virtualidx exclusive lock

I've googled, but there's 0 hits.

I have an issue with a ton of "idle in transaction" backends.
What I noticed is when I look at pg_locks, pretty much all of the processes being idle in transaction have an exclusive lock of locktype "virtualidx".

Well, that doesn't make sense to me, but maybe someone here can tell me where a "virtualidx" locktype would come from. I'm sure it has to be some type of query.
There is no info about table or anything, all the records look like:

locktype | database | relation | page | tuple | virtualxid | transactionid | classid | objid | objsubid | virtualtransaction | pid | mode | granted
---------------+----------+----------+------+-------+------------+---------------+---------+-------+----------+--------------------+-------+------------------+---------
virtualxid | | | | | 63/10150 | | | | | 63/10150 | 31932 | ExclusiveLock | t
virtualxid | | | | | 48/48530 | | | | | 48/48530 | 31323 | ExclusiveLock | t
virtualxid | | | | | 47/52387 | | | | | 47/52387 | 31321 | ExclusiveLock | t
virtualxid | | | | | 76/4086 | | | | | 76/4086 | 32074 | ExclusiveLock | t
virtualxid | | | | | 15/6007096 | | | | | 15/6007096 | 31169 | ExclusiveLock | t
virtualxid | | | | | 10/5689919 | | | | | 10/5689919 | 31595 | ExclusiveLock | t
virtualxid | | | | | 32/603998 | | | | | 32/603998 | 31213 | ExclusiveLock | t
virtualxid | | | | | 42/117511 | | | | | 42/117511 | 31270 | ExclusiveLock | t
virtualxid | | | | | 39/279415 | | | | | 39/279415 | 31267 | ExclusiveLock | t

Is there a way to find out what query, or in lack of that at least what table is involved?

Thanks

Uwe