title: Error occured when calling ffill in DolphinDB reactive state engine
body: <p>Please tell me why the error occurred when I called <code>&lt;ffill(Ticker)&gt;</code> in DolphinDB reactive state engine. I also used <code>select ffill(Ticker)</code> in MySQL and it returned the expected result.</p>
  <pre><code>//Code `Time`Date`Ticker`Open`NumberOfTrades
  //`SYMBOL`TIME`DATE`SYMBOL`FLOAT`INT
  metrics = array (ANY, 5)
  metrics[0] = &lt;Time&gt;
  metrics[1] = &lt;Date&gt;
  metrics[2] = &lt;ffill(Ticker)&gt; // &lt;Error, &lt;Ticker&gt; ok
  metrics[3] = &lt;ffill(Open)&gt;
  metrics[4] = &lt;ffill(NumberOfTrades)&gt;
  
  select Ticker, ffill(Ticker) as TickerFill from toutput //ok
  
  tt = createReactiveStateEngine(name=&quot;Demo&quot;, metrics=metrics, dummyTable=tinput, outputTable=toutput, keyColumn=`Code)
  
  subscribeTable(tableName=`tinput, actionName=&quot;testsub&quot;, handler=tableInsert{tt})
  </code></pre>