I'll try to depict this here, though ascii form doesn't really help (this is where a
blackboard and some chalk do a better job)
Assume you have a table which is physically stored as:
ID TAG
-------------
1 lala
2 koko
3 haha
4 toto
If I create an index on ID, then this would be considered to be a primary index. (Even
more if ID is unique you'd call that field an ordering KEY and if ID is not unique
you'd call the field an ordering FIELD but I have only found this terminology once so
I wouldn't bet on it). The main idea is that a primary index is an index that allows
me to read the records in an order that closely corresponds to the physical order.
You B+ would look like:
/ root \
/ / \ \
/ / / \ \
| | | | | <--Notice that the ptrs from the
| | | | | leaf nodes go straight down
[physical blocks: records sorted as in index]
On the other hand if I create an index on TAG, it would be a secondary index. That is:
the leaf level of my B+ would have the records sorted as (haha->koko->lala->toto)
whereas this is not their physical order. Secondary indeces are also called
non-clustered indeces thus leading to clustering index implying a primary index
(although I don't like this when the ID for example if unique)
(no drawing for the seconday B+ cause I found it impossible ;-)
>From my understanding I don't see Mysql using primary index as described above. Only
>secondary indeces on unique or non-unique fields.
Awaiting comments.
regards,
thalis
On Tue, 27 Mar 2001, Cal Evans wrote:
> How do you differentiate between primary and secondary indexes? I'm not
> familiar with the distinction.
>
> Cal
> http://www.calevans.com
>
>
> -----Original Message-----
> From: Thalis A. Kalfigopoulos [mailto:[EMAIL PROTECTED]]
> Sent: Tuesday, March 27, 2001 10:55 AM
> To: Cal Evans
> Cc: Duke; [EMAIL PROTECTED]
> Subject: RE: keywords in CREATE TABLE?
>
>
> On Tue, 27 Mar 2001, Cal Evans wrote:
>
> > Key = Creates an index
> > Index = Same as above? (Not sure, the section of the manual I just glanced
> > at seems to indicate that they are the same thing)
> > PRIMARY KEY = Creates a primary key for the table. Each table should have
> a
> > primary key and it should be unique for that row in that table. (For more
> on
> > PK theory, checkout
> >
> http://www1.fatbrain.com/asp/BookInfo/BookInfo.asp?theisbn=0764504150&from=T
> > GD352 )
> > UNIQUE = Like primary key in that it creates a unique index on that field
> > but it is not considered a primary key.
>
> Hello Cal,
>
> First: I don't think it is necessary that each table should have a primary
> key.
>
> Second: there seems to be some confusion between the concepts of primary key
> and primary index.
> >From the theory (at least the one I know) a primary index is NOT an index
> on a primary key. Primary key is just the one of all the candidate keys that
> was chosen by the DBA. Primary index on the other hand is the index whose
> search key specifies the sequential order of the file.
> So someone please bother to explain what happens exactly in Mysql. From what
> I gather, when you specify a field(s) as PK, you just get another secondary
> index on that field.
> So we come to the conclusion that primary key is just another unique key
> with a special name. Therefore the concept of primary index doesn't exist in
> Mysql and the only flavor you have is secondary indexes either on unique or
> non-unique keys.
>
> Are the above conclusions correct?
>
> regards,
> thalis
>
>
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