RE: Re: how does oracle manage hash maps?
- From: <ryan.gaffuri@xxxxxxx>
- To: oracle-l@xxxxxxxxxxxxx
- Date: Fri, 13 Feb 2004 11:02:55 -0500
how many buffer latch chains does oracle maintain? Im assuming its version
dependent.
>
> From: "Khedr, Waleed" <Waleed.Khedr@xxxxxxx>
> Date: 2004/02/13 Fri AM 09:25:14 EST
> To: "'oracle-l@xxxxxxxxxxxxx'" <oracle-l@xxxxxxxxxxxxx>
> Subject: RE: Re: how does oracle manage hash maps?
>
> Hash chains/buckets used for managing the buffer cache has nothing to do
> with the hash join. They are completely different topics.
> For a data blocks to come to buffer cache, a hash function gets applied to
> block address to determine which buckets it will end up in.
> For a hash a join to happen, the data needed are read either from buffer
> cache or directly from files, a hash function is applied to the values of
> the joined columns themselves, and the join process starts from here.
> The hash areas/buckets for the hash join is unrelated to hash buckets of the
> buffer cache.
>
> Regards,
>
> Waleed
>
> -----Original Message-----
> From: ryan.gaffuri@xxxxxxx [mailto:ryan.gaffuri@xxxxxxx]
> Sent: Friday, February 13, 2004 8:58 AM
> To: oracle-l@xxxxxxxxxxxxx
> Subject: Re: Re: how does oracle manage hash maps?
>
>
> I know how the LRU works with linked lists. I've written linked list code
> before.
>
> From your diagram I take it that, oracle builds the hash map in the pga. The
> number of buckets is a prime number.
>
> Its hard to read your diagram. I'm not quite sure how the hash map links to
> the buffer chain. In basic hashing methodology each hash bucket points to a
> specific value.
>
> Jonathan lewis stated that in making the hash map oracle grabs the actually
> values you want to return. However, those values don't go into the PGA with
> the hash map do they? I thought all data elements went into the buffer
> cache?
> >
> > From: Scott <oraracdba@xxxxxxxxx>
> > Date: 2004/02/13 Fri AM 12:14:48 EST
> > To: oracle-l@xxxxxxxxxxxxx
> > CC: ryan.gaffuri@xxxxxxx
> > Subject: Re: how does oracle manage hash maps?
> >
> > I hope this diagram shows up. Yahoo is not the best
> > for creating diagrams.
> >
> > This example applies to Oracle7 and Oracle8, although
> > in reality there may be multiple LRU chains. In
> > Oracle8i the LRU concept is replaced by a touch-count
> > algorithm but the idea is the same.
> >
> > LRU Chain:
> > Most Recent Doubly linked list Least
> > Recent
> > +------+------+------+ - - - -
> > +------+------+------+------+------+
> > | | | | | | PTR |
> > | | |
> > +------+------+------+ - - - -
> > +------+------+------+------+------+
> > Newly used buffers | We search
> for
> > are placed at this end. | LRU just FREE buffers
> > | points from this
> end
> > LRUW Chain (Dirty List): | to of the LRU.
> > +------+------+ - - +------+ | buffer
> Dirty
> > buffers
> > | | | | | | headers. are
> moved
> > to
> > +------+------+ - - +------+ | LRUW
> if there
> > (Cleared by DBWR) | is room.
> > |
> > |
> > Hashing is |
> > based on DBA |
> > modulo the |
> > number of buckets. |
> > A LATCH protects |
> > each hash chain |
> > |
> > ,-------.
> > --------------------------------/----------------------+
> > Double
> > | Hash |
> > <------------------------------/---------------------+
> > | linked
> > | Bucket| ,--------. ,---------V----.
> > ,--------. | | hash
> > | 1 | ----->| Buffer |--->| Buffer
> > |---->| Buffer |--+ | chain.
> > `-------' <-----| Header |<---| Header
> > |<----| Header |<---+
> > | 40 | | 2 |
> > | 999 |
> > | | | |
> > | |
> > |--------| | |
> > | |
> > |Usr|Wait| | |
> > | |
> > `--------' `--------------'
> > `--------'
> > ,-------. | | |
> > :
> > |SO. |--------------' | |
> >
> > | | | |
> >
> > `-------' | |
> >
> > Buffer Handle | |
> >
> > state object | |
> > | |
> > (actual DB blocks)
> > | |
> > ,-------------------.
> > ,-------. | |
> > | 1 |
> > | Hash | | |
> > | |
> > | Bucket| | |
> > +-------------------+
> > | 2 | -->... |
> > `---------->| 2 |
> > `-------' <--... |
> > | |
> > |
> > +-------------------+
> > |
> > : :
> > |
> > +-------------------+
> > :
> > `-------------------------->| 40 Data Block |
> > :
> > | |
> >
> > +-------------------+
> >
> > : :
> >
> > +-------------------+
> > ,-------.
> > | 999 |
> > | Hash |
> > | |
> > | Bucket|
> > +-------------------+
> > | N |
> > | 1000 |
> > `-------'
> > | |
> > There are a PRIME
> > +-------------------+
> > number of Hash buckets.
> >
> >
> > --- Ryan <ryan.gaffuri@xxxxxxx> wrote:
> > >
> > >
> > > When Oracle performs a hash-join it first creates a
> > > hashmap. This hash map is placed in the PGA. Now,
> > > Oracle uses linked lists to manage the buffer cache.
> > > Traditional hashing uses an array for the hash, then
> > > a linked list for the collisions. How does Oracle
> > > manage this?
> > >
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