Micha Nelissen wrote:
On Sat, 01 Apr 2006 18:33:10 -0300
Luiz Americo Pereira Camara <[EMAIL PROTECTED]> wrote:
And what about recno? Just define it as -1, or do a quick count to get
the current record number?
Take a look at TCustomSqliteDataset implementation. In its
implementation a count is done each time.
Store the recno with/in the linked list entry ?
It has two drawbacks: increase memory usage and when inserting, deleting
all following records must be updated.
An array of records has the same problem, no ? So these are not drawbacks
IMHO.
I think, when using an array, the RecNo is implicit from the position in the
array.
Implementing the RecordCount property is not so much a problem (using
increments/decrements on changes), but updating RecNo's can be troublesome I
think.
I can imagine it is possible to store some kind of relative RecNo's with only a
sparse subset of the record in a smart way, allowing O(log N) times for
inserting/deleting records and O(log N) time for getting RecNo's. I do not know
whether this is indeed possible, and the implementation would not be easy I
think.
Another option is to update the RecNo's only when requested, i.e.:
- each record stores a record number
- when inserting/deleting, all RecNo's from that record on are marked as
'dirty' (this needs only storage of a LowestDirtyRecNoIndex)
- when requesting the RecNo of an item with 'dirty' stored RecNo, one could
recalculate the RecNo's of record LowestDirtyRecNoIndex until the record whose
RecNo is requested at that time
To prevent the memory overhead of storing RecNo's, it is even possible to only
allocate the RecNo storage space only when some RecNo is retrieved for the
first time. This allows people to save memory by never using RecNo, but still
use RecNo if they really need.
Of course, using Bookmarks is a very good substitute for RecNo's! And if one
wants to enumerate the records, one can still do so by keeping a private
counter while traversing the dataset with calls to Next. (In one of my
applications I even have an array of Bookmarks -- I think I overlooked the
RecNo property, which would take away the necessity of using such an array, if
I'm understanding things correctly).
Regards,
Bram Kuijvenhoven
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