[fpc-devel]Some questions about tdataset and a small patch

Joost van der Sluis joost at cnoc.nl
Sun Aug 1 13:32:22 CEST 2004


I want to discuss something before i make all sort of changes...

Now TDataset uses a buffer of default 10 records. If you go to the 11th
record, the first record is removed from memory. If you go to the first
record, all buffers are cleared, and then they are read again from the
first record. All this is done by tdataset.

This works good for tdbf but it gives some problems with SQL-based
descendants, especially those for DB-servers that don't support
bidirectional cursors. (like Firebird)

If you fetch records from a table using a sql-query, you'll get all the
records, beginning with the first one, then the second one etcetera. But
you can't go back. If you've just fetched the 1000 record, and you need
record 999, you have to start all over again, beginning with record number

This gives some problems if you're buffer is holding the records
1000-1010. If you scroll back from record 999 to 1 the use of bandwith
will be huge.

The solution is to hold all records into memory once they are
fetched. At first sight you will say that that cost an huge amount of
memory, but i doesn't. Because if you used the SQL server right, you only
ask for the records you need by using a 'where' statement.

I see three solutions:

1: Make the tdataset descendents so that they do the fetch from the first
record if you want the prior record. Which is the worst solution, I hope
you all agree with that. (It's the current solution, though. Only it don't
2: Leave the creation and maintenance of the buffer to the tdataset
descendants, in stead of tdataset itself. So that every tdataset
descendent can use it's own buffer-system. (This solution is the one i was
trying, and it can be done. Some small changes to tdbf are needed,
though. And all other descendents have to been looked at too.)
3: Add a property to TDataset, which changes the behaviour of the buffers,
so that they are 'infinitive'. All fetched records are kept in memory. The
descendents that need this can set this property in their
create-functions. Delphi has something like this, if you set the
buffercount to -1.

I think i'm going to try to implement option three. But what do you all
think about this issue?

Further i've fixed a bug that made it impossible to show an integer field
in a tdbedit.

fields.inc: tlongintfield.gettext now uses getvalue
so that the typecasting is used correctly.

diff is attached.

  Joost van der Sluis.

-------------- next part --------------
Index: fields.inc
RCS file: /FPC/CVS/fpc/fcl/db/fields.inc,v
retrieving revision 1.13
diff -u -r1.13 fields.inc
--- fields.inc	19 Jul 2004 20:27:29 -0000	1.13
+++ fields.inc	1 Aug 2004 09:06:44 -0000
@@ -855,7 +855,7 @@
-  If Not GetData(@l) then exit;
+  If Not GetValue(l) then exit;
   If ADisplayText or (FEditFormat='') then

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