force index in Iteration Range End Value queries to fix execution plans

[wangzihuacool]

I recently found that gh-ost occasionally executes slowly in MySQL 8.0.
When copying data to the _gho table, querys for Iteration Range End Values occationally use the wrong index due to inappropriate optimizer selection.

I think we can force the unique-key index to avoid inappropriate optimizer selection, in BuildUniqueKeyRangeEndPreparedQueryViaOffset and BuildUniqueKeyRangeEndPreparedQueryViaTemptable. Just like unique key range queries do in #1237 .

I'd like to contribute the patch, if you agree.

[timvaillancourt]

@wangzihuacool thanks for this report. I believe #1237 will resolve this issue, can you please confirm?

[timvaillancourt]

in BuildUniqueKeyRangeEndPreparedQueryViaOffset and BuildUniqueKeyRangeEndPreparedQueryViaTemptable

@wangzihuacool sorry I misread your issue summary, this is similar but different to #1237 as you mentioned 👍

I'd like to contribute the patch, if you agree.

A PR would be appreciated thanks!

[wangzihuacool]

I have created a PR #1245 , hope you take the time to review it.

[timvaillancourt]

@wangzihuacool I believe this is resolved by #1237. Please test and re-open if this is not the case. Thanks!