Query Execution/Elapsed Time and Oracle Data Blocks

Question

Hi,
I have created 3 tables with one column only. As an example Table 1 below:
SQL> create table T1 ( x char(2000));
So 3 tables are created in this way i.e. T1,T2 and T3.
T1 = in the default database tablespace of 8k (11g v11.1.0.6.0 - Production) (O.S=Windows).
T2 = I created in a Tablespace with Blocksize 16k.
T3 = I created in a Tablespace with Blocksize 4k. In the same Instance.
Each table has approx. 500 rows (So, table sizes are same in all the cases to test Query execution time ). As these 3 tables are created under different data block sizes so the ALLOCATED no. of data blocks are different in all cases.
T1  =   8k  = 256 Blocks =  00:00:04.76 (query execution time/elapsed time)
T2  = 16k=121 Blocks =  00:00:04.64
T3 =   4k =  490 Blocks =  00:00:04.91
Table Access is FULL i.e. I have used select * from table_name; in all 3 cases. No Index nothing.
My Question is why query execution time is nearly the same in all 3 cases because Oracle has to read all the data blocks in each case to fetch the records and there is a much difference in the allocated no. of blocks ???
In 4k block size example, Oracle has to read just 121 blocks and it's taking nearly the same time as it's taking to read 490 blocks???
This is just 1 example of different data blocks. I have around 40 tables in each block size tablespace and the result are nearly the same. It's very strange for me because there is a much difference in the no. of allocated blocks but execution time is almost the same, only difference in milliseconds.
I'll highly appreciate the expert opinions.
Bundle of thanks in advance.
Best Regards,

Answer 1

Hi Chris,
No I'm not using separate databases, it's 8k database with non-standard blocksizes of 16k and 4k.
Actually I wanted to test the Elapsed time of these 3 tables, so for that I tried to create the same size
tables.
And how I equalize these is like I have created one column table with char(2000).
555 MB is the figure I wanted to use for these 3 tables ( no special figure, just to make it bigger than the
RAM used for my db at the db startup to be sure of not retrieving the records from cache).
so row size with overhead is 2006 * 290,000 rows = 581740000(bytes) / 1024 = 568105KB / 1024 = 555MB.
Through this math calculation I thought It will be the total table size. So I Created the same no. of rows in 3 blocksizes.
If it's wrong then what a mes because I was calculating tables sizes in the same way from the last few months.
Can you please explain a little how you found out the tables sizes in different block sizes.Though I understood how you
calculated size in MB from these 3 block sizes
T8K =97177 BLOCKS=759MB *( 97177*8 = 777416KB / 1024 = 759MB )*
T16K=41639 BLOCKS=650MB
BT4K=293656 BLOCKS=1147MB
For me it's new to calculate the size of a table. Can you please tell me then how many rows I can create in each of
these 3 tables to make them equal in MB to test for elapsed time.
Then I'll again run my test and put the results here. Because If I've wrongly calculated table sizes then there is no need to talk about elapsed time. First I must equalize the table sizes properly.
SQL> select sum(bytes)/1024/1024 "Size in MB" from dba_segments> 2 where segment_name = 'T16K';
Size in MB
655
Is above SQL is correct to calculate the size or is it the correct alternative way to your method of calculating the size??
I created the same table again with everything same and the result is :
SQL> select num_rows,blocks from user_tables where table_name = 'T16K';NUM_ROWS BLOCKS
290000 41703
64 more blocks are allocated this time so may be that's y it's showing total size of 655 instead of 650.
Thanks alot for your help.
Best Regards,
KAm
Edited by: kam555 on Nov 20, 2009 5:57 PM