Re: db_file_multiblock_read parameter in init.ora file
Date: 1996/03/25
Message-ID: <4j4qgo$ivo_at_inet-nntp-gw-1.us.oracle.com>#1/1
Chetan Sheth <chetan_at_iscp.bellcore.com> wrote:
>If I set db_file_multiblock_read_count parameter to some high value, let
>say 16. The meaning is Oracle will read 16 blocks in buffer cache in one
>I/O read.
>Question: If a query required let say only 2 blocks to read, does Oracle
>still read 16 as db_file_multiblock_read_count set to 16 or it
>will just read 2.
db_file_multiblock_read_count dictates how many blocks Oracle will read during full tablescans. If a query plan includes "FULL(T)", then table T will be read N blocks at a time.
Indexes, on the other hand, are read 1 block at a time during Index Range Scans and other index read opterations. db_file_multiblock_read_count has no effect on index reading (or data blocks read via an index range scan).
If a query required lets say only 2 blocks to be read and the query plan involved a full table scan, the entire table would be read 16 blocks at a time. If the table contained but 2 blocks, only two would be read. If the table contained 1,000 blocks, they would all be read 16 at a time.
If the query required lets say only 2 blocks to be read and the query plan involved an index range scan, only two blocks would *probably* be read (there are extenuating circumstances and conditions that will make this not be true) from the table. It is indeterminate how many blocks would be read from the index given the question.
(actually 16 is not very high 32 or 64 is not out of the question for this setting if you do lots of full scans)....
>The answer to this question will be highly appreciable.
I don't know if this answer will appreciate in value over time :)
>Chetan
Thomas Kyte
tkyte_at_us.oracle.com
Oracle Government
opinions and statements are mine and do not necessarily reflect the opinions of Oracle Corporation. Received on Mon Mar 25 1996 - 00:00:00 CET