*: remove evaluate.Eval from ddl (pingcap#2264)

ddl/ddl.go

@@ -34,6 +34,7 @@ import (
 	"github.com/pingcap/tidb/meta/autoid"
 	"github.com/pingcap/tidb/model"
 	"github.com/pingcap/tidb/mysql"
+	"github.com/pingcap/tidb/plan"
 	"github.com/pingcap/tidb/sessionctx/variable"
 	"github.com/pingcap/tidb/table"
 	"github.com/pingcap/tidb/terror"
@@ -541,7 +542,7 @@ func columnDefToCol(ctx context.Context, offset int, colDef *ast.ColumnDef) (*ta
 				col.Flag |= mysql.OnUpdateNowFlag
 				setOnUpdateNow = true
 			case ast.ColumnOptionComment:
-				value, err := evaluator.Eval(ctx, v.Expr)
+				value, err := plan.EvalAstExpr(v.Expr, ctx)
 				if err != nil {
 					return nil, nil, errors.Trace(err)
 				}
@@ -591,7 +592,7 @@ func getDefaultValue(ctx context.Context, c *ast.ColumnOption, tp byte, fsp int)
 
 		return value, nil
 	}
-	v, err := evaluator.Eval(ctx, c.Expr)
+	v, err := plan.EvalAstExpr(c.Expr, ctx)
 	if err != nil {
 		return nil, errors.Trace(err)
 	}

plan/expression_rewriter.go

@@ -20,6 +20,24 @@ import (
 // EvalSubquery evaluates incorrelated subqueries once.
 var EvalSubquery func(p PhysicalPlan, is infoschema.InfoSchema, ctx context.Context) ([]types.Datum, error)
 
+// EvalAstExpr evaluates ast expression directly.
+func EvalAstExpr(expr ast.ExprNode, ctx context.Context) (types.Datum, error) {
+	if val, ok := expr.(*ast.ValueExpr); ok {
+		return val.Datum, nil
+	}
+	b := &planBuilder{
+		ctx:       ctx,
+		allocator: new(idAllocator),
+		colMapper: make(map[*ast.ColumnNameExpr]int),
+		is:        ctx.GetSessionVars().TxnCtx.InfoSchema.(infoschema.InfoSchema),
+	}
+	newExpr, _, err := b.rewrite(expr, nil, nil, true)
+	if err != nil {
+		return types.Datum{}, errors.Trace(err)
+	}
+	return newExpr.Eval(nil, ctx)
+}
+
 // rewrite function rewrites ast expr to expression.Expression.
 // aggMapper maps ast.AggregateFuncExpr to the columns offset in p's output schema.
 // asScalar means whether this expression must be treated as a scalar expression.