row pieces, 255 columns, intra-block row chaining in details

Posted on by Posted in oracle, trace, undocumented 11 Comments

You might be familiar with intra-block row chaining, which can occur when a table has more than 255 columns. However, did you know that intra-block chaining only works with inserts, not updates? (Upd: This is not valid since version 12.2).

Documentation says:

When a table has more than 255 columns, rows that have data after the 255th column are likely to be chained within the same block. This is called intra-block chaining. A chained row’s pieces are chained together using the rowids of the pieces. With intra-block chaining, users receive all the data in the same block. If the row fits in the block, users do not see an effect in I/O performance, because no extra I/O operation is required to retrieve the rest of the row.

A bit more details:
1. A single row piece can store up to 255 columns.
2. Oracle splits fields into row pieces in reverse order.
3. Oracle doesn’t store trailing NULLs in a row (not in a row piece)
4. The next row piece can be stored in the same block only with inserts. When you run an update, oracle will place the new row piece into a different block. (not valid since 12.2)

I’ll show in examples with dumps:


Example 1:
1. Create table TEST with 355 columns (c_1, c_2, …, c_355)
2. insert into test(c_300) values(2)
3. dump data blocks

test code 

drop table test purge;
set serverout on;
alter session set tracefile_identifier='test1';
declare
   c varchar2(32000);
   v varchar2(32000);
   cols varchar2(32000):='c_1 number(1,0)';
   vals varchar2(32000):='null';

   ndf int;
   nbl int;
begin
   for i in 2..355 loop
      cols:=cols||',c_'||i||' number(1,0)';
      vals:=vals||','||case
                         when i = 300 then '2'
                         else 'null'
                       end;
   end loop;
   c:='create table test('||cols||')';
   v:='insert into test values('||vals||')';
   dbms_output.put_line(c);
   dbms_output.put_line(v);
   execute immediate (c);
   execute immediate (v);

   for r in (select 'alter system dump datafile '||file#||' block '||block# cmd
             from (
                   select distinct file#,block#
                   from v$bh
                   where class#=1
                   and objd in (select o.data_object_id from user_objects o where object_name='TEST')
                   order by 1,2
                  )
            )
   loop
       execute immediate r.cmd;
   end loop;
end;
/
disc;





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grep -P "^(bdba|block_row_dump|tl: |col )" test1.trc

bdba: 0x018019f3
block_row_dump:

bdba: 0x018019f4
block_row_dump:

bdba: 0x018019f5
block_row_dump:

bdba: 0x018019f6
block_row_dump:

bdba: 0x018019f7
    block_row_dump:
        tl: 260 fb: -----L-- lb: 0x1  cc: 255
            col  0: *NULL*
            col  1: *NULL*
            col  2: *NULL*
            col  3: *NULL*
               ...
            col 252: *NULL*
            col 253: *NULL*
            col 254: [ 2]  c1 03
        tl: 54 fb: --H-F--- lb: 0x1  cc: 45
            col  0: *NULL*
            col  1: *NULL*
            col  2: *NULL*
            col  3: *NULL*
               ...
            col 42: *NULL*
            col 43: *NULL*
            col 44: *NULL*


The resulted dump file shows us:
1. Both row pieces are located in the same block 0x018019f4
2. They contain only the first 300 columns, (trailing 55 columns are NULLs)
3. The first row piece contains columns c_46 - c_300,
4. The second row piece contains columns c_1 - c_45 (they all are NULLs)

Example 2.
But let's test an update with the same table:
1. Create table TEST with 355 columns (c_1, c_2, ..., c_355)
2. insert into test(c_1) values(null)
3. update test set c_300=2
4. dump data blocks

Test code 2 

drop table test purge;
set serverout on;
alter session set tracefile_identifier='test2';
declare
   c varchar2(32000);
   cols varchar2(32000):='c_1 number(1,0)';
begin
   for i in 2..355 loop
      cols:=cols||',c_'||i||' number(1,0)';
   end loop;
   c:='create table test('||cols||')';
   execute immediate (c);
   execute immediate ('insert into test(c_1) values(null)');
   execute immediate 'update test set c_300=3';
   commit;
   for r in (select 'alter system dump datafile '||file#||' block '||block# cmd
             from (
                   select distinct file#,block#
                   from v$bh
                   where class#=1
                   and objd in (select o.data_object_id from user_objects o where object_name='TEST')
                   order by 1,2
                  )
            )
   loop
       execute immediate r.cmd;
   end loop;
end;
/
disc;





[collapse]

Dump:

bdba: 0x018019f3
    block_row_dump:
        tl: 260 fb: -----L-- lb: 0x1  cc: 255
            col  0: *NULL*
            col  1: *NULL*
            col  2: *NULL*
               ...
            col 251: *NULL*
            col 252: *NULL*
            col 253: *NULL*
            col 254: [ 2]  c1 04

bdba: 0x018019f4
block_row_dump:

bdba: 0x018019f5
block_row_dump:

bdba: 0x018019f6
block_row_dump:

bdba: 0x018019f7
    block_row_dump:
        tl: 54 fb: --H-F--- lb: 0x1  cc: 45
            col  0: *NULL*
            col  1: *NULL*
            col  2: *NULL*
               ...
            col 42: *NULL*
            col 43: *NULL*
            col 44: *NULL*


As you can see, there is no intra-block chaining - second row piece was created in another block.

Example 3.
Now I want to show how oracle splits already chained rows:
1. Create table TEST with 355 columns (c_1, c_2, ..., c_355)
2. insert into test(c_1) values(1)
3. update test set c_300=2
4. update test set c_301=3
5. update test set c_302=4
6. dump data blocks

Test code 3 

drop table test purge;
set serverout on
alter session set tracefile_identifier='test3';

declare
   cols varchar2(32000):='c_1 number(1,0)';

   procedure print_and_exec(c varchar2) as
   begin
      dbms_output.put_line(c);
      execute immediate c;
   end;
begin
   for i in 2..355 loop
      cols:=cols||',c_'||i||' number(1,0)';
   end loop;
   print_and_exec ('create table test('||cols||')');
   print_and_exec ('insert into test(c_1) values(1)');
   print_and_exec ('update test set c_300=2');
   print_and_exec ('update test set c_301=3');
   print_and_exec ('update test set c_302=4');
   commit;
   execute immediate 'alter system flush buffer_cache';
   execute immediate 'select count(*) from test';
   for r in (select 'alter system dump datafile '||file#||' block '||block# cmd
             from (
                   select distinct file#,block#
                   from v$bh
                   where class#=1
                   and objd in (select o.data_object_id from user_objects o where object_name='TEST')
                   order by 1,2
                  )
            )
   loop
       execute immediate r.cmd;
   end loop;
end;
/
disc;





[collapse]


bdba: 0x018019f3
    block_row_dump:
        tl: 10 fb: -------- lb: 0x1  cc: 1
            col  0: *NULL*

bdba: 0x018019f4
    block_row_dump:
        tl: 264 fb: -----L-- lb: 0x1  cc: 255
            col  0: *NULL*
            col  1: *NULL*
               ...
            col 249: *NULL*
            col 250: *NULL*
            col 251: *NULL*
            col 252: [ 2]  c1 03
            col 253: [ 2]  c1 04
            col 254: [ 2]  c1 05

bdba: 0x018019f5
block_row_dump:

bdba: 0x018019f6
    block_row_dump:
        tl: 10 fb: -------- lb: 0x1  cc: 1
            col  0: *NULL*

bdba: 0x018019f7
    block_row_dump:
        tl: 56 fb: --H-F--- lb: 0x1  cc: 45
            col  0: [ 2]  c1 02
            col  1: *NULL*
            col  2: *NULL*
            col  3: *NULL*
               ...
            col 42: *NULL*
            col 43: *NULL*
            col 44: *NULL*


This dump shows us 4 row pieces: First row piece contains 255 columns, second - 45, and 2 row pieces - just by one row.
So we can analyze it step-by-step:
2. insert into test(c_1) values(1)
After insert we have just one row piece with 1 field.

3. update test set c_300=2
After this update, we have 2 row pieces:
1) c_1-c_45
2) c_46-c_300

4. update test set c_301=3
This update split row piece c_46-c_300 into 2 row pieces:
1) c_46
2) c_47-c_301
So we have 3 row pieces now: c_1-c_45, c_46, c_47-c_301

5. update test set c_302=4
This update split row piece c_47-c_301 into 2 row pieces:
1) c_47
2) c_48-c_302
And we've got 4 row pieces: c_1-c_45, c_46, c_47, c_48-c_302

You can try Example 4 and see how many blocks you can get, and all of them (except last one) will have only 1 column each:

Test code 4 

drop table test purge;
set serverout on
alter session set tracefile_identifier='test4';

declare
   cols varchar2(32000):='c_1 char(3)';

   procedure print_and_exec(c varchar2) as
   begin
      dbms_output.put_line(c);
      execute immediate c;
   end;
begin
   for i in 2..355 loop
      cols:=cols||',c_'||i||' char(3)';
   end loop;
   print_and_exec ('create table test('||cols||')');
   print_and_exec ('insert into test(c_1) values(null)');
   commit;
   for i in 256..355 loop
      execute immediate 'update test set c_'||i||'='||i;
   end loop;
   commit;
   execute immediate 'alter system flush buffer_cache';
   for r in (select 'alter system dump datafile '||file#||' block '||block# cmd
             from (
                   select distinct file#,block#
                   from v$bh
                   where class#=1
                   and objd in (select o.data_object_id from user_objects o where object_name='TEST')
                   order by 1,2
                  )
            )
   loop
       execute immediate r.cmd;
   end loop;
end;
/
disc;





[collapse]
grep 

bdba: 0x01801281
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x01801282
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x01801283
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x01801284
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x01801285
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x01801286
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x01801287
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x01801288
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x01801289
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x0180128a
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x0180128b
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x0180128c
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x0180128d
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x0180128e
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x0180128f
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x01801291
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x01801292
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x01801293
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x01801294
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x01801295
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x01801296
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x01801297
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x01801298
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x01801299
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x0180129a
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x0180129b
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x0180129c
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x0180129d
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x0180129e
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x0180129f
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012a1
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012a2
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012a3
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012a4
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012a5
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012a6
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012a7
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012a8
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012a9
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012aa
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012ab
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012ac
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012ad
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012ae
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012af
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012b1
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012b2
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012b3
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012b4
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012b5
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012b6
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012b7
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012b8
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012b9
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012ba
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012bb
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012bc
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012bd
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012be
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012bf
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012c1
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012c2
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012c3
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012c4
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012c5
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012c6
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012c7
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012c8
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012c9
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012ca
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012cb
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012cc
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012cd
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012ce
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012cf
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012d1
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012d2
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012d3
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012d4
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012d5
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012d6
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012d7
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012d8
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012d9
block_row_dump:
bdba: 0x018012da
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012db
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012dc
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012dd
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018012de
block_row_dump:
tl: 558 fb: -----L-- lb: 0x1  cc: 255
col  0: *NULL*
col  1: *NULL*
col  2: *NULL*
   ...
col 152: *NULL*
col 153: *NULL*
col 154: *NULL*
col 155: [ 3]  32 35 36
col 156: [ 3]  32 35 37
col 157: [ 3]  32 35 38
col 158: [ 3]  32 35 39
  ...
col 251: [ 3]  33 35 32
col 252: [ 3]  33 35 33
col 253: [ 3]  33 35 34
col 254: [ 3]  33 35 35
bdba: 0x018012df
block_row_dump:
bdba: 0x018019f3
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018019f4
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018019f5
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018019f6
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018019f7
block_row_dump:
tl: 10 fb: --H-F--- lb: 0x2  cc: 1
col  0: *NULL*
bdba: 0x018019f8
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018019f9
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018019fa
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018019fb
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018019fc
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018019fd
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018019fe
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*
bdba: 0x018019ff
block_row_dump:
tl: 10 fb: -------- lb: 0x1  cc: 1
col  0: *NULL*





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