Add a small hint on filter databases listed from tsh

tool/tsh/common/db_print.go

@@ -141,3 +141,28 @@ func formatDatabaseRolesForDB(database types.Database, accessChecker services.Ac
 	}
 	return ""
 }
+
+func shouldShowListDatabasesHint(cf *CLIConf, numRows int) bool {
+	selector := newDatabaseResourceSelectors(cf)
+
+	return numRows >= minNumRowsToShowListDatabasesHint &&
+		cf.command == "db ls" &&
+		cf.SearchKeywords == "" &&
+		selector.IsEmpty()
+}
+
+func maybeShowListDatabasesHint(cf *CLIConf, w io.Writer, numRows int) {
+	if !shouldShowListDatabasesHint(cf, numRows) {
+		return
+	}
+
+	fmt.Fprint(w, listDatabaseHint)
+}
+
+// minNumRowsToShowListDatabasesHint is an arbitrary number selected to show
+// filtering hint for `tsh db ls` command when too many databases are listed.
+const minNumRowsToShowListDatabasesHint = 20
+
+const listDatabaseHint = "" +
+	"hint: use 'tsh db ls --search foo,bar' to search keywords\n" +
+	"      use 'tsh db ls key1=value1,key2=value2' to filter databases by labels\n\n"

tool/tsh/common/db_print_test.go

@@ -19,6 +19,7 @@
 package common
 
 import (
+	"bytes"
 	"strings"
 	"testing"
 
@@ -196,3 +197,80 @@ func Test_formatDatabaseRolesForDB(t *testing.T) {
 		})
 	}
 }
+
+func Test_maybeShowListDatabaseHint(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name     string
+		cf       *CLIConf
+		numRows  int
+		wantHint bool
+	}{
+		{
+			name: "show hint when number is big",
+			cf: &CLIConf{
+				command: "db ls",
+			},
+			numRows:  25,
+			wantHint: true,
+		},
+		{
+			name: "no hint for tsh db connect",
+			cf: &CLIConf{
+				command: "db connect",
+			},
+			numRows:  25,
+			wantHint: false,
+		},
+		{
+			name: "no hint when number is small",
+			cf: &CLIConf{
+				command: "db ls",
+			},
+			numRows:  15,
+			wantHint: false,
+		},
+		{
+			name: "no hint when search flag exists",
+			cf: &CLIConf{
+				command:        "db ls",
+				SearchKeywords: "foo",
+			},
+			numRows:  25,
+			wantHint: false,
+		},
+		{
+			name: "no hint when query flag exists",
+			cf: &CLIConf{
+				command:             "db ls",
+				PredicateExpression: "labels[\"key\"] == \"value\"",
+			},
+			numRows:  25,
+			wantHint: false,
+		},
+		{
+			name: "no hint when labels exist",
+			cf: &CLIConf{
+				command: "db ls",
+				Labels:  "key=value",
+			},
+			numRows:  25,
+			wantHint: false,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			var buf bytes.Buffer
+
+			maybeShowListDatabasesHint(test.cf, &buf, test.numRows)
+
+			if test.wantHint {
+				require.Contains(t, buf.String(), "hint")
+			} else {
+				require.Empty(t, buf.String())
+			}
+		})
+	}
+}

tool/tsh/common/tsh.go

@@ -2987,6 +2987,8 @@ func showDatabasesAsText(cf *CLIConf, w io.Writer, databases []types.Database, a
 		rows:    rows,
 		verbose: verbose,
 	})
+
+	maybeShowListDatabasesHint(cf, w, len(rows))
 }
 
 func printDatabasesWithClusters(cf *CLIConf, dbListings []databaseListing, active []tlsca.RouteToDatabase) {
@@ -3007,6 +3009,8 @@ func printDatabasesWithClusters(cf *CLIConf, dbListings []databaseListing, activ
 		showProxyAndCluster: true,
 		verbose:             cf.Verbose,
 	})
+
+	maybeShowListDatabasesHint(cf, cf.Stdout(), len(rows))
 }
 
 func formatActiveDB(active tlsca.RouteToDatabase, displayName string) string {