single table hash clusters

I created a hash cluster single table like this:

create tablespace mssm datafile 'c:\app\mssm01.dbf' size 100 m
Segment space management manual;


create the cluster hash_cluster_4k
(id number (2))
size 8192 single hash table is id hashkeys 4 tablespace mssm;

--Also created a table cluster with the line size such as single record corresponds to a block and inserted 5 records each with a separate key value


CREATE TABLE hash_cluster_tab_8k
(number (2) id,)
txt1 tank (2000).
txt2 tank (2000).
tank (2000) txt3
)
CLUSTER hash_cluster_8k (id);


Begin
because loop me in 1.5
Insert in the values of hash_cluster_tab_8k (i, 'x', 'x', 'x');
end loop;
end;
/
exec dbms_stats.gather_table_stats (WATERFALL of the USER 'HASH_CLUSTER_TAB_8K' = > true);


Now, if I try to access the folder with id = 1 - it shows 2 I / O (cr = 2) instead of the single e/s as provided in a hash cluster.



Rows Row Source operation
------- ---------------------------------------------------
1 ACCESS HASH_CLUSTER_TAB_8K HASH TABLE (cr = 2 pr = 0 pw = time 0 = 0 US)


If I run the query, even after the creation of a unique index on hash_cluster_tab (id), the execution plan specifies access hash and single e/s (cr = 1).

This means that for a single e/s in a single table hash cluster, we create a unique index? It will not create an additional burden to maintain an index?

What is the second I/o necessary for in the case where a unique index is absent?

I would be very grateful if gurus could explain this behavior.

Thanks in advance...

user12288492 wrote:
I ran the query with all 5 id values and the results have been more confusing.

During the first inning, I had VC = 2 for two values of keys, the Czech Republic rest = 1
During the second inning, I = 2 for a key value, the Czech Republic cr rest = 1
In the third set, I had VC = 1 for all values of keys

The effects vary depending on the number of previous runs and the number of times you reconnect.
The extra CR is a cleansing of the block effect. If you check the access of the buffer (events 10200-10203), then you can see the details. Simplistically, if you create your data, then connect and interrogate one of the lines (but not id = 5, because that will be cleaned on the collection of statistics) you should be able to see the following numbers:

cleanouts only - consistent read gets                                        1
immediate (CR) block cleanout applications                                   1
commit txn count during cleanout                                             1
cleanout - number of ktugct calls                                            1
Commit SCN cached                                                            1
redo entries                                                                 1
redo size                                                                   80

On the first block visited, Oracle made a visit to buffer to untangle a YVERT cleaning (ktugct - get the commit time). This cleans up the block and caches the acquired RCS. The rest of the blocks that visit you in the same session should not be cleaned because the session can use the updated SNA caching to avoid needing a cleanup operation. Finally all the blocks will have been cleaned up (which means that they will be written to the disk) and the extra CR stops happening.

There is a little quirk - drain plug seems to apply to the format block calls - and I do not understand why this was did not each row inserted thereafter.

Concerning
Jonathan Lewis

Tags: Database

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