I was trying to use getrawmempool:
bitcoin-cli getrawmempool
But it just returns empty result:
[]
I am running bitcoind in prune mode. Could this be the reason or is there another explanation?
No, running bitcoind in prune mode indeed does not let you use "getrawmempool".
See the answer below which describes it better than i ever could.
The mempool is where transactions are stored until they get put into a block. It would be hard to determine you had the correct mempool if you didn't keep track of the whole block chain, though. For example, someone could broadcast an older transaction, and you would have no way to determine that you shouldn't just add it to your mempool if you didn't keep track of the whole block chain. Transactions also have to pass some basic validation before they can be added to the mempool, validation which requires having all the blockchain data.
If there were a program to implement this, it would at least have to process all new blocks as they came in, in order to determine which transactions to take out of the mempool as blocks get solved.
https://bitcoin.stackexchange.com/questions/35811/bitcoind-getrawmempool-without-downloading-the-full-blockchain