planner: eliminate if null on non null column

cmd/explaintest/r/explain_easy.result

@@ -385,3 +385,40 @@ Projection_5	8000.00	root	test.ta.a
     └─TableReader_9	10000.00	root	data:TableScan_8
       └─TableScan_8	10000.00	cop	table:ta, range:[-inf,+inf], keep order:false, stats:pseudo
 rollback;
+drop table if exists t;
+create table t(a int, nb int not null, nc int not null);
+explain select ifnull(a, 0) from t;
+id	count	task	operator info
+Projection_3	10000.00	root	ifnull(test.t.a, 0)
+└─TableReader_5	10000.00	root	data:TableScan_4
+  └─TableScan_4	10000.00	cop	table:t, range:[-inf,+inf], keep order:false, stats:pseudo
+explain select ifnull(nb, 0) from t;
+id	count	task	operator info
+TableReader_5	10000.00	root	data:TableScan_4
+└─TableScan_4	10000.00	cop	table:t, range:[-inf,+inf], keep order:false, stats:pseudo
+explain select ifnull(nb, 0), ifnull(nc, 0) from t;
+id	count	task	operator info
+TableReader_5	10000.00	root	data:TableScan_4
+└─TableScan_4	10000.00	cop	table:t, range:[-inf,+inf], keep order:false, stats:pseudo
+explain select ifnull(a, 0), ifnull(nb, 0) from t;
+id	count	task	operator info
+Projection_3	10000.00	root	ifnull(test.t.a, 0), test.t.nb
+└─TableReader_5	10000.00	root	data:TableScan_4
+  └─TableScan_4	10000.00	cop	table:t, range:[-inf,+inf], keep order:false, stats:pseudo
+explain select ifnull(nb, 0), ifnull(nb, 0) from t;
+id	count	task	operator info
+Projection_3	10000.00	root	test.t.nb, test.t.nb
+└─TableReader_5	10000.00	root	data:TableScan_4
+  └─TableScan_4	10000.00	cop	table:t, range:[-inf,+inf], keep order:false, stats:pseudo
+explain select 1+ifnull(nb, 0) from t;
+id	count	task	operator info
+Projection_3	10000.00	root	plus(1, test.t.nb)
+└─TableReader_5	10000.00	root	data:TableScan_4
+  └─TableScan_4	10000.00	cop	table:t, range:[-inf,+inf], keep order:false, stats:pseudo
+explain select 1+ifnull(a, 0) from t;
+id	count	task	operator info
+Projection_3	10000.00	root	plus(1, ifnull(test.t.a, 0))
+└─TableReader_5	10000.00	root	data:TableScan_4
+  └─TableScan_4	10000.00	cop	table:t, range:[-inf,+inf], keep order:false, stats:pseudo
+drop table if exists t;
+drop table if exists t;

cmd/explaintest/t/explain_easy.test

@@ -81,3 +81,15 @@ begin;
 insert tb values ('1');
 explain select * from ta where a = 1;
 rollback;
+
+# https://github.com/pingcap/tidb/issues/7918
+drop table if exists t;
+create table t(a int, nb int not null, nc int not null);
+explain select ifnull(a, 0) from t;
+explain select ifnull(nb, 0) from t;
+explain select ifnull(nb, 0), ifnull(nc, 0) from t;
+explain select ifnull(a, 0), ifnull(nb, 0) from t;
+explain select ifnull(nb, 0), ifnull(nb, 0) from t;
+explain select 1+ifnull(nb, 0) from t;
+explain select 1+ifnull(a, 0) from t;
+drop table if exists t;

expression/builtin.go

@@ -162,6 +162,10 @@ func (b *baseBuiltinFunc) getArgs() []Expression {
 	return b.args
 }
 
+func (b *baseBuiltinFunc) setArgs(args []Expression) {
+	b.args = args
+}
+
 func (b *baseBuiltinFunc) evalInt(row chunk.Row) (int64, bool, error) {
 	panic("baseBuiltinFunc.evalInt() should never be called.")
 }
@@ -274,6 +278,8 @@ type builtinFunc interface {
 	evalJSON(row chunk.Row) (val json.BinaryJSON, isNull bool, err error)
 	// getArgs returns the arguments expressions.
 	getArgs() []Expression
+	// setArgs set the arguments expressions to builtFunc.
+	setArgs(args []Expression)
 	// equal check if this function equals to another function.
 	equal(builtinFunc) bool
 	// getCtx returns this function's context.

expression/scalar_function.go

@@ -46,6 +46,11 @@ func (sf *ScalarFunction) GetArgs() []Expression {
 	return sf.Function.getArgs()
 }
 
+// SetArgs sets arguments of function.
+func (sf *ScalarFunction) SetArgs(args []Expression) {
+	sf.Function.setArgs(args)
+}
+
 // GetCtx gets the context of function.
 func (sf *ScalarFunction) GetCtx() sessionctx.Context {
 	return sf.Function.getCtx()

planner/core/logical_plan_builder.go

@@ -576,6 +576,38 @@ func (b *PlanBuilder) buildProjectionField(id, position int, field *ast.SelectFi
 	}
 }
 
+func eliminateIfNullOnNotNullCol(p LogicalPlan, expr expression.Expression) expression.Expression {
+	ds, isDs := p.(*DataSource)
+	if !isDs {
+		return expr
+	}
+
+	scalarExpr, isScalarFunc := expr.(*expression.ScalarFunction)
+	if !isScalarFunc {
+		return expr
+	}
+	exprChildren := scalarExpr.GetArgs()
+	for i := 0; i < len(exprChildren); i++ {
+		exprChildren[i] = eliminateIfNullOnNotNullCol(p, exprChildren[i])
+	}
+
+	if scalarExpr.FuncName.L == ast.Ifnull {
+		colRef, isColRef := exprChildren[0].(*expression.Column)
+		if !isColRef {
+			return expr
+		}
+
+		colInfo := model.FindColumnInfo(ds.Columns, colRef.ColName.L)
+		if !mysql.HasNotNullFlag(colInfo.Flag) {
+			return expr
+		}
+
+		return colRef
+	}
+	scalarExpr.SetArgs(exprChildren)
+	return scalarExpr
+}
+
 // buildProjection returns a Projection plan and non-aux columns length.
 func (b *PlanBuilder) buildProjection(p LogicalPlan, fields []*ast.SelectField, mapper map[*ast.AggregateFuncExpr]int) (LogicalPlan, int, error) {
 	b.optFlag |= flagEliminateProjection
@@ -585,6 +617,7 @@ func (b *PlanBuilder) buildProjection(p LogicalPlan, fields []*ast.SelectField,
 	oldLen := 0
 	for _, field := range fields {
 		newExpr, np, err := b.rewrite(field.Expr, p, mapper, true)
+		newExpr = eliminateIfNullOnNotNullCol(p, newExpr)
 		if err != nil {
 			return nil, 0, errors.Trace(err)
 		}