Actually, the event names db file sequential read and db file scattered read describe how blocks are stored in memory, not how they are read from disk. The names are analogous teo two classes of UNIX read calls, represented by the functions read() and readv(). The unix read() function reads a contiguous portion of a file and stores it contiguously in a single memory area. The readv() function reads a contiguous portion of a file and stores pieces of that data in different areas of memory dictated by an array of memory references. Oracle disk-reads executed through calls like read() are recorded as db_file_sequential_read events, and disk-reads executed through calls like readv() are recorded as db_file_scattered_read events.
In summary, a db_file_sequential_read occurs when the memory receiving the contents of a disk read is contiguous. A db_file_scattered_read event occurs when the memory receiving the contents of a disk is not guaranteed to be contiguous.
This information was taken from a great article from the Hotsos guys.
> -----Original Message-----
> From: Govindan K [mailto:[Email Address Removed]
> Sent: Saturday, January 25, 2003 11:04 AM
> To: LazyDBA.com Discussion
> Subject: RE: Ref. : db file scattered read..help needed
> urgent..(taking a side road)
>
>
> That is because, the data may be scattered over the disk and
> may not be contiguous.
>
> GovindanK
>
>
> On Fri, 24 Jan 2003 22:56:01
> Robert Freeman wrote:
> >Anyone know WHY Oracle calls full scans "scattered" reads? I
> feel like
> >I should know the reasoning behind the choice of names, but I don't.
> >
> >Cheers!
> >
> >RF
> >
> >-----Original Message-----
> >From: Christophe Tsobgny
> >[mailto:christophe.[Email Address Removed] >Sent: Friday, January 10, 2003 7:51 AM
> >To: LazyDBA.com Discussion
> >Subject: Ref. : db file scattered read..help needed urgent..
> >
> >
> >
> >
> >you are doing 'FULL TABLE SCANS'
> >
> >if your tables are large, your disks heavily loaded
> >then this could be the reason why it takes such a long time.
> >
> >should you be doign full table scans ?
> >do you have indexes ?
> >
> >trace your query within sqlplus , try the following :
> >
> >sqlplus>set autot traceonly explain
> >sqlplus>your query
> >
> >
> >
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tplus
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