RE: Cost of hash join
- From: <Peter.Hitchman@xxxxxxxxxxx>
- To: <ORACLE-L@xxxxxxxxxxxxx>
- Date: Tue, 7 Jun 2005 15:30:09 +0100
Hi,
Part of the problem must be related to the size of the table being =
hashed, I remember reading that if it cannot all sit in memory that =
Oracle creates many hash maps that get paged in and out of memory.
Regards
Pete
-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx
[mailto:oracle-l-bounce@xxxxxxxxxxxxx]On Behalf Of Christian Antognini
Sent: 07 June 2005 15:09
To: new_dba_on_the_block@xxxxxxxxx
Cc: ORACLE-L@xxxxxxxxxxxxx
Subject: RE: Cost of hash join
Hi Naveen
=20
>cost=3D (access cost of A * number of hash partitions of
>B) + access cost of B.
>=20
>Why is the access cost of A multiplied by no. of hash
>partitions?
=20
In my opinion this is not a good one (perhaps for this reason it's no =
more in the 10g documentation...).
In fact the cost is something like:
=20
cost =3D access cost A + access cost B + cost join.
=20
If the hash table is "small", the cost of the join itself will be close =
to 0.
If the hash table is "large", the cost is strongly dependent on the =
amount of "available" PGA memory.=20
=20
>Is "A" read for each hash partition of B?
=20
Each table is read only once.
=20
=20
HTH
Chris
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