Statistics
https://play.clickhouse.com/play?user=play#c2VsZWN0IAp0b1N0YXJ0T2ZIb3VyKGNoZWNrX3N0YXJ0X3RpbWUpIGFzIGQsCmNvdW50KCksICBncm91cFVuaXFBcnJheShwdWxsX3JlcXVlc3RfbnVtYmVyKSwgIGFueShyZXBvcnRfdXJsKQpmcm9tIGNoZWNrcyB3aGVyZSAnMjAyMi0wNi0wMScgPD0gY2hlY2tfc3RhcnRfdGltZSBhbmQgdGVzdF9uYW1lIGxpa2UgJyUwMjQ5MV9wYXJ0X2xvZ19oYXNfdGFibGVfdXVpZCUnIGFuZCB0ZXN0X3N0YXR1cyBpbiAoJ0ZBSUwnLCAnRkxBS1knKSBncm91cCBieSBkIG9yZGVyIGJ5IGQgZGVzYw==

Some times there are not output from requests.
{
    if (txn)
        return txn->tid;
    return Tx::PrehistoricTID;
}
void MergeTreeData::Transaction::addPart(MutableDataPartPtr & part)
{
    precommitted_parts.insert(part);
}
void MergeTreeData::Transaction::rollback()
{
    if (!isEmpty())
    {
        WriteBufferFromOwnString buf;
        buf << "Removing parts:";
        for (const auto & part : precommitted_parts)
            buf << " " << part->getDataPartStorage().getPartDirectory();
        buf << ".";
        LOG_DEBUG(data.log, "Undoing transaction {}. {}", getTID(), buf.str());