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COOL data overhead

For a folder with 150 days of data (SV MC example) here are the statistics :

COLUMN_NAME                    AVG_COL_LEN
------------------------------ -----------
CHANNEL_ID                               4
IOV_SINCE                                8
IOV_UNTIL                                8
LASTMOD_DATE                            34
NEW_HEAD_ID                              2
OBJECT_ID                                6
ORIGINAL_ID                              2
S                                      101
SYS_INSTIME                             34
USER_TAG_ID                              2
S is the payload.

SQL> exec DBMS_STATS.gather_table_stats('ST_CLIENT', 'PDBST005_F0001_IOVS', NULL, 100);

SQL> select avg_row_len from user_tables where table_name = 'PDBST005_F0001_IOVS';

AVG_ROW_LEN
-----------
        198

Actually, using the user_extents table we find another result :

SQL> select sum(bytes) from user_extents where SEGMENT_NAME like 'PDBST005_F0001_IOVS';

SUM(BYTES)
----------
2080374784

SQL> select num_rows from user_tables where table_name = 'PDBST005_F0001_IOVS';

  NUM_ROWS
----------
   8640200

And 2080374784/86402Main.00 = 240Bytes/row

For the indexes :

SQL> select sum(bytes) from user_extents where SEGMENT_NAME like 'PDBST005_F0001_IOVS%INDX' OR  SEGMENT_NAME like 'PDBST005_F0001_IOVS%PK';

SUM(BYTES)
----------
 947978240
947978240/86402Main.00 = 109Bytes/row

So in total 349Bytes/row meaning a factor of 3.5 for a payload of 100Bytes.

In the current COOL schema, the string (VARCHAR2(255)) used to represent a date takes 33 Bytes.

SQL> select SYS_INSTIME, length(SYS_INSTIME) from PDBST005_F0001_IOVS where rownum < 2;

SYS_INSTIME                              LENGTH(SYS_INSTIME)
---------------------------------------- -------------------
2007-06-09_05:12:26.880713000 GMT                         33

SQL> desc PDBST005_F0001_IOVS

 Name                                                  Null?    Type
 ----------------------------------------------------- -------- ------------------------------------
 OBJECT_ID                                             NOT NULL NUMBER(10)
 CHANNEL_ID                                                     NUMBER(10)
 IOV_SINCE                                                      NUMBER(20)
 IOV_UNTIL                                                      NUMBER(20)
 USER_TAG_ID                                                    NUMBER(10)
 SYS_INSTIME                                                    VARCHAR2(255)
 LASTMOD_DATE                                                   VARCHAR2(255)
 ORIGINAL_ID                                                    NUMBER(10)
 NEW_HEAD_ID                                                    NUMBER(10)
 S                                                              VARCHAR2(4000)

From the Oracle documentation : Oracle Database stores dates in its own internal format. Date data is stored in fixed-length fields of seven bytes each, corresponding to century, year, month, day, hour, minute, and second.

So we would save 52 Bytes per row by switching to Oracle date.

After moving one of the tables to an "enhanced" version with :

create table PDBST005_F0001_IOVS_switch as
(
select  OBJECT_ID,
        CHANNEL_ID,
        IOV_SINCE,
        IOV_UNTIL,
        USER_TAG_ID,
        TO_DATE(SUBSTR(SYS_INSTIME,0,19), 'YYYY-MM-DD_HH24:MI:SS') "SYS_INSTIME",
        TO_DATE(SUBSTR(LASTMOD_DATE,0,19), 'YYYY-MM-DD_HH24:MI:SS') "LASTMOD_DATE",
        ORIGINAL_ID,
        NEW_HEAD_ID,
        S
from    PDBST005_F0001_IOVS
);

And the result in term of space is the following :

SQL> select sum(bytes)/power(2,30)||' GB' "Size" from user_extents where segment_name = 'PDBST005_F0001_IOVS';

Size
-------------------------------------------
1.9375 GB

SQL> select sum(bytes)/power(2,30)||' GB' "Size" from user_extents where segment_name = 'PDBST005_F0001_IOVS_SWITCH';

Size
-------------------------------------------
1.4375 GB

It should not affects indexes because none of them is based on the modified columns.

Here there was a 100 Bytes payload and we saved around 18% of space (taking into account the indexes).

Actually, the current precision is a fraction of a second so we should use the TIMESTAMP datatype which takes from 7 to 11 Bytes depending on the precision.

(see : http://download-uk.oracle.com/docs/cd/B19306_01/server.102/b14200/sql_elements001.htm#i54330 )

With the full precision (11 Bytes) we still would save 44 Bytes per IOV.

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Topic revision: r5 - 2007-07-25 - RomainBasset
 
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