From 401e79e0d878799c73a62d61d13538e89b91f762 Mon Sep 17 00:00:00 2001
From: Aliaksandr Valialkin <valyala@victoriametrics.com>
Date: Mon, 27 May 2024 14:22:18 +0200
Subject: [PATCH] wip

---
 docs/VictoriaLogs/CHANGELOG.md    |  2 ++
 docs/VictoriaLogs/LogsQL.md       |  6 ++++++
 lib/logstorage/parser_test.go     | 10 ++++------
 lib/logstorage/pipe_limit.go      | 17 +++++++++++------
 lib/logstorage/pipe_limit_test.go | 12 +++++++++++-
 5 files changed, 34 insertions(+), 13 deletions(-)

diff --git a/docs/VictoriaLogs/CHANGELOG.md b/docs/VictoriaLogs/CHANGELOG.md
index b569f6195..eec03ea26 100644
--- a/docs/VictoriaLogs/CHANGELOG.md
+++ b/docs/VictoriaLogs/CHANGELOG.md
@@ -19,6 +19,8 @@ according to [these docs](https://docs.victoriametrics.com/victorialogs/quicksta
 
 ## tip
 
+* FEATURE: allow [`head` pipe](https://docs.victoriametrics.com/victorialogs/logsql/#limit-pipe) without number. For example, `error | head`. In this case 10 last values are returned as `head` Unix command does by default.
+
 ## [v0.12.1](https://github.com/VictoriaMetrics/VictoriaMetrics/releases/tag/v0.12.1-victorialogs)
 
 Released at 2024-05-26
diff --git a/docs/VictoriaLogs/LogsQL.md b/docs/VictoriaLogs/LogsQL.md
index 8b6e17467..b1455e9e0 100644
--- a/docs/VictoriaLogs/LogsQL.md
+++ b/docs/VictoriaLogs/LogsQL.md
@@ -1463,6 +1463,12 @@ _time:5m | limit 100
 
 `head` keyword can be used instead of `limit` for convenience. For example, `_time:5m | head 100` is equivalent to `_time:5m | limit 100`.
 
+The `N` in `head N` can be omitted - in this case up to 10 matching logs are returned:
+
+```logsql
+error | head
+```
+
 By default rows are selected in arbitrary order because of performance reasons, so the query above can return different sets of logs every time it is executed.
 [`sort` pipe](#sort-pipe) can be used for making sure the logs are in the same order before applying `limit ...` to them.
 
diff --git a/lib/logstorage/parser_test.go b/lib/logstorage/parser_test.go
index 4b80b66b8..4ba801927 100644
--- a/lib/logstorage/parser_test.go
+++ b/lib/logstorage/parser_test.go
@@ -869,8 +869,10 @@ func TestParseQuerySuccess(t *testing.T) {
 	f(`* | DELETE foo, bar`, `* | delete foo, bar`)
 
 	// limit and head pipe
-	f(`foo | limit 10`, `foo | limit 10`)
-	f(`foo | head 10`, `foo | limit 10`)
+	f(`foo | limit`, `foo | limit 10`)
+	f(`foo | head`, `foo | limit 10`)
+	f(`foo | limit 20`, `foo | limit 20`)
+	f(`foo | head 20`, `foo | limit 20`)
 	f(`foo | HEAD 1_123_432`, `foo | limit 1123432`)
 	f(`foo | head 10K`, `foo | limit 10000`)
 
@@ -1313,10 +1315,6 @@ func TestParseQueryFailure(t *testing.T) {
 	f(`foo | delete foo,`)
 	f(`foo | delete foo,,`)
 
-	// missing limit and head pipe value
-	f(`foo | limit`)
-	f(`foo | head`)
-
 	// invalid limit pipe value
 	f(`foo | limit bar`)
 	f(`foo | limit -123`)
diff --git a/lib/logstorage/pipe_limit.go b/lib/logstorage/pipe_limit.go
index 2f4054393..964307cfc 100644
--- a/lib/logstorage/pipe_limit.go
+++ b/lib/logstorage/pipe_limit.go
@@ -88,15 +88,20 @@ func parsePipeLimit(lex *lexer) (*pipeLimit, error) {
 	if !lex.isKeyword("limit", "head") {
 		return nil, fmt.Errorf("expecting 'limit' or 'head'; got %q", lex.token)
 	}
+	lex.nextToken()
 
-	lex.nextToken()
-	n, err := parseUint(lex.token)
-	if err != nil {
-		return nil, fmt.Errorf("cannot parse rows limit from %q: %w", lex.token, err)
+	limit := uint64(10)
+	if !lex.isKeyword("|", ")", "") {
+		n, err := parseUint(lex.token)
+		if err != nil {
+			return nil, fmt.Errorf("cannot parse rows limit from %q: %w", lex.token, err)
+		}
+		lex.nextToken()
+		limit = n
 	}
-	lex.nextToken()
+
 	pl := &pipeLimit{
-		limit: n,
+		limit: limit,
 	}
 	return pl, nil
 }
diff --git a/lib/logstorage/pipe_limit_test.go b/lib/logstorage/pipe_limit_test.go
index bc7afa4fd..23b176ee1 100644
--- a/lib/logstorage/pipe_limit_test.go
+++ b/lib/logstorage/pipe_limit_test.go
@@ -20,7 +20,6 @@ func TestParsePipeLimitFailure(t *testing.T) {
 		expectParsePipeFailure(t, pipeStr)
 	}
 
-	f(`limit`)
 	f(`limit -10`)
 	f(`limit foo`)
 }
@@ -30,6 +29,17 @@ func TestPipeLimit(t *testing.T) {
 		t.Helper()
 		expectPipeResults(t, pipeStr, rows, rowsExpected)
 	}
+	f("limit", [][]Field{
+		{
+			{"_msg", `{"foo":"bar"}`},
+			{"a", `test`},
+		},
+	}, [][]Field{
+		{
+			{"_msg", `{"foo":"bar"}`},
+			{"a", `test`},
+		},
+	})
 
 	f("limit 100", [][]Field{
 		{