Re: Primary keys don't work?

From: Peter Laursen <pl_at_mail1.remove.this.stofanet.dk>
Date: Thu, 20 Sep 2001 18:58:47 +0200
Message-ID: <3baa213d$0$14816$ba624c82@nntp02.dk.telia.net>

Jan Doggen skrev i meddelelsen ...
>1) I cannot determine my own SQL query, because it is a Delphi TTable that
>opens.
<SNIPPED>

Jan,

This is not really an Oracle issue but a BDE/Delphi one. You will get better help in borland.public.database.sqlservers or borland.public.bde on the newsserver
newsgroups.borland.com.

I know its not the answer you are looking for but using BDE, an older Delphi and specifically TTables to access an Oracle database is asking for trouble! TTables were never meant to be used against relational databases. The text below by Wayne Niddery is often qouted in
borland.public.database.sqlserver.

TTables vs. TQuerys

The whole reason there is both a TTable and a TQuery component is due to the
fact there table-oriented databases like Dbase, Paradox, or Access, and
there are set-oriented databases like Interbase, Oracle, and MSSQL. These
different types of database systems work and behave completely different
from one another and the same methods of access cannot be equally applied.

TTable is specifically designed to work best with table-oriented systems -
it is "native" to them. Using a TQuery against such databases is slower
because they do not understand SQL and so the BDE must interpret the SQL and
convert it to table calls for that database.

TQuery is specifically designed to work best with set-oriented databases
that understand SQL directly and were designed to work this way. Using a
TTable against such a system is slower because the BDE must convert the
table functions into SQL instructions to be sent off to the database.

Some of the things that TTables do that eat time and resources over a
network with an SQL system are:
- on Opening, always sends many queries to the database to
get all the
metadata for all fields and indexes in the selected table in order to
provide you with a selection of these (only *Live* TQuerys do this).
- if you have large records with many fields, TTable will
always select ALL
fields even if you only want one or two. This is especially bad if the table
contains blobs and you do not need them.
- Using Locate or FindKey or RecordCount forces all records
to be fetched
because such searching / counting has to be done on the client side. This
can be eased by using a good filter (in the Filter property, not the
OnFilter event) to limit the records that need to be fetched (Filters are
turned into SQL where clauses by the BDE).
- if used in a grid, TTable must frequently execute multiple
queries to fill
the grid whenever you change record positions.
- Tables prevent you from using the power of SQL when
working against a real
SQL server - they only see physical tables (or views in SQL systems),
whereas you can write TQueries to slect any raltionships between and number
of tables and get only *exactly* the data you need.

With TQuerys, you still need to use them right to get the most out of them,
but the point is that you *can* use them right with regard to SQL databases.
- with the exception of extremely small "lookup" type tables
(e.g. State
codes) *always* use where clauses to limit the number of records brought
back, if you do not then you are defeating the whole purpose of using them.
- unless you *really* need to every field in a table, always
specify the
fields you actually need (e.g. "select cust_id, cust_name from...", not
"select * from..."). A tip here is to avoid editing records in a grid, use
grids only for selection. This allows you to only select the minimum fields
needed for selection, then use another query to select all fields for that
ONE selected record for editing purposes.
- Never use the Filter property or OnFilter event, or call
RecordCount with
a TQuery, this forces the entire record set to be fetched. If you really
need the record count, use another query to get it so the server will do the
counting and send back the count itself instead of all the records.

--
Wayne Niddery (WinWright Inc.)
RADBooks - http://members.home.net/wniddery/
I love deadlines. I like the whooshing sound they make as
they pass by -
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Peter Laursen

Received on Thu Sep 20 2001 - 11:58:47 CDT