diff --git a/models/db/common.go b/models/db/common.go
index ea628bf2a0..b3c43f8b62 100644
--- a/models/db/common.go
+++ b/models/db/common.go
@@ -12,30 +12,30 @@ import (
 	"xorm.io/builder"
 )
 
-// BuildCaseInsensitiveLike returns a condition to check if the given value is like the given key case-insensitively.
-// Handles especially SQLite correctly as UPPER there only transforms ASCII letters.
+// BuildCaseInsensitiveLike returns a case-insensitive LIKE condition for the given key and value.
+// Cast the search value and the database column value to the same case for case-insensitive matching.
+// * SQLite: only cast ASCII chars because it doesn't handle complete Unicode case folding
+// * Other databases: use database's string function, assuming that they are able to handle complete Unicode case folding correctly
 func BuildCaseInsensitiveLike(key, value string) builder.Cond {
+	// ToLowerASCII is about 7% faster than ToUpperASCII (according to Golang's benchmark)
 	if setting.Database.Type.IsSQLite3() {
-		return builder.Like{"UPPER(" + key + ")", util.ToUpperASCII(value)}
+		return builder.Like{"LOWER(" + key + ")", util.ToLowerASCII(value)}
 	}
-	return builder.Like{"UPPER(" + key + ")", strings.ToUpper(value)}
+	return builder.Like{"LOWER(" + key + ")", strings.ToLower(value)}
 }
 
 // BuildCaseInsensitiveIn returns a condition to check if the given value is in the given values case-insensitively.
-// Handles especially SQLite correctly as UPPER there only transforms ASCII letters.
+// See BuildCaseInsensitiveLike for more details
 func BuildCaseInsensitiveIn(key string, values []string) builder.Cond {
-	uppers := make([]string, 0, len(values))
+	incaseValues := make([]string, len(values))
+	caseCast := strings.ToLower
 	if setting.Database.Type.IsSQLite3() {
-		for _, value := range values {
-			uppers = append(uppers, util.ToUpperASCII(value))
-		}
-	} else {
-		for _, value := range values {
-			uppers = append(uppers, strings.ToUpper(value))
-		}
+		caseCast = util.ToLowerASCII
 	}
-
-	return builder.In("UPPER("+key+")", uppers)
+	for i, value := range values {
+		incaseValues[i] = caseCast(value)
+	}
+	return builder.In("LOWER("+key+")", incaseValues)
 }
 
 // BuilderDialect returns the xorm.Builder dialect of the engine
diff --git a/models/repo/user_repo.go b/models/repo/user_repo.go
index 232087d865..08cf964bc8 100644
--- a/models/repo/user_repo.go
+++ b/models/repo/user_repo.go
@@ -151,7 +151,7 @@ func GetRepoAssignees(ctx context.Context, repo *Repository) (_ []*user_model.Us
 func GetIssuePostersWithSearch(ctx context.Context, repo *Repository, isPull bool, search string, isShowFullName bool) ([]*user_model.User, error) {
 	users := make([]*user_model.User, 0, 30)
 	var prefixCond builder.Cond = builder.Like{"lower_name", strings.ToLower(search) + "%"}
-	if isShowFullName {
+	if search != "" && isShowFullName {
 		prefixCond = prefixCond.Or(db.BuildCaseInsensitiveLike("full_name", "%"+search+"%"))
 	}
 
diff --git a/modules/util/util.go b/modules/util/util.go
index f197d4d6a4..d7702439d6 100644
--- a/modules/util/util.go
+++ b/modules/util/util.go
@@ -90,12 +90,12 @@ func CryptoRandomBytes(length int64) ([]byte, error) {
 	return buf, err
 }
 
-// ToUpperASCII returns s with all ASCII letters mapped to their upper case.
-func ToUpperASCII(s string) string {
+// ToLowerASCII returns s with all ASCII letters mapped to their lower case.
+func ToLowerASCII(s string) string {
 	b := []byte(s)
 	for i, c := range b {
-		if 'a' <= c && c <= 'z' {
-			b[i] -= 'a' - 'A'
+		if 'A' <= c && c <= 'Z' {
+			b[i] += 'a' - 'A'
 		}
 	}
 	return string(b)
diff --git a/modules/util/util_test.go b/modules/util/util_test.go
index 38876276e3..fd677f5c11 100644
--- a/modules/util/util_test.go
+++ b/modules/util/util_test.go
@@ -178,30 +178,26 @@ type StringTest struct {
 	in, out string
 }
 
-var upperTests = []StringTest{
+var lowerTests = []StringTest{
 	{"", ""},
-	{"ONLYUPPER", "ONLYUPPER"},
-	{"abc", "ABC"},
-	{"AbC123", "ABC123"},
-	{"azAZ09_", "AZAZ09_"},
-	{"longStrinGwitHmixofsmaLLandcAps", "LONGSTRINGWITHMIXOFSMALLANDCAPS"},
-	{"long\u0250string\u0250with\u0250nonascii\u2C6Fchars", "LONG\u0250STRING\u0250WITH\u0250NONASCII\u2C6FCHARS"},
-	{"\u0250\u0250\u0250\u0250\u0250", "\u0250\u0250\u0250\u0250\u0250"},
-	{"a\u0080\U0010FFFF", "A\u0080\U0010FFFF"},
-	{"lél", "LéL"},
+	{"ABC", "abc"},
+	{"AbC123_", "abc123_"},
+	{"LONG\u0250string\u0250WITH\u0250non-ascii\u2C6FCHARS\u0080\uFFFF", "long\u0250string\u0250with\u0250non-ascii\u2C6Fchars\u0080\uFFFF"},
+	{"lél", "lél"},
+	{"LÉL", "lÉl"},
 }
 
-func TestToUpperASCII(t *testing.T) {
-	for _, tc := range upperTests {
-		assert.Equal(t, ToUpperASCII(tc.in), tc.out)
+func TestToLowerASCII(t *testing.T) {
+	for _, tc := range lowerTests {
+		assert.Equal(t, ToLowerASCII(tc.in), tc.out)
 	}
 }
 
-func BenchmarkToUpper(b *testing.B) {
-	for _, tc := range upperTests {
+func BenchmarkToLower(b *testing.B) {
+	for _, tc := range lowerTests {
 		b.Run(tc.in, func(b *testing.B) {
 			for b.Loop() {
-				ToUpperASCII(tc.in)
+				ToLowerASCII(tc.in)
 			}
 		})
 	}