Fix Impossible Query to account for Group By clauses

[bbeaudreault]

We had a query which uses an aggregation and group by, and our mysql uses sql_mode=only_full_group_by. The impossible query generated by vitess did not include the group by clause, and so the query violated the conditions of only_full_group_by:

In aggregated query without GROUP BY, expression #1 of SELECT list contains nonaggregated column 'Foo.sr.id'; this is incompatible with sql_mode=only_full_group_by

This PR does three things:

• Include the GroupBy in the select query, if the GroupBy is not nil
• Refactors the code path so that both the vttablet and vtgate use the same impossible query generation code.
• Update all tests to expect a group by in the generated field queries

In terms of refactoring, creating a new file in the sqlparser packaged seemed cleanest. I have not created new tests for this file, because it seems to be covered by the existing planbuilder and route tests. But if you'd like me to add new tests, I can do so.

Note: my CLA should be ready some time this week, waiting on legal department

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
• If you signed the CLA as a corporation, please let us know the company's name.

[sougou]

I think this part can be simplified. How about something like this instead:

formatter := func(buf *sqlparser.TrackedBuffer, node sqlparser.SQLNode) {
  switch node := node.(type) {
  case *sqlparser.ColName:
    // like before
  case * sqlparser.TableName:
    // like before
  }
  // Call impossible query formatter instead of node.Format.
  sqlparser.ImpossibleQueryFormatter(buf, node)
}

Then, all you need to do is implement ImpossibleQueryFormatter in the parser.

In the case of vttablet, you can directly instantiate sqlparser.NewTrackedBuffer(sqlparser.ImpossibleQueryFormatter).

[bbeaudreault]

FYI my Google CLA has been signed by the VP of Engineering at my company. Company name is HubSpot, Inc

[sougou]

@michael-berlin has limited availability. We can have him check on this when he comes back.

[bbeaudreault]

Looks like our CLA was confirmed, I got an email and see it active at https://cla.developers.google.com/clas. Thanks @michael-berlin!

I'll be wrapping up this PR soon, have been stuck doing some other things. Thanks for the comments.

[googlebot]

CLAs look good, thanks!

[bbeaudreault]

@sougou simplified

[bbeaudreault]

thanks!