executor, plan: fix a bug occurred in `set @varname = (select a from …

…t limit 1)` (pingcap#4326)

* fix infoschema not support unsigned

* change the comment of FieldType's InfoSchemaStr for specification

* fix issue pingcap#4302

* use mockPlan instend of process subquery directly

* add test case for issue pingcap#4302

executor/set_test.go

@@ -80,6 +80,13 @@ func (s *testSuite) TestSetVar(c *C) {
 	tk.MustExec(testSQL)
 	tk.MustQuery(`select @issue998b, @@global.autocommit;`).Check(testkit.Rows("6 1"))
 
+	// For issue 4302
+	testSQL = "use test;drop table if exists x;create table x(a int);insert into x value(1);"
+	tk.MustExec(testSQL)
+	testSQL = "SET @issue4302=(select a from x limit 1);"
+	tk.MustExec(testSQL)
+	tk.MustQuery(`select @issue4302;`).Check(testkit.Rows("1"))
+
 	// Set default
 	// {ScopeGlobal | ScopeSession, "low_priority_updates", "OFF"},
 	// For global var

plan/planbuilder.go

@@ -219,7 +219,9 @@ func (b *planBuilder) buildSet(v *ast.SetStmt) Plan {
 			IsSystem: vars.IsSystem,
 		}
 		if _, ok := vars.Value.(*ast.DefaultExpr); !ok {
-			assign.Expr, _, b.err = b.rewrite(vars.Value, nil, nil, true)
+			mockTablePlan := TableDual{}.init(b.allocator, b.ctx)
+			mockTablePlan.SetSchema(expression.NewSchema())
+			assign.Expr, _, b.err = b.rewrite(vars.Value, mockTablePlan, nil, true)
 			if b.err != nil {
 				return nil
 			}