Fix WAL stats parse error on PG 18 (issue #132)

internal/query/wal.go

@@ -21,10 +21,12 @@ const (
 		"FROM pg_stat_wal"
 )
 
-// SelectStatWALQuery returns the proper query and column count for pg_stat_wal based on PG version.
-func SelectStatWALQuery(version int) (string, int) {
+// SelectStatWALQuery returns the proper query, column count and diff interval for pg_stat_wal based on PG version.
+func SelectStatWALQuery(version int) (string, int, [2]int) {
 	if version >= 180000 {
-		return PgStatWALPG18, 7
+		// PG 18 removed wal_write/wal_sync columns; stats_age is col 6 and must not be diffed.
+		return PgStatWALPG18, 7, [2]int{2, 5}
 	}
-	return PgStatWALDefault, 11
+	// cols 2-9: wal,KiB..buffers_full; col 10 (stats_age) excluded.
+	return PgStatWALDefault, 11, [2]int{2, 9}
 }

internal/query/wal_test.go

@@ -7,13 +7,35 @@ import (
 	"testing"
 )
 
-// Test_StatWALQueries tests query, executing it against all supported Postgres versions.
+func Test_SelectStatWALQuery(t *testing.T) {
+	testcases := []struct {
+		version      int
+		wantNcols    int
+		wantDiffIntvl [2]int
+	}{
+		{version: 140000, wantNcols: 11, wantDiffIntvl: [2]int{2, 9}},
+		{version: 150000, wantNcols: 11, wantDiffIntvl: [2]int{2, 9}},
+		{version: 170000, wantNcols: 11, wantDiffIntvl: [2]int{2, 9}},
+		// PG 18: removed wal_write/wal_sync; stats_age must be outside the diff interval.
+		{version: 180000, wantNcols: 7, wantDiffIntvl: [2]int{2, 5}},
+	}
+
+	for _, tc := range testcases {
+		t.Run(fmt.Sprintf("version/%d", tc.version), func(t *testing.T) {
+			_, gotNcols, gotDiffIntvl := SelectStatWALQuery(tc.version)
+			assert.Equal(t, tc.wantNcols, gotNcols)
+			assert.Equal(t, tc.wantDiffIntvl, gotDiffIntvl)
+		})
+	}
+}
+
+// Test_StatWALQueries tests query execution against all supported Postgres versions.
 func Test_StatWALQueries(t *testing.T) {
 	versions := []int{140000, 150000, 160000, 170000, 180000}
 
 	for _, version := range versions {
 		t.Run(fmt.Sprintf("pg_stat_wal/%d", version), func(t *testing.T) {
-			tmpl, _ := SelectStatWALQuery(version)
+			tmpl, _, _ := SelectStatWALQuery(version)
 
 			opts := NewOptions(version, "f", "off", 256, "public")
 			q, err := Format(tmpl, opts)

internal/stat/postgres_test.go

@@ -335,6 +335,53 @@ func Test_diff(t *testing.T) {
 	assert.Error(t, err)
 }
 
+// Test_diff_pg18_wal_stats_age reproduces issue #132: when showing WAL statistics on PG 18,
+// the stats_age column ('19 days 02:52:00') was incorrectly included in the diff interval
+// and caused a parse error. The correct DiffIntvl for PG 18 WAL view is [2, 5].
+func Test_diff_pg18_wal_stats_age(t *testing.T) {
+	// PG 18 pg_stat_wal columns: source, waldir_size, wal_KiB, records, fpi, buffers_full, stats_age
+	prev := PGresult{
+		Valid: true, Ncols: 7, Nrows: 1,
+		Cols: []string{"source", "waldir_size", "wal,KiB", "records", "fpi", "buffers_full", "stats_age"},
+		Values: [][]sql.NullString{
+			{
+				{String: "WAL", Valid: true},
+				{String: "64 MB", Valid: true},
+				{String: "12000.00", Valid: true},
+				{String: "1000", Valid: true},
+				{String: "50", Valid: true},
+				{String: "10", Valid: true},
+				{String: "19 days 02:52:00", Valid: true},
+			},
+		},
+	}
+	curr := PGresult{
+		Valid: true, Ncols: 7, Nrows: 1,
+		Cols: []string{"source", "waldir_size", "wal,KiB", "records", "fpi", "buffers_full", "stats_age"},
+		Values: [][]sql.NullString{
+			{
+				{String: "WAL", Valid: true},
+				{String: "64 MB", Valid: true},
+				{String: "12100.00", Valid: true},
+				{String: "1010", Valid: true},
+				{String: "55", Valid: true},
+				{String: "11", Valid: true},
+				{String: "19 days 03:01:38", Valid: true},
+			},
+		},
+	}
+
+	// Old (buggy) interval [2, 9]: stats_age at col 6 is inside the diff range and causes parse error.
+	_, err := diff(curr, prev, 1, [2]int{2, 9}, 0)
+	assert.Error(t, err, "stats_age should not be diffable with interval [2,9] on PG18 WAL data")
+
+	// Correct interval [2, 5]: stats_age at col 6 is outside the diff range.
+	got, err := diff(curr, prev, 1, [2]int{2, 5}, 0)
+	assert.NoError(t, err)
+	assert.Equal(t, "19 days 03:01:38", got.Values[0][6].String, "stats_age should be copied as-is")
+	assert.Equal(t, "100.00", got.Values[0][2].String, "wal,KiB delta should be computed")
+}
+
 func Test_sort(t *testing.T) {
 	res := newTestPGresult()
 	testcases := []struct {

internal/view/view.go

@@ -304,7 +304,7 @@ func (v Views) Configure(opts query.Options) error {
 			view.QueryTmpl = query.SelectStatStatementsTimingQuery(opts.Version)
 			v[k] = view
 		case "wal":
-			view.QueryTmpl, view.Ncols = query.SelectStatWALQuery(opts.Version)
+			view.QueryTmpl, view.Ncols, view.DiffIntvl = query.SelectStatWALQuery(opts.Version)
 			v[k] = view
 		}
 	}