Consistent gets and db block gets

Hi...
I wanted to know the difference between consistent gets and db block gets in v$sess_io.I have read that consistent gets is the blocks in consistent mode..so here what does consistent mode means????
Thanks in Advance,
Anand

Here's the complete text of the answer I originally wrote nearly 5 years ago on the Oracle-L mailing list:
A 'db block get' is a current mode get. That is, it's the most up-to-date copy of the data in that block, as it is right now, or currently. There can only be one current copy of a block in the buffer cache at any time. Db block gets generally are used when DML changes data in the database. In that case, row-level locks are implicitly taken on the updated rows. There is also at least one well-known case where a select statement does a db block get, and does not take a lock. That is, when it does a full table scan or fast full index scan, Oracle will read the segment header in current mode (multiple times, the number varies based on Oracle version).
A 'consistent get' is when Oracle gets the data in a block which is consistent with a given point in time, or SCN. The consistent get is at the heart of Oracle's read consistency mechanism. When blocks are fetched in order to satisfy a query result set, they are fetched in consistent mode. If no block in the buffer cache is consistent to the correct point in time, Oracle will (attempt to) reconstruct that block using the information in the rollback segments. If it fails to do so, that's when a query errors out with the much dreaded, much feared, and much misunderstood ORA-1555 "snapshot too old".
As to latching, and how it relates, well, consider that the block buffers are in the SGA, which is shared memory. To avoid corruption, latches are used to serialize access to many linked lists and data structures that point to the buffers as well as the buffers themselves. It is safe to say that each consistent get introduces serialization to the system, and by tuning SQL to use more efficient access paths, you can get the same answer to the same query but do less consistent gets. This not only consumes less CPU, it also can significantly reduce latching which reduces serialization and makes your system more scalable.
Well, that turned out longer than I planned. If you're still reading, I hope it helped!
Hope that helps,
-Mark
PS The original question asked about latching as well, which explains the reason for the third paragraph.
Edited by: mbobak on Sep 2, 2008 11:07 PM

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Reduce Logical IO [db block gets/consistent gets]

Hi,
Still I'm unsure about the Logical IO (db block gets + consistent gets).
I want to reduce 'consistent gets' for this query
SQL> set autotrace traceonly
SQL> select * from cm_per_phone_vw;
905 rows selected.
Execution Plan
Plan hash value: 524433310
| Id | Operation                    | Name         | Rows | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT             |              |   868 | 38192 |     8   (0)| 00:00:01 |
|   1 | SORT GROUP BY NOSORT        |              |   868 | 38192 |     8   (0)| 00:00:01 |
|   2 |   TABLE ACCESS BY INDEX ROWID| CI_PER_PHONE | 1238 | 54472 |     8   (0)| 00:00:01 |
|   3 |    INDEX FULL SCAN           | CM172C0      | 1238 |       |     1   (0)| 00:00:01 |
Statistics
          8 recursive calls
          0 db block gets
        922 consistent gets
          4 physical reads
          0 redo size
      39151 bytes sent via SQL*Net to client
       1045 bytes received via SQL*Net from client
         62 SQL*Net roundtrips to/from client
          0 sorts (memory)
          0 sorts (disk)
        905 rows processedFollowing is the view it's accessing
CREATE OR REPLACE VIEW CM_PER_PHONE_VW
AS
SELECT
per_id
, MAX(DECODE(TRIM(phone_type_cd), 'MOB', phone)) AS MOB
, MAX(DECODE(TRIM(phone_type_cd), 'HOME', phone)) AS HOME
, MAX(DECODE(TRIM(phone_type_cd), 'BUSN', TRIM(phone) || ' ' || TRIM(extension))) AS BUSN
, MAX(DECODE(TRIM(phone_type_cd), 'FAX', phone)) AS FAX
, MAX(DECODE(TRIM(phone_type_cd), 'INT', phone)) AS INT
FROM
ci_per_phone
GROUP BY
per_idI have following indexes on table ci_per_phone
INDEX_NAME                     COLUMN_NAME                    COLUMN_POSITION
XM172P0                        PER_ID                                       1
XM172P0                        SEQ_NUM                                      2
XM172S1                        PHONE                                        1
CM172C0                        PER_ID                                       1I tried creating indexes on PER_ID and PHONE_TYPE_CD but the consistent gets reduces to 920 instead of 922.
Just for curiosity, how can I reduce this?
secondly, is there any explanation on 'OPERATION' break of the plan, e.g. TABLE ACCESS BY INDEX ROWID ?
Please advice.
Luckys.

Further I'm having problem with another query which is a view
CREATE OR REPLACE VIEW CM_PER_CHAR_VW
AS
SELECT
/*+ full (a) */
a.acct_id
, MAX(DECODE(a.char_type_cd, 'ACCTYPE', a.char_val)) acct_type
, MAX(DECODE(a.char_type_cd, 'PRVBLCYC', a.adhoc_char_val)) prev_bill_cyc
FROM
ci_acct_char a
WHERE
a.effdt =
    (SELECT
      MAX(a1.effdt)
    FROM
      ci_acct_char a1
    WHERE a1.acct_id = a.acct_id
    AND a1.char_type_cd = a.char_type_cd)
GROUP BY
a.acct_idI'm not able to reduce the consistent gets and even the filter appears.
I've analyzed the table as well as the index on the table.
cisadm@CCBDEV> select * from cm_acct_char_vw;
2649 rows selected.
Execution Plan
Plan hash value: 132362271
| Id | Operation              | Name         | Rows | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT       |              |    27 | 4536 |    14   (8)| 00:00:01 |
|   1 | HASH GROUP BY         |              |    27 | 4536 |    14   (8)| 00:00:01 |
|   2 |   VIEW                 |              |    27 | 4536 |    14   (8)| 00:00:01 |
|* 3 |    FILTER              |              |       |       |            |          |
|   4 |     HASH GROUP BY      |              |    27 | 2916 |    14   (8)| 00:00:01 |
|   5 |      NESTED LOOPS      |              | 2686 |   283K|    13   (0)| 00:00:01 |
|   6 |       TABLE ACCESS FULL| CI_ACCT_CHAR | 2686 |   157K|    12   (0)| 00:00:01 |
|* 7 |       INDEX RANGE SCAN | XM064P0      |     1 |    48 |     1   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   3 - filter("A"."EFFDT"=MAX("A1"."EFFDT"))
   7 - access("A1"."ACCT_ID"="A"."ACCT_ID" AND
              "A1"."CHAR_TYPE_CD"="A"."CHAR_TYPE_CD")
Statistics
          0 recursive calls
          0 db block gets
       2754 consistent gets
          0 physical reads
          0 redo size
      76517 bytes sent via SQL*Net to client
       2321 bytes received via SQL*Net from client
        178 SQL*Net roundtrips to/from client
          0 sorts (memory)
          0 sorts (disk)
       2649 rows processedhere's the tkprof
select *
from
cm_acct_char_vw
call     count       cpu    elapsed       disk      query    current        rows
Parse        2      0.00       0.00          0          0          0           0
Execute      1      0.00       0.00          0          0          0           0
Fetch      178      0.07       0.05          0       2754          0        2649
total      181      0.07       0.05          0       2754          0        2649
Misses in library cache during parse: 1
Optimizer mode: CHOOSE
Parsing user id: 63 (CISADM)
Rows     Execution Plan
      0 SELECT STATEMENT   MODE: CHOOSE
      0   HASH (GROUP BY)
      0    VIEW
      0     FILTER
      0      HASH (GROUP BY)
      0       NESTED LOOPS
      0        TABLE ACCESS   MODE: ANALYZED (FULL) OF 'CI_ACCT_CHAR'
                   (TABLE)
      0        INDEX   MODE: ANALYZED (RANGE SCAN) OF 'XM064P0'
                   (INDEX (UNIQUE))
Elapsed times include waiting on following events:
Event waited on                             Times   Max. Wait Total Waited
----------------------------------------   Waited ---------- ------------
SQL*Net message to client                     179        0.00          0.00
SQL*Net message from client                   179        0.00          0.08
********************************************************************************I've an similar query for another table, where there are 1110 rows, but in the explain, no filter appears in the predicate
Predicate Information (identified by operation id):
   2 - access("P"."EFFDT"="VW_COL_1" AND "PER_ID"="P"."PER_ID" AND
              "CHAR_TYPE_CD"="P"."CHAR_TYPE_CD")Both the queries have somewhat similar views.
I've got 2 questions,
Is there a way I can reduce the consistent gets( I've tried with/without HINTS),
secondly whats the predicate access shows as 'VW_COL_1'
please advice.

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what is the difference between Consistent gets and buffer gets ?
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Consistent gets and db block gets

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