Thomas Steinmaurer wrote:

> Hello,
>
> I'm not sure how to interpret the following result in the monitoring
> tables regarding some transaction counters. I'm querying the monitoring
> tables of a Firebird 2.1.2 database (Classic on Windows) with an
> explicitely started client transaction (using IBO) set to read-only,
> repeatable read, auto-commit = false.
>
> With the following result.
>
> * In MON$DATABASE
>
> MON$OLDEST_TRANSACTION => 29117
> MON$OLDEST_ACTIVE => 29710
> MON$OLDEST_SNAPSHOT => 29710
> MON$NEXT_TRANSACTION => 29710
>
> * In MON$TRANSACTIONS
>
> I see two records with the following MON$TRANSACTION_IDs:
>
> 29710

This is the your transaction

> 29698

My guess is that this is an IBO's service read-committed
readonly transaction.

> Both marked with MON$STATE = 1
>
>
> * Querying the header page gives the following result:
>
> OIT: 29117
> OAT: 29709
> OST: 29709
> NT: 29710
>
>
> A few questions:
>
> - Why isn't there a gap between MON$OLDEST_ACTIVE and
> MON$NEXT_TRANSACTION? I would expect at least a gap of 1.

Tx numbers are recalculated at any transaction start. The
process run as follows :
- lock and read header page
- increment Next value
- assign Next value to the new tx number
- update OIT\OAT\OST by values from in-memory variables
- release and write header page
- create transaction lock
- calculate new values of OIT\OAT\OST and save it into in-memory variables

As you can see all tx numbers (except of Next) have the latest
values at in-memory variables and previous values at header page.

Should i say that MON$DATABASE reflects in-memory variables ? :)

> - Why do I see a record in MON$TRANSACTIONS with MON$TRANSACTION_ID =
> 29698 but not in any transaction counters either via MON$DATABASE or
> querying the header page, even if this transaction in MON$TRANSACTIONS
> is marked as active (MON$STATE = 1?)

If my guess is correct, then
a) as RC RO transaction is pre-committed (marked as committed in TIP at
tx start) it not affected OAT\OIT\OST
b) as this transaction is still active you see it in MON$TRANSACTIONS

> - Why is there a difference regarding transaction counters when querying
> MON$DATABASE and the header page?

See above

Regards,
Vlad