planner: fix the fail when we compare multi fields in the subquery (#21699)#21808

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qw4990 merged 3 commits intopingcap:release-4.0from ti-srebot:release-4.0-e01a1422377c

Jan 26, 2021

+36

Commits on Dec 15, 2020

cherry pick pingcap#21699 to release-4.0

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Commits on Jan 26, 2021