Fix show processlist panic (#1991)

Author: max-hoffman

enginetest/queries/queries.go

@@ -742,6 +742,10 @@ var SpatialQueryTests = []QueryTest{
 }
 
 var QueryTests = []QueryTest{
+	{
+		Query:    "show full processlist",
+		Expected: []sql.Row{},
+	},
 	{
 		Query: "select * from (select i, i2 from niltable) a(x,y) union select * from (select 1, NULL) b(x,y) union select * from (select i, i2 from niltable) c(x,y)",
 		ExpectedColumns: sql.Schema{

sql/planbuilder/show.go

@@ -34,6 +34,7 @@ func (b *Builder) buildShow(inScope *scope, s *ast.Show, query string) (outScope
 	showType := strings.ToLower(s.Type)
 	switch showType {
 	case "processlist":
+		outScope = inScope.push()
 		outScope.node = plan.NewShowProcessList()
 	case ast.CreateTableStr, "create view":
 		return b.buildShowTable(inScope, s, showType)