🚀 go-pugleaf

"Tim" <tim.hoebeek@NOSPAM.advalvas.be> schreef in bericht
news:u33cfsojbhlif6g35ba31k27pgqhm11j4s@4ax.com...
> >I can see you copied this from the example...
>
> Well, that was the point I started from, but I have difficulties
> seeing the bigger picture. I think I understand what all the
> statements individually do, but I haven't got a clue about how all
> those statements relate to each other and whom is a descendant of
> whom.
>
> >The example shows transactions AND cached updates. You are not using
cached
> >updates, so you would only use:
>
> What is the defference between the two and can't they be mingled? I
> thought one was using cached updates when the CachedUpdates property
> of the TQuery component was set to true.

Transaction handling is a feature of the database you are using. If your
database doesn't support transactions or if you don't want to burden the
database server with the overhead of transactions you can use local caching.
With local caching, all updates to a specified dataset are stored and only
executed on the database with ApplyUpdates. Cached changes are not really
stored with the table, but locally to the dataset, so if you retreive data
from the same table with another dataset you will get the old values ! For
transaction handling you can specify weither you want to read old or new
data with the TransIsolation property of the TDatabase.

>
> >Database.StartTransaction;
> >try
> >  Query.ExecSQL;
> >   Database.Commit;
>
> Aren't those SQL statementsexecuted at that moment and commited? Does
> StartTransaction put some kind of marker in the database, so it knows
> to what place it should roll back?

StartTransaction initiates a kind of separated memory space for you to store
your changes. Those changes are applied to the complete database on a
Commit. The memory is discarded with a Rollback.

>
> >   Ok := True;
> >except
> >   Database1.Rollback;
> >   Ok := False;
> >end;
>
> In the other case the rollback is executed and none of the code is
> executed, right?

Yes

>
> Would it be possible then to start a transaction, call the same
> procedure for a couple of times (running inserts against the database)
> from within the try block and when anything goes wrong, performing a
> rollback as if nothing happened?

You can only call StartTransaction ONCE. After StartTransaction you can make
all the changes you want (as long as the database server has enough memory
available). Finally you should either call Commit or Rollback.

>
> What's this 'Database' then in Database.StartTransaction? Is it a
> database from the type TDatabase?

Yes. You will need a separate TDatabase object to call transaction handling
routines.

>
> The fog in my head is clearing now, but some things remain unclear.
>
> Thank you very much for your help,
>
>
> Tim.
>