After brief look into bitcoinj library, I found out that wallet class keeps all transactions in one big hash map. That make me think about transaction limits of this library. How many transaction can I make with multibit wallet on computer with just 1 gig of RAM ?
I noticed that there is BlockStore implementation for Postgres and other SQL servers but seems to me it's only for mem pool transactions. Wallet transaction are all in memory and saved into .wallet file.