Re: sequential read on full-table scan?
- From: Paul Baumgartel <paul.baumgartel@xxxxxxxxx>
- To: Christian Antognini <Christian.Antognini@xxxxxxxxxxxx>
- Date: Fri, 20 May 2005 13:17:36 -0400
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@xxxxxxxxxxxx> 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@xxxxxxxxx
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