Reduce allocations when creating KeyCursors

The refs map was to increment the file references one time each.
It doesn't hurt to increment them multiple times though.

We also do not need to copy the files slice as we are accessing it
under a read lock so it can't be changed.

tsdb/engine/tsm1/file_store.go

@@ -724,14 +724,9 @@ func (f *FileStore) walkFiles(fn func(f TSMFile) error) error {
 // whether the key will be scan in ascending time order or descenging time order.
 // This function assumes the read-lock has been taken.
 func (f *FileStore) locations(key string, t int64, ascending bool) []*location {
-	filesSnapshot := make([]TSMFile, len(f.files))
-	for i := range f.files {
-		filesSnapshot[i] = f.files[i]
-	}
-
 	var entries []IndexEntry
-	locations := make([]*location, 0, len(filesSnapshot))
+	locations := make([]*location, 0, len(f.files))
-	for _, fd := range filesSnapshot {
+	for _, fd := range f.files {
 		minTime, maxTime := fd.TimeRange()
 
 		tombstones := fd.TombstoneRange(key)
@@ -748,7 +743,8 @@ func (f *FileStore) locations(key string, t int64, ascending bool) []*location {
 		// This file could potential contain points we are looking for so find the blocks for
 		// the given key.
 		fd.ReadEntries(key, &entries)
-		for _, ie := range entries {
+		for i := 0; i < len(entries); i++ {
+			ie := entries[i]
 
 			// Skip any blocks only contain values that are tombstoned.
 			var skip bool
@@ -884,9 +880,6 @@ type KeyCursor struct {
 	// If this is true, we need to scan the duplicate blocks and dedup the points
 	// as query time until they are compacted.
 	duplicates bool
-
-	// The distinct set of TSM files references by the cursor
-	refs map[string]TSMFile
 }
 
 type location struct {
@@ -943,7 +936,6 @@ func newKeyCursor(fs *FileStore, key string, t int64, ascending bool) *KeyCursor
 		seeks:     fs.locations(key, t, ascending),
 		ascending: ascending,
 	}
-	c.refs = make(map[string]TSMFile, len(c.seeks))
 
 	c.duplicates = c.hasOverlappingBlocks()
 
@@ -955,10 +947,7 @@ func newKeyCursor(fs *FileStore, key string, t int64, ascending bool) *KeyCursor
 
 	// Determine the distinct set of TSM files in use and mark then as in-use
 	for _, f := range c.seeks {
-		if _, ok := c.refs[f.r.Path()]; !ok {
 		f.r.Ref()
-			c.refs[f.r.Path()] = f.r
-		}
 	}
 
 	c.seek(t)
@@ -968,8 +957,8 @@ func newKeyCursor(fs *FileStore, key string, t int64, ascending bool) *KeyCursor
 // Close removes all references on the cursor.
 func (c *KeyCursor) Close() {
 	// Remove all of our in-use references since we're done
-	for _, f := range c.refs {
+	for _, f := range c.seeks {
-		f.Unref()
+		f.r.Unref()
 	}
 
 	c.buf = nil