builtin: add lpad built-in function (#2838)

expression/builtin_string.go

@@ -1599,5 +1599,37 @@ type builtinLpadSig struct {
 
 // See https://dev.mysql.com/doc/refman/5.6/en/string-functions.html#function_lpad
 func (b *builtinLpadSig) eval(row []types.Datum) (d types.Datum, err error) {
-	return d, errFunctionNotExists.GenByArgs("lpad")
+	args, err := b.evalArgs(row)
+	if err != nil {
+		return types.Datum{}, errors.Trace(err)
+	}
+	// LPAD(str,len,padstr)
+	// args[0] string, args[1] int, args[2] string
+	str, err := args[0].ToString()
+	if err != nil {
+		return d, errors.Trace(err)
+	}
+	length, err := args[1].ToInt64(b.ctx.GetSessionVars().StmtCtx)
+	if err != nil {
+		return d, errors.Trace(err)
+	}
+	l := int(length)
+
+	padStr, err := args[2].ToString()
+	if err != nil {
+		return d, errors.Trace(err)
+	}
+
+	if l < 0 || (len(str) < l && padStr == "") {
+		return d, nil
+	}
+
+	tailLen := l - len(str)
+	if tailLen > 0 {
+		repeatCount := tailLen/len(padStr) + 1
+		str = strings.Repeat(padStr, repeatCount)[:tailLen] + str
+	}
+	d.SetString(str[:l])
+
+	return d, nil
 }

expression/builtin_string_test.go

@@ -949,3 +949,37 @@ func (s *testEvaluatorSuite) TestField(c *C) {
 		c.Assert(r, testutil.DatumEquals, types.NewDatum(t.ret))
 	}
 }
+
+func (s *testEvaluatorSuite) TestLpad(c *C) {
+	tests := []struct {
+		str    string
+		len    int64
+		padStr string
+		expect interface{}
+	}{
+		{"hi", 5, "?", "???hi"},
+		{"hi", 1, "?", "h"},
+		{"hi", 0, "?", ""},
+		{"hi", -1, "?", nil},
+		{"hi", 1, "", "h"},
+		{"hi", 5, "", nil},
+		{"hi", 5, "ab", "abahi"},
+		{"hi", 6, "ab", "ababhi"},
+	}
+	fc := funcs[ast.Lpad]
+	for _, test := range tests {
+		str := types.NewStringDatum(test.str)
+		length := types.NewIntDatum(test.len)
+		padStr := types.NewStringDatum(test.padStr)
+		f, err := fc.getFunction(datumsToConstants([]types.Datum{str, length, padStr}), s.ctx)
+		c.Assert(err, IsNil)
+		result, err := f.eval(nil)
+		c.Assert(err, IsNil)
+		if test.expect == nil {
+			c.Assert(result.Kind(), Equals, types.KindNull)
+		} else {
+			expect, _ := test.expect.(string)
+			c.Assert(result.GetString(), Equals, expect)
+		}
+	}
+}

plan/typeinferer.go

@@ -372,7 +372,7 @@ func (v *typeInferrer) handleFuncCallExpr(x *ast.FuncCallExpr) {
 	case "dayname", "version", "database", "user", "current_user", "schema",
 		"concat", "concat_ws", "left", "lcase", "lower", "repeat",
 		"replace", "ucase", "upper", "convert", "substring",
-		"substring_index", "trim", "ltrim", "rtrim", "reverse", "hex", "unhex", "date_format", "rpad", "char_func", "conv":
+		"substring_index", "trim", "ltrim", "rtrim", "reverse", "hex", "unhex", "date_format", "rpad", "lpad", "char_func", "conv":
 		tp = types.NewFieldType(mysql.TypeVarString)
 		chs = v.defaultCharset
 	case "strcmp", "isnull", "bit_length", "char_length", "character_length", "crc32", "timestampdiff", "sign":

plan/typeinferer_test.go

@@ -214,6 +214,7 @@ func (ts *testTypeInferrerSuite) TestInferType(c *C) {
 		{"unhex(12)", mysql.TypeVarString, "utf8"},
 		{"DATE_FORMAT('2009-10-04 22:23:00', '%W %M %Y')", mysql.TypeVarString, "utf8"},
 		{"rpad('TiDB', 12, 'go')", mysql.TypeVarString, charset.CharsetUTF8},
+		{"lpad('TiDB', 12, 'go')", mysql.TypeVarString, charset.CharsetUTF8},
 		{"bit_length('TiDB')", mysql.TypeLonglong, charset.CharsetBin},
 		{"char(66)", mysql.TypeVarString, charset.CharsetUTF8},
 		{"char_length('TiDB')", mysql.TypeLonglong, charset.CharsetBin},