Consistent Gets Doubt

Question

Hello,
Consistent Gets

Answer 1

Aman.... wrote:
Well, as I said before, there is an overhead that would be always there, ALWAYS, when you would be reading the data from the table and this would be there with per execution! What does this mean that whenever you would read the data of the query, oracle would read the extent map of the table and for this , would incur additional ios and then will start fetching the data of the table. This overhead is going to be there when you have the data or won't have the data in the table. See the below demo and tell me what you have understood, forget your demo for a moment,I understood the point that there will ALWAYS be overhead but I am trying to find out how much of the total work is overhead.
Not sure what you are trying to show with your demo. It is similar to my test case except with more data spread over multiple blocks (which I want to avoid)
So here is your demo, executed on my environment
SQL> select * from v$version ;
BANNER
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Prod
PL/SQL Release 10.2.0.1.0 - Production
CORE     10.2.0.1.0     Production
TNS for Linux: Version 10.2.0.1.0 - Production
NLSRTL Version 10.2.0.1.0 - Production
SQL> drop table t_new purge ;
Table dropped.
SQL> create table t_new(a number) ;
Table created.
SQL> exec dbms_stats.gather_table_stats(user, 'T_NEW') ;
SQL> exec hr.show_space('T_NEW','HR') ;
Unformatted Blocks .....................            0
FS1 Blocks (0-25)  .....................            0
FS2 Blocks (25-50) .....................            0
FS3 Blocks (50-75) .....................            0
FS4 Blocks (75-100).....................            0
Full Blocks        .....................            0
Total Blocks............................            8
Total Bytes.............................       65,536
Total MBytes............................            0
Unused Blocks...........................            5
Unused Bytes............................       40,960
Last Used Ext FileId....................            6
Last Used Ext BlockId...................          169
Last Used Block.........................            3
PL/SQL procedure successfully completed.
SQL> set autotrace traceonly statistics
SQL> select * from t_new ;
no rows selected
Statistics
          1  recursive calls
          0  db block gets
          3  consistent gets
          0  physical reads
          0  redo size
        270  bytes sent via SQL*Net to client
        374  bytes received via SQL*Net from client
          1  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          0  rows processed
SQL> /
no rows selected
Statistics
          0  recursive calls
          0  db block gets
          3  consistent gets
          0  physical reads
          0  redo size
        270  bytes sent via SQL*Net to client
        374  bytes received via SQL*Net from client
          1  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          0  rows processed
SQL> select count(*) from t_new ;
Statistics
          1  recursive calls
          0  db block gets
          3  consistent gets
          0  physical reads
          0  redo size
        410  bytes sent via SQL*Net to client
        385  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed
SQL> /
Statistics
          0  recursive calls
          0  db block gets
          3  consistent gets
          0  physical reads
          0  redo size
        410  bytes sent via SQL*Net to client
        385  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed
SQL> set autotrace off
SQL> begin
  2  for i in 1..1000 loop
  3    insert into t_new values (i);
  4  end loop;
  5  end;
  6  /
PL/SQL procedure successfully completed.
SQL> commit ;
Commit complete.
SQL> exec dbms_stats.gather_table_stats(user, 'T_NEW') ;
PL/SQL procedure successfully completed.
SQL> exec hr.show_space('T_NEW','HR') ;
Unformatted Blocks .....................            0
FS1 Blocks (0-25)  .....................            0
FS2 Blocks (25-50) .....................            0
FS3 Blocks (50-75) .....................            0
FS4 Blocks (75-100).....................            4
Full Blocks        .....................            1
Total Blocks............................            8
Total Bytes.............................       65,536
Total MBytes............................            0
Unused Blocks...........................            0
Unused Bytes............................            0
Last Used Ext FileId....................            6
Last Used Ext BlockId...................          169
Last Used Block.........................            8
PL/SQL procedure successfully completed.
SQL> show arrays
arraysize 100
SQL> set autotrace traceonly statistics
SQL> select * from t_new ;
1000 rows selected.
Statistics
          0  recursive calls
          0  db block gets
         17  consistent gets
          0  physical reads
          0  redo size
      10495  bytes sent via SQL*Net to client
        484  bytes received via SQL*Net from client
         11  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
       1000  rows processed
SQL> select count(*) from t_new ;
Statistics
          0  recursive calls
          0  db block gets
          7  consistent gets
          0  physical reads
          0  redo size
        411  bytes sent via SQL*Net to client
        385  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed
SQL> set autotrace off
SQL> select blocks,empty_blocks from user_tables where table_name = 'T_NEW' ;
    BLOCKS EMPTY_BLOCKS
         5            0
SQL> select count(distinct dbms_rowid.rowid_block_number(rowid)) from t_new ;
COUNT(DISTINCTDBMS_ROWID.ROWID_BLOCK_NUMBER(ROWID))
                                                  2