From 02d36637de481051e9f3445210583f9d019a7cfa Mon Sep 17 00:00:00 2001
From: chrislu <chris.lu@gmail.com>
Date: Thu, 4 Sep 2025 09:13:54 -0700
Subject: [PATCH] limit with offset

---
 weed/query/engine/aggregations.go           | 63 +++++++++++----------
 weed/query/engine/engine.go                 |  6 +-
 weed/query/engine/hybrid_message_scanner.go | 29 +++++-----
 3 files changed, 53 insertions(+), 45 deletions(-)

diff --git a/weed/query/engine/aggregations.go b/weed/query/engine/aggregations.go
index 1bbeda456..44cf9fbcb 100644
--- a/weed/query/engine/aggregations.go
+++ b/weed/query/engine/aggregations.go
@@ -350,7 +350,8 @@ func (e *SQLEngine) executeAggregationQuery(ctx context.Context, hybridScanner *
 // executeAggregationQueryWithPlan handles SELECT queries with aggregation functions and populates execution plan
 func (e *SQLEngine) executeAggregationQueryWithPlan(ctx context.Context, hybridScanner *HybridMessageScanner, aggregations []AggregationSpec, stmt *SelectStatement, plan *QueryExecutionPlan) (*QueryResult, error) {
 	// Parse LIMIT and OFFSET for aggregation results (do this first)
-	limit := 0
+	// Use -1 to distinguish "no LIMIT" from "LIMIT 0"
+	limit := -1
 	offset := 0
 	if stmt.Limit != nil && stmt.Limit.Rowcount != nil {
 		if limitExpr, ok := stmt.Limit.Rowcount.(*SQLVal); ok && limitExpr.Type == IntVal {
@@ -391,29 +392,31 @@ func (e *SQLEngine) executeAggregationQueryWithPlan(ctx context.Context, hybridS
 			if isDebugMode(ctx) {
 				fmt.Printf("Using fast hybrid statistics for aggregation (parquet stats + live log counts)\n")
 			}
-			
+
 			// Apply OFFSET and LIMIT to fast path results too
-			if offset > 0 || limit > 0 {
+			// Limit semantics: -1 = no limit, 0 = LIMIT 0 (empty), >0 = limit to N rows
+			if offset > 0 || limit >= 0 {
 				rows := fastResult.Rows
-				// Apply OFFSET first
-				if offset > 0 {
-					if offset >= len(rows) {
-						rows = [][]sqltypes.Value{}
-					} else {
-						rows = rows[offset:]
+				// Handle LIMIT 0 first
+				if limit == 0 {
+					rows = [][]sqltypes.Value{}
+				} else {
+					// Apply OFFSET first
+					if offset > 0 {
+						if offset >= len(rows) {
+							rows = [][]sqltypes.Value{}
+						} else {
+							rows = rows[offset:]
+						}
 					}
-				}
-				// Apply LIMIT after OFFSET
-				if limit >= 0 { // Handle LIMIT 0 case
-					if limit == 0 {
-						rows = [][]sqltypes.Value{}
-					} else if len(rows) > limit {
+					// Apply LIMIT after OFFSET (only if limit > 0)
+					if limit > 0 && len(rows) > limit {
 						rows = rows[:limit]
 					}
 				}
 				fastResult.Rows = rows
 			}
-			
+
 			return fastResult, nil
 		}
 	}
@@ -484,22 +487,24 @@ func (e *SQLEngine) executeAggregationQueryWithPlan(ctx context.Context, hybridS
 	}
 
 	// Apply OFFSET and LIMIT to aggregation results
+	// Limit semantics: -1 = no limit, 0 = LIMIT 0 (empty), >0 = limit to N rows
 	rows := [][]sqltypes.Value{row}
-	if offset > 0 || limit > 0 {
-		// Apply OFFSET first
-		if offset > 0 {
-			if offset >= len(rows) {
-				rows = [][]sqltypes.Value{}
-			} else {
-				rows = rows[offset:]
+	if offset > 0 || limit >= 0 {
+		// Handle LIMIT 0 first
+		if limit == 0 {
+			rows = [][]sqltypes.Value{}
+		} else {
+			// Apply OFFSET first
+			if offset > 0 {
+				if offset >= len(rows) {
+					rows = [][]sqltypes.Value{}
+				} else {
+					rows = rows[offset:]
+				}
 			}
-		}
 
-		// Apply LIMIT after OFFSET
-		if limit >= 0 { // Handle LIMIT 0 case
-			if limit == 0 {
-				rows = [][]sqltypes.Value{}
-			} else if len(rows) > limit {
+			// Apply LIMIT after OFFSET (only if limit > 0)
+			if limit > 0 && len(rows) > limit {
 				rows = rows[:limit]
 			}
 		}
diff --git a/weed/query/engine/engine.go b/weed/query/engine/engine.go
index cd8c7d57c..985bcd4fe 100644
--- a/weed/query/engine/engine.go
+++ b/weed/query/engine/engine.go
@@ -1416,7 +1416,8 @@ func (e *SQLEngine) executeSelectStatement(ctx context.Context, stmt *SelectStat
 	}
 
 	// Parse LIMIT and OFFSET clauses
-	limit := 0
+	// Use -1 to distinguish "no LIMIT" from "LIMIT 0"
+	limit := -1
 	offset := 0
 	if stmt.Limit != nil && stmt.Limit.Rowcount != nil {
 		switch limitExpr := stmt.Limit.Rowcount.(type) {
@@ -1605,7 +1606,8 @@ func (e *SQLEngine) executeSelectStatementWithBrokerStats(ctx context.Context, s
 	}
 
 	// Parse LIMIT and OFFSET clauses
-	limit := 0
+	// Use -1 to distinguish "no LIMIT" from "LIMIT 0"
+	limit := -1
 	offset := 0
 	if stmt.Limit != nil && stmt.Limit.Rowcount != nil {
 		switch limitExpr := stmt.Limit.Rowcount.(type) {
diff --git a/weed/query/engine/hybrid_message_scanner.go b/weed/query/engine/hybrid_message_scanner.go
index 0f76de2fd..14c784615 100644
--- a/weed/query/engine/hybrid_message_scanner.go
+++ b/weed/query/engine/hybrid_message_scanner.go
@@ -233,25 +233,26 @@ func (hms *HybridMessageScanner) ScanWithStats(ctx context.Context, options Hybr
 	}
 
 	// Apply final OFFSET and LIMIT processing (done once at the end)
+	// Limit semantics: -1 = no limit, 0 = LIMIT 0 (empty), >0 = limit to N rows
 	if options.Offset > 0 || options.Limit >= 0 {
-		// Handle LIMIT 0 special case - return empty result immediately
+		// Handle LIMIT 0 special case first
 		if options.Limit == 0 {
-			results = []HybridScanResult{}
-		} else {
-			// Apply OFFSET first
-			if options.Offset > 0 {
-				if options.Offset >= len(results) {
-					results = []HybridScanResult{}
-				} else {
-					results = results[options.Offset:]
-				}
-			}
+			return []HybridScanResult{}, stats, nil
+		}
 
-			// Apply LIMIT after OFFSET
-			if options.Limit > 0 && len(results) > options.Limit {
-				results = results[:options.Limit]
+		// Apply OFFSET first
+		if options.Offset > 0 {
+			if options.Offset >= len(results) {
+				results = []HybridScanResult{}
+			} else {
+				results = results[options.Offset:]
 			}
 		}
+
+		// Apply LIMIT after OFFSET (only if limit > 0)
+		if options.Limit > 0 && len(results) > options.Limit {
+			results = results[:options.Limit]
+		}
 	}
 
 	return results, stats, nil