planner: fix union statements order

[mtunique]

What problem does this PR solve?

Fix #8202 union statements order because union result schema info isn't correct.

What is changed and how it works?

Because union schema is the same to its first children. MySQL UNION Syntax

func (b *PlanBuilder) buildProjection4Union(u *LogicalUnionAll) {
	unionCols := make([]*expression.Column, 0, u.children[0].Schema().Len())

	// Infer union result types by its children's schema.
	for i, col := range u.children[0].Schema().Columns {
		resultTp := col.RetType
		for j := 1; j < len(u.children); j++ {
			childTp := u.children[j].Schema().Columns[i].RetType
			resultTp = unionJoinFieldType(resultTp, childTp)
		}
		unionCols = append(unionCols, &expression.Column{
			ColName:  col.ColName,
			TblName:  col.TblName,
			RetType:  resultTp,
			UniqueID: b.ctx.GetSessionVars().AllocPlanColumnID(),
		})
	}

But when mix UNION ALL and UNION DISTINCT in the same query, allSelectPlans is in front of unionDistinctPlan. When call buildUnionAll , the schema become incorrect. We should change the order, it should be same to SQL.

func (b *PlanBuilder) buildUnion(union *ast.UnionStmt) (LogicalPlan, error) {
	distinctSelectPlans, allSelectPlans, err := b.divideUnionSelectPlans(union.SelectList.Selects)
	if err != nil {
		return nil, errors.Trace(err)
	}

	unionDistinctPlan := b.buildUnionAll(distinctSelectPlans)
	if unionDistinctPlan != nil {
		unionDistinctPlan = b.buildDistinct(unionDistinctPlan, unionDistinctPlan.Schema().Len())
		if len(allSelectPlans) > 0 {
			allSelectPlans = append(allSelectPlans, unionDistinctPlan)
		}
	}
	unionAllPlan := b.buildUnionAll(allSelectPlans)

Check List

Tests

• Unit test
  Related changes

• Need to cherry-pick to the release branch

[sre-bot]

Hi contributor, thanks for your PR.

This patch needs to be approved by someone of admins. They should reply with "/ok-to-test" to accept this PR for running test automatically.

[zz-jason]

LGTM

[zz-jason]

@lysu @winoros @eurekaka PTAL

[winoros]

@mtunique
You can use fix issue link grammer in your pr description so that issue can be auto closed when this pr is merged.
See https://help.github.com/articles/closing-issues-using-keywords/

[zz-jason]

@mtunique Should we cherrypick this PR to the release-2.1 and release-2.0 branch?

[mtunique]

@zz-jason Maybe it's necessary.

[mtunique]

@mtunique
You can use fix issue link grammer in your pr description so that issue can be auto closed when this pr is merged.
See https://help.github.com/articles/closing-issues-using-keywords/

Done.

[winoros]

lgtm

[winoros]

/run-all-tests

[XuHuaiyu]

LGTM

[XuHuaiyu]

@mtunique
And we should also cherry-pick this to release-2.0.

It should be noted that, the directory structure planner/core in master corresponds to the directory plan in release-2.0.

[mtunique]

@mtunique
And we should also cherry-pick this to release-2.0.

It should be noted that, the directory structure planner/core in master corresponds to the directory plan in release-2.0.

Done. #8318