> May be this is needed to support savepoints/subtransactions? Or is it
> something else that i am missing?
It's for two-phase commit. A prepared transaction can hold locks that need
to be recovered.
When a transaction enters (successfully) the prepared state it only retains its exclusive locks and releases any shared locks (i.e. multixacts)... or, at least, that's how it should be in principle according to serializiaton theory, i haven't yet checked out if this is what is done in postgresql .
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