[SQL] Client and server cursors

[Andreas Grosam]

On Mittwoch, 5. März 2003, Ruslan Zasukhin <sunshine at public.kherson.ua> wrote:
>on 3/5/03 6:13 PM, Andreas Grosam at agrosam at computerworks.ch
>wrote:
>
>>>> b) fetching one row by one UNTIL ALL rows have been fetched. Then the client
>>>> application code is responsible for managing the data.
>>> 
>>> Andreas, but this sounds like Sever-Side ForwardOnly cursors.
>>> No ?
>> 
>> YesNo.
>> 
>> Server versus Client Side Cursors says something bout the location of the
>> resources which are required for the cursors to operate as long as it is
>> opened.
>> 
>> If the server is not required to hold any resources when creating a cursor -
>> there is no question of server or client side.
>> 
>> For instance, consider this simple query:
>> Select * from Persons;
>> 
>> If the cursor is specified Sensitive and not updateable and also shall have
>> only forward iterator capabilities - then the server is not required to
>> allocate any resources because it can select the candidate records for that
>> cursor "on the fly" when the client fetches the rows.
>> 
>> The client does also not store the resources, otherwise it would not be
>> sensitive.
>> 
>> As a result, no resources will be allocated at all.
>> 
>> 
>> If the cursor is now specified Insensitive, the server must guarantee this
>> behavior by either creating the complete set and transfering  it all to the
>> client which becomes responsible for the records (Client Side Cursor) or it
>> must create a lokal copy of the records in the server side which then yields
>> to a Server-Side Cursor.
>> 
>> Certainly, in any case, the server may or may not require to temporarily
>> create and hold resources for creating the result set on the server side - but
>> IMO, what counts is the fact which - Server or Client  - becomes responsibel
>> after the creation of the result set and is responsible for it as long the
>> cursor is opened.
>> 
>> If the server releases the resources on its side after they have been fetched
>> from the cursor, then this is a Client-Side Cursor - which also implicitly
>> means that the Client side is actually willing to manage the resources and
>> become the owner of it once they have been fetched.
>> 
>> If a Cursor manages resources on the client side, even if the server was not
>> required to hold and allocate resources while creating the result set - for
>> instance, if it it was able to create the set on the fly, IMO this is a
>> Client-Side Cursor since it manages resources while it is open on the client
>> side.
>
>
>Well, I think it is possible to make more simple definition for this
>classification.
>
>-- IF to read next/prev record cursor must ask for data Server, then this is
>Server-side cursor. Otherwise this is client-side cursor.
>
>Question of resources is SECONDARY, and your example with SELECT * is just
>an optimization... 
>
>----
>In the same time it is obvious that may be really introduced hybrid --
>Differed Client Cursor.
>
>IF user ask for Bidirectional, Deferred, ReadOnly client cursor,
>We can at first keep cursor as Server side. During fetches, client cursor
>STORE records into buffers, so it can faster return to already read records,
>and after all records read the server drop its version of
>cursor.
>
>I don't know if this have sense...may be only for faster reaction of system
>to introduce first records of result.

Exactly:
Question of Server or Client Side is an implementation detail - not a feature.
Best performance is a mixture:
Server allocates the records until the client fetches a record for the first time. Then the server releases it immediately. The client now becomes responsible for holding the fetched record.

>
>
>-- 
>Best regards,
>Ruslan Zasukhin      [ I feel the need...the need for speed ]
>-------------------------------------------------------------
>e-mail: ruslan at paradigmasoft.com
>web: http://www.paradigmasoft.com
>
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