Fragmentation. Inserts not using free space

Question

Hi,
We are running Oracle version 10.2.04, and we have a tablespace with locally managed, uniform, extents with ASSM set to manual. As part of our ETL process, we delete rows one at a time in a loop and then commit every 10,000 rows. We then insert using the same method.
Can anyone think of reasons why the inserts would not use the free space from the deletion statements?
I'm going to try and run a few tests, so any recommendations for this would be appreciated.
Thanks,
Rob

Answer 1

Thank you both for your comments. I have now performed some testing and concluded that it was due to the PCTUSED parameter being set to 40 which has caused the lack of space re-use. I did some testing and here are the results:
The reason for using the MOD function is because I wanted to delete from different blocks and not just the same one because they are ordered by the sequence numner (PK_COL in my example).
Test 1
Manual SSM
Default PCTFREE and PCTUSED
Look at the NUM_ROWS and BLOCKS values
dba@TEST> create table dba.rj_del_ins_test
  2  tablespace Tbs_tables_x128K
  3  as
  4  select *
  5  from table
  6  where PK_COL > 1135268669
  7  and PK_COL is not null
  8  order by PK_COL ;
Table created.
dba@TEST> exec dbms_stats.gather_table_stats('DBA', tabname=>'RJ_DEL_INS_TEST',estimate_percent => 100, method_opt => 'FOR ALL COLUMNS SIZE 1', cascade=>TRUE);
PL/SQL procedure successfully completed.
dba@TEST> select TABLE_NAME, FREELISTS, NUM_ROWS, BLOCKS, EMPTY_BLOCKS, AVG_SPACE, AVG_ROW_LEN, NUM_FREELIST_BLOCKS
  2  from dba_tables
  3  where table_name = 'RJ_DEL_INS_TEST';
TABLE_NAME                      FREELISTS   NUM_ROWS     BLOCKS EMPTY_BLOCKS  AVG_SPACE AVG_ROW_LEN NUM_FREELIST_BLOCKS
RJ_DEL_INS_TEST                         1      84217        849            0          0         141           0
dba@TEST> delete from dba.rj_del_ins_test
  2  where PK_COL > 1135268669
  3  and mod(PK_COL ,20) = 10;
4143 rows deleted.
dba@TEST> commit;
Commit complete.
dba@TEST> exec dbms_stats.gather_table_stats('DBA', tabname=>'RJ_DEL_INS_TEST',estimate_percent => 100, method_opt => 'FOR ALL COLUMNS SIZE 1', cascade=>TRUE);
PL/SQL procedure successfully completed.
dba@TEST> select TABLE_NAME, FREELISTS, NUM_ROWS, BLOCKS, EMPTY_BLOCKS, AVG_SPACE, AVG_ROW_LEN, NUM_FREELIST_BLOCKS
  2  from dba_tables
  3  where table_name = 'RJ_DEL_INS_TEST';
TABLE_NAME                      FREELISTS   NUM_ROWS     BLOCKS EMPTY_BLOCKS  AVG_SPACE AVG_ROW_LEN NUM_FREELIST_BLOCKS
RJ_DEL_INS_TEST                         1      80074        849            0          0         141           0
dba@TEST>
dba@TEST> insert into dba.rj_del_ins_test
  2  select *
  3  from table
  4  where PK_COL > 1135268669
  5  and PK_COL is not null
  6  and mod(PK_COL ,20) = 6;
4277 rows created.
dba@TEST> commit;
Commit complete.
dba@TEST> exec dbms_stats.gather_table_stats('DBA', tabname=>'RJ_DEL_INS_TEST',estimate_percent => 100, method_opt => 'FOR ALL COLUMNS SIZE 1', cascade=>TRUE);
PL/SQL procedure successfully completed.
dba@TEST> select TABLE_NAME, FREELISTS, NUM_ROWS, BLOCKS, EMPTY_BLOCKS, AVG_SPACE, AVG_ROW_LEN, NUM_FREELIST_BLOCKS
  2  from dba_tables
  3  where table_name = 'RJ_DEL_INS_TEST';
TABLE_NAME                      FREELISTS   NUM_ROWS     BLOCKS EMPTY_BLOCKS  AVG_SPACE AVG_ROW_LEN NUM_FREELIST_BLOCKS
RJ_DEL_INS_TEST                         1      84351        892            0          0         141           0TEST 2
New tablespace created with same extent size as previous tablespace being used for TEST1
One change: Automatic Segment Space Management (ASSM) is set to AUTO instead of manual, as in TEST1
As you can see from the results below, we have the same problem here where the deleted space is not being reused when the new inserts come along
dba@TEST> create table dba.rj_del_ins_test
  2  tablespace rj_test_X128K
  3  as
  4  select *
  5  from table
  6  where PK_COL > 1135268669
  7  and PK_COL is not null;
Table created.
dba@TEST> exec dbms_stats.gather_table_stats('DBA', tabname=>'RJ_DEL_INS_TEST',estimate_percent => 100, method_opt => 'FOR ALL COLUMNS SIZE 1', cascade=>TRUE);
PL/SQL procedure successfully completed.
dba@TEST> select TABLE_NAME, FREELISTS, NUM_ROWS, BLOCKS, EMPTY_BLOCKS, AVG_SPACE, AVG_ROW_LEN, NUM_FREELIST_BLOCKS
  2  from dba_tables
  3  where table_name = 'RJ_DEL_INS_TEST';
TABLE_NAME                      FREELISTS   NUM_ROWS     BLOCKS EMPTY_BLOCKS  AVG_SPACE AVG_ROW_LEN NUM_FREELIST_BLOCKS
RJ_DEL_INS_TEST                                84217        868            0          0         141           0
dba@TEST> delete from dba.rj_del_ins_test
  2  where PK_COL > 1135268669
  3  and mod(PK_COL ,20) = 10;
4143 rows deleted.
dba@TEST> commit;
Commit complete.
dba@TEST> exec dbms_stats.gather_table_stats('DBA', tabname=>'RJ_DEL_INS_TEST',estimate_percent => 100, method_opt => 'FOR ALL COLUMNS SIZE 1', cascade=>TRUE);
PL/SQL procedure successfully completed.
dba@TEST> select TABLE_NAME, FREELISTS, NUM_ROWS, BLOCKS, EMPTY_BLOCKS, AVG_SPACE, AVG_ROW_LEN, NUM_FREELIST_BLOCKS
  2  from dba_tables
  3  where table_name = 'RJ_DEL_INS_TEST';
TABLE_NAME                      FREELISTS   NUM_ROWS     BLOCKS EMPTY_BLOCKS  AVG_SPACE AVG_ROW_LEN NUM_FREELIST_BLOCKS
RJ_DEL_INS_TEST                                80074        868            0          0         141           0
dba@TEST> insert into dba.rj_del_ins_test
  2  select *
  3  from table
  4  where PK_COL > 1135268669
  5  and PK_COL is not null
  6  and mod(PK_COL ,20) = 6;
4277 rows created.
dba@TEST> commit;
Commit complete.
dba@TEST> exec dbms_stats.gather_table_stats('DBA', tabname=>'RJ_DEL_INS_TEST',estimate_percent => 100, method_opt => 'FOR ALL COLUMNS SIZE 1', cascade=>TRUE);
PL/SQL procedure successfully completed.
dba@TEST> select TABLE_NAME, FREELISTS, NUM_ROWS, BLOCKS, EMPTY_BLOCKS, AVG_SPACE, AVG_ROW_LEN, NUM_FREELIST_BLOCKS
  2  from dba_tables
  3  where table_name = 'RJ_DEL_INS_TEST';
TABLE_NAME                      FREELISTS   NUM_ROWS     BLOCKS EMPTY_BLOCKS  AVG_SPACE AVG_ROW_LEN NUM_FREELIST_BLOCKS
RJ_DEL_INS_TEST                                84351        911            0          0         141           0TEST 3
Manual segment space management
Specifying the PCTFREE and PCTUSED parameters for the table being created
The first sign that something has changed is the number of blocks for the same number of rows is reduced; we are only using 764 instead of 868
Secondly, upon re-inserting the rows we are only using 3 additional blocks which shows that we are in fact re-using the space we freed up
dba@TEST> create table dba.rj_del_ins_test PCTFREE 0 PCTUSED 99
  2  tablespace Tbs_tables_X128K
  3  as
  4  select *
  5  from table
  6  where PK_COL > 1135268669
  7  and PK_COL is not null;
Table created.
dba@TEST> exec dbms_stats.gather_table_stats('DBA', tabname=>'RJ_DEL_INS_TEST',estimate_percent => 100, method_opt => 'FOR ALL COLUMNS SIZE 1', cascade=>TRUE);
PL/SQL procedure successfully completed.
dba@TEST> select TABLE_NAME, FREELISTS, NUM_ROWS, BLOCKS, EMPTY_BLOCKS, AVG_SPACE, AVG_ROW_LEN, NUM_FREELIST_BLOCKS
  2  from dba_tables
  3  where table_name = 'RJ_DEL_INS_TEST';
TABLE_NAME                      FREELISTS   NUM_ROWS     BLOCKS EMPTY_BLOCKS  AVG_SPACE AVG_ROW_LEN NUM_FREELIST_BLOCKS
RJ_DEL_INS_TEST                         1      84217        764            0          0         141           0
dba@TEST> delete from dba.rj_del_ins_test
  2  where PK_COL > 1135268669
  3  and mod(PK_COL ,20) = 10;
4143 rows deleted.
dba@TEST> commit;
Commit complete.
dba@TEST> exec dbms_stats.gather_table_stats('DBA', tabname=>'RJ_DEL_INS_TEST',estimate_percent => 100, method_opt => 'FOR ALL COLUMNS SIZE 1', cascade=>TRUE);
PL/SQL procedure successfully completed.
dba@TEST> select TABLE_NAME, FREELISTS, NUM_ROWS, BLOCKS, EMPTY_BLOCKS, AVG_SPACE, AVG_ROW_LEN, NUM_FREELIST_BLOCKS
  2  from dba_tables
  3  where table_name = 'RJ_DEL_INS_TEST';
TABLE_NAME                      FREELISTS   NUM_ROWS     BLOCKS EMPTY_BLOCKS  AVG_SPACE AVG_ROW_LEN NUM_FREELIST_BLOCKS
RJ_DEL_INS_TEST                         1      80074        764            0          0         141           0
dba@TEST> insert into dba.rj_del_ins_test
  2  select *
  3  from table
  4  where PK_COL > 1135268669
  5  and PK_COL is not null
  6  and mod(PK_COL ,20) = 6;
4277 rows created.
dba@TEST> commit;
Commit complete.
dba@TEST> exec dbms_stats.gather_table_stats('DBA', tabname=>'RJ_DEL_INS_TEST',estimate_percent => 100, method_opt => 'FOR ALL COLUMNS SIZE 1', cascade=>TRUE);
PL/SQL procedure successfully completed.
dba@TEST> select TABLE_NAME, FREELISTS, NUM_ROWS, BLOCKS, EMPTY_BLOCKS, AVG_SPACE, AVG_ROW_LEN, NUM_FREELIST_BLOCKS
  2  from dba_tables
  3  where table_name = 'RJ_DEL_INS_TEST';
TABLE_NAME                      FREELISTS   NUM_ROWS     BLOCKS EMPTY_BLOCKS  AVG_SPACE AVG_ROW_LEN NUM_FREELIST_BLOCKS
RJ_DEL_INS_TEST                         1      84351        767            0          0         141           0TEST 4
Automatic segment space management, using same tablespace as in TEST2
PCTFREE and PCTUSED has been changed to be the same as in TEST2
The result of this shows that the space is not re-used when using manual segment space management
dba@TEST> create table dba.rj_del_ins_test PCTFREE 0 PCTUSED 99
  2  tablespace rj_test_X128K
  3  as
  4  select *
  5  from table
  6  where PK_COL > 1135268669
  7  and PK_COL is not null;
Table created.
dba@TEST> exec dbms_stats.gather_table_stats('DBA', tabname=>'RJ_DEL_INS_TEST',estimate_percent => 100, method_opt => 'FOR ALL COLUMNS SIZE 1', cascade=>TRUE);
PL/SQL procedure successfully completed.
dba@TEST> select TABLE_NAME, FREELISTS, NUM_ROWS, BLOCKS, EMPTY_BLOCKS, AVG_SPACE, AVG_ROW_LEN, NUM_FREELIST_BLOCKS
  2  from dba_tables
  3  where table_name = 'RJ_DEL_INS_TEST';
TABLE_NAME                      FREELISTS   NUM_ROWS     BLOCKS EMPTY_BLOCKS  AVG_SPACE AVG_ROW_LEN NUM_FREELIST_BLOCKS
RJ_DEL_INS_TEST                                84217        782            0          0         141           0
dba@TEST> delete from dba.rj_del_ins_test
  2  where PK_COL > 1135268669
  3  and mod(PK_COL ,20) = 10;
4143 rows deleted.
dba@TEST> commit;
Commit complete.
dba@TEST> exec dbms_stats.gather_table_stats('DBA', tabname=>'RJ_DEL_INS_TEST',estimate_percent => 100, method_opt => 'FOR ALL COLUMNS SIZE 1', cascade=>TRUE);
PL/SQL procedure successfully completed.
dba@TEST> select TABLE_NAME, FREELISTS, NUM_ROWS, BLOCKS, EMPTY_BLOCKS, AVG_SPACE, AVG_ROW_LEN, NUM_FREELIST_BLOCKS
  2  from dba_tables
  3  where table_name = 'RJ_DEL_INS_TEST';
TABLE_NAME                      FREELISTS   NUM_ROWS     BLOCKS EMPTY_BLOCKS  AVG_SPACE AVG_ROW_LEN NUM_FREELIST_BLOCKS
RJ_DEL_INS_TEST                                80074        782            0          0         141           0
dba@TEST> insert into dba.rj_del_ins_test
  2  select *
  3  from table
  4  where PK_COL > 1135268669
  5  and PK_COL is not null
  6  and mod(PK_COL ,20) = 6;
4277 rows created.
dba@TEST> commit;
Commit complete.
dba@TEST> exec dbms_stats.gather_table_stats('DBA', tabname=>'RJ_DEL_INS_TEST',estimate_percent => 100, method_opt => 'FOR ALL COLUMNS SIZE 1', cascade=>TRUE);
PL/SQL procedure successfully completed.
dba@TEST> select TABLE_NAME, FREELISTS, NUM_ROWS, BLOCKS, EMPTY_BLOCKS, AVG_SPACE, AVG_ROW_LEN, NUM_FREELIST_BLOCKS
  2  from dba_tables
  3  where table_name = 'RJ_DEL_INS_TEST';
TABLE_NAME                      FREELISTS   NUM_ROWS     BLOCKS EMPTY_BLOCKS  AVG_SPACE AVG_ROW_LEN NUM_FREELIST_BLOCKS
RJ_DEL_INS_TEST                                84351        823            0          0         141           0So, that does prove one point that you are right, Robert. However, can you think of why when using AUTO segment space management we don't utilitize the space as efficiently as with manual SSM?
Thanks,
Rob