Fix the bug that results of where <string> is wrong because it will be converted to int type. (#3463)

[ti-chi-bot]

This is an automated cherry-pick of #3463

What problem does this PR solve?

Issue Number: close #3447

Problem Summary:

The result of where <string> is wrong because it will be converted to int type.

What is changed and how it works?

Proposal: xxx

What's Changed: convert it to double type to keep compatible with tidb/tikv.

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• Need to cherry-pick to the release branch:

Check List

Tests

• Unit test

Side effects

Release note

Fix the bug that results of `where <string>` is wrong because it will be converted to int type.

[ti-chi-bot]

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[ti-chi-bot]

@ti-chi-bot: This cherry pick PR is for a release branch and has not yet been approved by release team.
Adding the do-not-merge/cherry-pick-not-approved label.

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