[yunqa.de] Re: Determining size of TDISQLite3WideStringField
- From: Delphi Inspiration <delphi@xxxxxxxx>
- To: yunqa@xxxxxxxxxxxxx
- Date: Sat, 05 Apr 2008 20:00:25 +0200
Yeoh Ray Mond wrote:
>How can I retrieve the size of a specific column in a SQLite table using
>DISQLite? I am retrieving data, and need to store a subset of that data in an
>in-memory table. In order to create that table, I need to know the
>definitions of each field. For a field identified as a
>TDISQLite3WideStringField, the size returned is always 20.
Your question points to an old TDataSet limitation which dates back to when
database could store fixed length strings only. To support those engines (BDE
mostly), a TStringField.Size property to limit the length of string fields made
complete sense.
Not so for DISQLite3! SQLite has no practical limits on string fields: It
happily stores strings from 0 to 1000000000 characters in length. But even this
number is only intended to avoid abuse -- the technical limit is as high as
2E31-1.
So the answer to your question is: There is no maximum field size in DISQLite3,
and TDISQLite3WideStringField takes care to reflect this by storing strings of
any length, no mattern what its Size property specifies. The
TDISQLite3WideStringField.Size = 20 only reflects the default size for
TStringField.Size to give applications some posivite value instead of just
returning 0.
>Is it possible to retrieve the maximum size of that field in the returned
>result set, without having to calculate it manually?
No, this is not provided for because it could cause data loss when retrieving
strings from the database which exceed the maximum size. So if you really must
set a limit for your in-memory-table, you need to scan through all records or
execute "SELECT max(length(StringField)) FROM Table;"
As an alternative, you might just want to set the field size to the maximum
value. Best would of course be an in-memory table implementation which
allocates memory only as required and/or does not pose any limitations on
string field size in the first place.
Ralf
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