Re: [HACKERS] sp-gist porting to postgreSQL

[Ramy M. Hassan]

On Wed, 10 Nov 2004, Oleg Bartunov wrote:
On Wed, 10 Nov 2004, Ramy M. Hassan wrote:
I believe that it is still possible to have several index access methods 
for
the same type and the same operations. But this requires that each index
access method has its own tuple in the pg_am relation and therefore
postgresql recognizes it by itself. But this is not the case with GiST 
based
indices. They are all recognized by postgresql as same index access method,
and from here comes the limitation.
It's possible, see contrib/intarray, for example. You can specify
opclass in CREATE INDEX command:
CREATE INDEX text_idx  on test__int using gist ( a gist__int_ops );
CREATE INDEX text_idx2 on test__int using gist ( a gist__intbig_ops );
Here  gist__int_ops and gist__intbig_ops  are different opclasses for the
same type and intended to use with different cardinality. The problem is how 
to use them (indices) automatically, how planner/optimizer could
select which indices to use.
This is great. I didn't know that. Thanks.

Also, I think GiST and SP-GiST are better viewed as index classes not as
indices by themselves. So may be it is more logical to say:
CREATE INDEX index_name ON table_name USING spgist_trie(field)
Where spgist_trie is an spgist based index instance.
Than to say:
CREATE INDEX index_name ON table_name USING spgist(field)
And depend on the operator classes to define the required support methods
for the trie function.

why not use existed syntax  ?
CREATE INDEX index_name ON table_name USING spgist (fiels trie_ops)
That's ok now.
The only concern now is the portability of the extensions. Currently Are there any 
plans to introduce GiST to some other DBMS ? If yes, then I think all GiST 
based indexes will have to be rewritten or atleast modified to a great 
extent, as they depend on postgresql API and how index access methods work in postgresql.
Do you see any value in defining an SP-GiST API for the extensions 
to completely isolate the extensions code from postgresql ?
Such isolation will require that SP-GiST code loads the extensions instead 
of relying on postgresql to do that so it will no longer be a matter of 
operator classes that defines extension.



I am not sure I have a complete vision, but this is what I see. I would
appreciate your opinions regarding to this design issue.
Teodor is rather busy right now, but he certainly knows better GiST 
internals,
so we'll wait his comments.

Thanks
Ramy

-----Original Message-----
From: Oleg Bartunov [mailto:[EMAIL PROTECTED]
Sent: Wednesday, November 10, 2004 5:21 AM
To: Ramy M. Hassan
Cc: 'Pgsql Hackers'; 'Teodor Sigaev'; 'Walid G. Aref'
Subject: RE: sp-gist porting to postgreSQL
On Wed, 10 Nov 2004, Ramy M. Hassan wrote:
Oleg,
Thanks for your prompt reply.
Actually, I am able to create a new access method for testing and add an
operator class for the type "integer" using the new access method. Then
created a table with two integer fields, one indexed using the new access
method and the other using a btree index, and everything is ok so far.
Even
using EXPLAIN statement for queries show that the indexes are used
correctly
as they should.
I am using postgresql version 8.0.0beta3 from CVS.
I was wrong, Ramy. You could have several indices for the same type as soon
as they support different operations. I don't know if it's possible
to have them for the same operation but for different conditions.
Thanks
Ramy

-----Original Message-----
From: Oleg Bartunov [mailto:[EMAIL PROTECTED]
Sent: Wednesday, November 10, 2004 12:35 AM
To: Ramy M. Hassan; Pgsql Hackers
Cc: Teodor Sigaev; Walid G. Aref
Subject: Re: sp-gist porting to postgreSQL
Ramy,
glad to hear from you !
AFAIK, posgresql doesnt' supports several indices for the same type.
I think this is a problem of optimizer. Probably other hackers know
better. I forward your message to -hackers mailing list which is a
relevant place for GiST discussion.
regards,
Oleg
On Tue, 9 Nov 2004, Ramy M. Hassan wrote:
Dear Oleg and Teodor,
Thanks for offering help.
I have a design question for now.
Currently in the postgresql GiST implementation, I noticed that the way
to

have a GiST based index is to define an operator class for a certain type
using GiST index. There is no new index type defined from the point of
view
of postgresql ( nothing is added to pg_am ). This means that for a
certain

type there could only be one GiST based index. I mean that there is no
way
in
the same server to use gist to implement an xtree index and a ytree  for
the
same  type even if they index different fields in different relations. is
that correct ?
What about doing it the other way ( I am talking about SP-GiST now ) , by
providing the extension writer with an API to use it to instantiate a
standalone SP-GiST based index ( for example trie index ) that has a
record
in the pg_am relation. In my point of view this would give more
flexibility,
and also would not require the extension writer to learn the postgresql
API (
maybe oneday SP-GiST will be ported to another database engine )  he will
just need to learn the SP-GiST API which will propably be less amount of
study  (and this is what GiST and SP-GiST is all about if I correctly
understand ).
Please let me know your opinions regarding to this.
Thanks
Ramy
Regards,
                Oleg
_____________________________________________________________
Oleg Bartunov, sci.researcher, hostmaster of AstroNet,
Sternberg Astronomical Institute, Moscow University (Russia)
Internet: [EMAIL PROTECTED], http://www.sai.msu.su/~megera/
phone: +007(095)939-16-83, +007(095)939-23-83
Regards,
                Oleg
_____________________________________________________________
Oleg Bartunov, sci.researcher, hostmaster of AstroNet,
Sternberg Astronomical Institute, Moscow University (Russia)
Internet: [EMAIL PROTECTED], http://www.sai.msu.su/~megera/
phone: +007(095)939-16-83, +007(095)939-23-83
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Regards,
                Oleg
_____________________________________________________________
Oleg Bartunov, sci.researcher, hostmaster of AstroNet,
Sternberg Astronomical Institute, Moscow University (Russia)
Internet: [EMAIL PROTECTED], http://www.sai.msu.su/~megera/
phone: +007(095)939-16-83, +007(095)939-23-83

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