for advanced index compression (low):

SET SERVEROUTPUT ON
DECLARE
  1_blkcnt_cmp	   PLS_INTEGER;
  1_blkcnt_uncmp   PLS_INTEGER
  1_row_cmp	   PLS_INTEGER;
  1_row_uncmp	   PLS_INTEGER;
  1_cmp_ratio	   NUMBER;
  1_comptype_str   VARCHAR2(32767);
BEGIN
   DBMS_COMPRESSION.GET_COMPRESSION_RATIO (
    scratchtbsname =>       'USERS' ,
    ownname	   =>       'TEST' ,
    objname	   =>       'SALES_IDX' ,
    subobjname     =>        NULL ,
    comptype       =>        DBMS_COMPRESSION.COMP_INDEX_ADVANCED_LOW,
    blkcnt_cmp     =>        1_blkcnt_cmp,
    blkcnt_uncmp   =>        1_blkcnt_uncmp,
    row_cmp	   =>        1_row_cmp,
    row_uncmp      =>        1_row_uncmp,
    cmp_ratio      =>        1_cmp_ratio,
    comptype_str   =>        1_comptype_str,
    subset_numrows =>        DBMS_COMPRESSION.comp_ratio_minrows,
    objtype	   =>        DBMS_COMPRESSION.objtype_index
   );
DBMS_OUTPUT.put_line( 'Number of blocks used by the compressed sample of the object   :  ' || 1_blkcnt_cmp);
DBMS_OUTPUT.put_line( 'Number of blocks used by the uncompressed sample of the object :  ' || 1_blkcnt_uncmp);
DBMS_OUTPUT.put_line( 'Number of rows in a block in compressed sample of the object   :  ' || 1_row_cmp);
DBMS_OUTPUT.put_line( 'Number of rows in a block in uncompressed sample of the object :  ' || 1_row_uncmp);
DBMS_OUTPUT.put_line( 'Estimated Compression Ratio of Sample                          :  ' || 1_cmp_ratio);
DBMS_OUTPUT.put_line( 'Compression Type						      :  ' || 1_comptype_str);
END;
/

Output of compression advisor estimate for advanced index compression (Low):

Number of blocks used by the compressed sample of the object     : 243
Number of blocks used by the uncompressed sample of the object   : 539
Number of rows in a block in compressed sample of the object     : 499
Number of rows in a block in uncompressed sample of the object   : 145
Estimated Compression Ratio of Sample				 : 2.2
Compression Type						 : “Compress Advanced Low”

Example: Estimate the compression ratio for LOBs with medium compression type

The following example shows how to estimate the compression ratio for advanced LOB compression (medium):

SET SERVEROUTPUT ON
DECLARE
  1_blkcnt_cmp     PLS_INTEGER;
  1_blkcnt_uncmp   PLS_INTEGER;
  1_row_cmp	   PLS_INTEGER;
  1_lobcnt	   PLS_INTEGER;
  1_cmp_ratio      NUMBER;
  1_comptype_str   VARCHAR2(32767);
BEGIN
   DBMS_COMPRESSION.GET_COMPRESSION_RATIO (
    scratchtbsname => 'USERS' ,
    tabowner       => 'TEST' ,
    tabname	   => 'PARTS' ,
    lobname	   =>  'PART_DESCRIPTION' ,
    partname       =>  NULL ,
    comptype       =>  DBMS_COMPRESSION.COMP_LOB_MEDIUM,
    blkcnt_cmp     => 1_blkcnt_cmp,
    blkcnt_uncmp   => 1_blkcnt_uncmp,
    row_cmp	   => 1_row_cmp,
    lobcnt	   => 1_lobcnt,
    cmp_ratio      => 1_cmp_ratio,
    comptype_str   => 1_comptype_str,
    subset_numrows => DBMS_COMPRESSION.comp_ratio_lob_maxrows
   );
DBMS_OUTPUT.put_line( 'Number of blocks used by the compressed sample of the object    :  ' || 1_blkcnt_cmp);
DBMS_OUTPUT.put_line( 'Number of blocks used by the uncompressed sample of the object  :  ' || 1_blkcnt_uncmp);
DBMS_OUTPUT.put_line( 'Number of rows in a block in compressed sample of the object    :  ' || 1_row_cmp);
DBMS_OUTPUT.put_line( 'Number of LOBS actually sampled				      :  ' || 1_lobcnt);
DBMS_OUTPUT.put_line( 'Estimated Compression Ratio of Sample                           :  ' || 1_cmp_ratio);
DBMS_OUTPUT.put_line( 'Compression Type                                                :  ' || 1_comptype_str);
END;
/

Output of compression advisor estimate for advanced LOB compression (Medium):

Number of blocks used by the compressed sample of the object   : 199
Number of blocks used by the uncompressed sample of the object     : 389
Number of rows in a block in compressed sample of the object       : 293
Number of LOBS actually sampled 			               : 55
Estimated Compression Ratio of Sample			          : 1.9
Compression Type					                 : “Compress Medium”

Example: Estimate the compression ratio for IoTs

The following example shows how to estimate the compression ratio for IOTs:

SET SERVEROUTPUT ON
DECLARE
  bcmp       INTEGER;
  buncmp     INTEGER;
  rowcmp     INTEGER;
  rowuncmp   INTEGER;
  cr         NUMBER;
  cstr       VARCHAR2(2000);
  iotcomp_cr DBMS_COMPRESSION.COMPRECLIST;
 BEGIN
    DBMS_COMPRESSION.GET_COMPRESSION_RATIO (
    scratchtbsname        => 'USERS',
    ownname               => 'TEST',
    objname               => 'SALES',
    subobjname            =>  NULL,
    comptype              =>  DBMS_COMPRESSION.COMP_INDEX_ADVANCED_LOW,
    iotcomp_cr            =>  iotcomp_cr,
    comptype_str          =>  cstr,
    subset_numrows        =>  DBMS_COMPRESSION.COMP_RATIO_ALLROWS
    ); 
 --information about the index and the overflow segment 
 DBMS_OUTPUT.put_line( 'Number of blocks used by the compressed sample of the IOT table                                    : ' || iotcomp_cr(1).blkcnt_cmp);
 DBMS_OUTPUT.put_line( 'Number of blocks used by the uncompressed sample of the IOT table                                  : ' || iotcomp_cr(1).blkcnt_uncmp);          
 DBMS_OUTPUT.put_line( 'Average number of rows in a block in the compressed sample of the IOT table                        : ' || iotcomp_cr(1).row_cmp);
 DBMS_OUTPUT.put_line( 'Average number of rows in a block in the uncompressed sample of the IOT table                      : ' || iotcomp_cr(1).row_uncmp);
 DBMS_OUTPUT.put_line( 'Estimated Compression Ratio of the sample                                                          : ' || iotcomp_cr(1).cmp_ratio);  
 --information about the index segment 
 DBMS_OUTPUT.put_line( 'Number of blocks used by the compressed sample of the index segment of the IOT table               : ' || iotcomp_cr(2).blkcnt_cmp);
 DBMS_OUTPUT.put_line( 'Number of blocks used by the uncompressed sample of the index segment of the IOT table             : ' || iotcomp_cr(2).blkcnt_uncmp);
 DBMS_OUTPUT.put_line( 'Average number of rows in a block in the compressed sample of the index segment of the IOT table   : ' || iotcomp_cr(2).row_cmp);
 DBMS_OUTPUT.put_line( 'Average number of rows in a block in the uncompressed sample of the index segment of the IOT table : ' || iotcomp_cr(2).row_uncmp);
 DBMS_OUTPUT.put_line( 'Estimated Compression Ratio of the sample                                                          : ' || iotcomp_cr(2).cmp_ratio);
 END;
 /

Output of the compression ratio for IOTs:

Number of blocks used by the compressed sample of the IOT table                                    : 5027
Number of blocks used by the uncompressed sample of the IOT table                                  : 7950
Average number of rows in a block in the compressed sample of the IOT table                        : 199
Average number of rows in a block in the uncompressed sample of the IOT table                      : 126
Estimated Compression Ratio of the sample                                                          : 1.58
Number of blocks used by the compressed sample of the index segment of the IOT table               : 3238
Number of blocks used by the uncompressed sample of the index segment of the IOT table             : 6161
Average number of rows in a block in the compressed sample of the index segment of the IOT table   : 309
Average number of rows in a block in the uncompressed sample of the index segment of the IOT table : 162
Estimated Compression Ratio of the sample                                                          : 1.9

Usage Notes

• The procedure creates different tables in the scratch tablespace and runs analysis on these objects. It does not modify anything in the user-specified tables.

• From 23ai onwards, this feature has been enhanced to estimate the compression ratio faster for LOBs while using less space. To get a more accurate result, run the following command to switch to the old method. The older method to calculate the compression ratio takes more time to return the results and uses more space.

  alter session set "_kdlf_new_compression_adv"= FALSE;

• To understand the impact of compression, use the value of the byte compression ratio for inline LOBs and for out-of-line LOBs, use the value of the block compression ratio and space used.
• You can get more benefits when you compress large volume of data as compared to small volumes of data. If you want to compress small volumes of data, look at the byte ratio instead of the block ratio to understand the impact of compression.

Syntax

DBMS_COMPRESSION.GET_COMPRESSION_TYPE (
   ownname      IN    VARCHAR2, 
   tabname      IN    VARCHAR2, 
   row_id       IN    ROWID,
   subobjname   IN    VARCHAR2 DEFAULT NULL))
  RETURN NUMBER;

Parameters

Return Values

Flag to indicate the compression type (see Table 53-1).