Enable pushdown for json_unquote

[kolbe]

Feature Request

Is your feature request related to a problem? Please describe:

For some reason, 721782d "forbid json unquote push down".

There's no indication of why it was necessary to forbid this or whether there is any feasible fix for the issue. We should address this limitation so that json_unquote can be pushed down.

[kolbe]

@lonng can you provide any additional information about why it was necessary to forbid pushdown of json_unquote to the coprocessor?

[lonng]

@kolbe I forbade the json_unquote pushdown switcher because I found the implementation inconsistent between TiKV and TiDB when I develop the copr-test testing framework last year. I just simply comment the json_unquote to make the subsequent works going on because I have no time to take an investigation at that time.

I am not sure does this inconsistent have been fixed in the last year. So I think we can uncomment the json_unquote to check the copr-test result.

And it's time to take an investigation if the json_unquote function if it's necessary for some scenarios.

[guo-shaoge]

The pushdown of json_unquote will be enabled after this PR is merged.

[kolbe]

@guo-shaoge can we demonstrate that this passes any set of tests to verify correctness? Are there any MySQL or MariaDB tests we can adopt?

[guo-shaoge]

@guo-shaoge can we demonstrate that this passes any set of tests to verify correctness? Are there any MySQL or MariaDB tests we can adopt?

We have some tests for json_unquote in copr-test and I verified that these cases are passed.

But the test coverage is very small(just 50 SQL). So maybe we should only enable the pushdown on master branch. And add more testing for json_XXX before next release.