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09 May, 2016

Compression -- 7 : Updating after BASIC Compression

In the first blog post on compression, I had shown BASIC Compression and also the effect of executing an UPDATE on a table with BASIC compression.

To expand on the them of UPDATEs of BASIC compression blocks ....

SQL> select count(*) from source_data;

  COUNT(*)
----------
    367156

SQL> create table target_comp row store compress basic as select * from source_data where 1=2;

Table created.

SQL> select pct_free from user_tables where table_name = 'TARGET_COMP';

  PCT_FREE
----------
         0

SQL> insert /*+ APPEND */ into target_comp
  2  select * from source_data;

367156 rows created.

SQL> commit;

Commit complete.

SQL> insert /*+ APPEND */ into target_comp
  2  select * from source_data;

367156 rows created.

SQL> commit;

Commit complete.

SQL> insert /*+ APPEND */ into target_comp
  2  select * from source_data;

367156 rows created.

SQL> commit;

Commit complete.

SQL> 
SQL> analyze table target_comp compute statistics;

Table analyzed.

SQL> select chain_cnt, blocks from user_tables where table_name = 'TARGET_COMP';

 CHAIN_CNT     BLOCKS
---------- ----------
         0       4452

SQL> 
SQL> exec dbms_stats.gather_table_stats('','TARGET_COMP');

PL/SQL procedure successfully completed.

SQL> select num_rows, chain_cnt, blocks from user_tables where table_name = 'TARGET_COMP';

  NUM_ROWS  CHAIN_CNT     BLOCKS
---------- ---------- ----------
   1101468          0       4452

SQL> 


So we have a table with 1.1million rows and no Row Chaining.

What happens if we update about 20% of the rows ?

SQL> begin
  2  for rec in (select rowid from target_comp where rownum < 220001)
  3  loop
  4    update target_comp set owner=owner where rowid=rec.rowid;
  5    commit;
  6  end loop;
  7  end;
  8  /

PL/SQL procedure successfully completed.

SQL> 
SQL> commit;

Commit complete.

SQL> analyze table target_comp compute statistics;

Table analyzed.

SQL> select chain_cnt, blocks from user_tables where table_name = 'TARGET_COMP';

 CHAIN_CNT     BLOCKS
---------- ----------
    202189       7882

SQL> exec dbms_stats.gather_table_stats('','TARGET_COMP');

PL/SQL procedure successfully completed.

SQL> select num_rows, chain_cnt, blocks from user_tables where table_name = 'TARGET_COMP';

  NUM_ROWS  CHAIN_CNT     BLOCKS
---------- ---------- ----------
   1101468     202189       7882

SQL> 


I have updated 220,000 rows without actually increasing the notional length of each row (I set OWNER=OWNER).  Yet, The CHAIN_CNT is now 202K and the table's HighWaterMark has expanded from 4,452 blocks to 7,882 blocks.  A significant increase !
(YMMV may vary in your tests !)

It was Jonathan Lewis who suggested getting the Chain Count (or LIST CHAINED ROWS) to understand the impact of UPDATEs on a table with BASIC compression.
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