querying data from the buffer_cache

Hello
I use an experience on my own database of House to understand how data is retrieved from the buffer_cache. Using Oracle 10 g 10.2.0.3.0. I have completed 2 tables with 10 million documents each. The structure of the table is as follows.

create the bigtable table (varchar2 (50) col1, col2 varchar2 (50))
create table bigtablechild (varchar2 (50) col1, col2 varchar2 (50))

bigtablechild.Col1 is a foreign key to bigtable.col1, and there is no index on the tables.

When I run this query, it takes about 30 seconds to return the data whenever it is running. I can also see the the hard disk activity running on the usefulness of my practical hard drive monitor each time. I thought that once the data is loaded into the buffer_cache, the query would not scan the hard drive for data. Could someone please help me understand what is happening?

Select bigtable b.col2 a, bigtablechild b where a.col1 = b.col1 and a.col1 = "ABC8876."
-------------------------------------------------------------------------------------------------------
| Id  | Operation          | Name          | Starts | E-Rows | A-Rows |   A-Time   | Buffers | Reads  |
-------------------------------------------------------------------------------------------------------
|*  1 |  HASH JOIN         |               |      1 |      5 |      5 |00:00:23.00 |   93671 |  90663 |
|*  2 |   TABLE ACCESS FULL| BIGTABLE      |      1 |      1 |      1 |00:00:14.26 |   57799 |  54931 |
|*  3 |   TABLE ACCESS FULL| BIGTABLECHILD |      1 |      5 |      5 |00:00:08.74 |   35872 |  35732 |
-------------------------------------------------------------------------------------------------------
Published by: arizona9952 on January 19, 2013 09:38

Jonathan Lewis wrote:
One of the diffficulties we face when trying to figure out how to size the table affects caching on the full tablescans, is that algorithms exist to protect the blocks being scrapped from the cache by excessive scanning - but if you clear the cache of the buffer before testing, then there is no blocks which need protection , and the algorithm has a branch that takes advantage of this fact to load more blocks in the cache.

Oracle is smarter than I realized. Thank you for this release, if I ever get a test methodology reproducible for it I'll post it here.

And in the meantime:

@arizona9952, I hope see you why it is more that likely that your repeated scans large tables will always drive. The only exception to this that I can see would be if you were to cache tables in a pool to keep, like stefan suggested.
(by the way, I have to warn you that the situation is complicated in the current version, due to in part to changes in treatment parallel and direct series of e/s)

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