title: Select on tag return nothing in CnosDB
body: <p>I am puzzled that since a tag is a indexed column, why CnosDB returns nothing when I am doing a select on it.</p>
  <p>Using the sample data <a href="https://cnosdb.github.io/docs/content/cnosdb/latest/cnosql/cnosql_sample.html" rel="nofollow noreferrer">here</a> and running the following query:</p>
  <pre><code>&gt; select * from test_air
  name: test_air
  time                         pressure station     temperature visibility
  ----                         -------- -------     ----------- ----------
  2022-07-01T08:42:04.4103282Z 75       XiaoMaiDao  79          68
  2022-07-01T08:42:09.6582015Z 78       LianYunGang 63          71
  2022-07-01T08:43:24.5802497Z 79       LianYunGang 63          71
  2022-07-01T08:43:24.6005666Z 79       LianYunGang 53          71
  2022-07-01T08:43:24.6228182Z 90       LianYunGang 63          71
  2022-07-01T08:43:24.6451171Z 75       XiaoMaiDao  79          78
  2022-07-01T08:43:25.2744992Z 75       XiaoMaiDao  77          78
  2022-07-05T11:34:04.8420635Z          XiaoMaiDao  70          85
  &gt; select station from test_air
  </code></pre>
  <p>Shouldn't it return the two tag values - XiaoMaiDao and LianYunGang instead?</p>