title: DolphinDB: error occurs running ploop statement
body: <p>When I running the following script in dolphindb, error occurs in the last line:</p>
  <pre class="lang-py prettyprint-override"><code>sym = `C`MS`MS`MS`IBM`IBM`C`C`C$symbol
  price= 49.6 29.46 29.52 30.02 174.97 175.23 50.76 50.32 51.29
  qty = 2200 1900 2100 3200 6800 5400 1300 2500 8800
  timestamp = [09:34:07,09:36:42,09:36:51,09:36:59,09:32:47,09:35:26,09:34:16,09:34:26,09:38:12]
  t0 = table(timestamp, sym, qty, price);
  
  t1 = select  sum((qty))  as qty from t0  group by  minute(timestamp) as ts,sym,price;
  t2 = select  log((qty))  as qty from t0  group by  minute(timestamp) as ts ,sym,price;
  
  name = `ts`q
  typeString = `MINUTE`INT
  tResult=table(1:0,name,typeString)
  share tResult as shareT
  
  ploop(myFunc,([t1,shareT],[t2, shareT]))
  </code></pre>
  <p>What did I understand wrong about the ploop statement?</p>
  <p><a href="https://i.stack.imgur.com/OJfb5.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/OJfb5.png" alt="enter image description here" /></a></p>