FLASH-479 select from empty table throw error in tiflash#223

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zanmato1984 merged 2 commits intopingcap:copfrom windtalker:empty_table

Sep 6, 2019

Commits on Sep 6, 2019

1. select from empty table throw error in tiflash, 2. add some logs, 3. disable timestamp literal in DAG request
windtalker
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revert unrelated change
windtalker
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