db: simplify L0 picked compaction input construction

The newPickedCompactionFromL0 function unnecessarily iterated over the provided
L0CompactionFiles, constructing a []*TableMetadata slice. The L0CompactionFiles
type already provides a []*TableMetadata slice. The slice is only needed to
construct the B-Tree that will back the manifest.LevelSlice, so there's no need
to clone it.

Author: jbowens

compaction_picker.go

@@ -291,13 +291,7 @@ func newPickedCompactionFromL0(
 	// any overlapping L0 SSTables that need to be added, and
 	// because compactions built by L0SSTables do not necessarily
 	// pick contiguous sequences of files in pc.version.Levels[0].
-	files := make([]*manifest.TableMetadata, 0, len(lcf.Files))
-	for f := range vers.Levels[0].All() {
-		if lcf.FilesIncluded[f.L0Index] {
-			files = append(files, f)
-		}
-	}
-	pc.startLevel.files = manifest.NewLevelSliceSeqSorted(files)
+	pc.startLevel.files = manifest.NewLevelSliceSeqSorted(lcf.Files)
 	return pc
 }