Is it possible to use db other than leveldb for bitcoin core? I'd like to make queries on running node and make views with decoded data
There is no code for that, and it would be either very slow or nearly useless.
Bitcoin Core assumes it has exclusive access to the -database, and caches data very aggresively. It may take hours before actual data from the chain is on disk.
The data structures are very compact and specifically designed for efficient use in Bitcoin Core. This means only unspent transaction outputs and block/transaction disk location are written to the database, in a custom byte-level format. You'd have to write your own parsers for these. In the 0.12 release most of this data will be scrambled by default even, to thwart overly aggressive virus scanners that detect virus patterns inside the database.
In short, that's not what the database is intended for. If you want something to index the blockchain or query the UTXO set, there exist other projects that build databases specifically designed for that.