Re: sequential read on full-table scan?

From: Paul Baumgartel <paul.baumgartel_at_gmail.com>
Date: Fri, 20 May 2005 13:17:36 -0400
Message-ID: <f8c4771050520101744c1bdb9@mail.gmail.com>

Christian,

The third point you made might be relevant here. There's pretty much no other activity on this instance; maybe the buffer cache is filled with blocks from this table, and Oracle just reads in single blocks as needed. I suppose that's more efficient than reading in the same blocks over and over in multi-block reads.

Paul

On 5/20/05, Christian Antognini <Christian.Antognini_at_trivadis.com> wrote:

> Hi Paul

>=20

> >Why in the world would Oracle be reading a block at a time rather than
> >performing full-table scans?

>=20

> If Oracle recognizes that a single block has to be read, even while execu= ting a full table scan, it performs a db file sequential read.
>=20

> Three common situations lead to multi-block reads that are smaller than t= he specified value:

> - Oracle reads headers with single-block reads
> - Oracle never does an I/O that spans more extents
> - Oracle never reads a block that is already in the buffer cache

>=20
>=20
>=20

> HTH
> Chris

>=20

--=20
Paul Baumgartel
paul.baumgartel_at_gmail.com

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Received on Fri May 20 2005 - 13:22:17 CDT