sqlite storage: Do ReadAt without using incremental I/O

storage/sqlite/sqlite-storage.go

@@ -221,6 +221,7 @@ func (i instance) Stat() (ret os.FileInfo, err error) {
 
 func (i instance) ReadAt(p []byte, off int64) (n int, err error) {
 	i.withConn(func(conn conn) {
+		if false {
 			var blob *sqlite.Blob
 			blob, err = i.openBlob(conn, false, true)
 			if err != nil {
@@ -235,6 +236,33 @@ func (i instance) ReadAt(p []byte, off int64) (n int, err error) {
 				p = p[:blob.Size()-off]
 			}
 			n, err = blob.ReadAt(p, off)
+		} else {
+			gotRow := false
+			err = sqlitex.Exec(
+				conn,
+				"select substr(data, ?, ?) from blob where name=?",
+				func(stmt *sqlite.Stmt) error {
+					if gotRow {
+						panic("found multiple matching blobs")
+					} else {
+						gotRow = true
+					}
+					n = stmt.ColumnBytes(0, p)
+					return nil
+				},
+				off+1, len(p), i.location,
+			)
+			if err != nil {
+				return
+			}
+			if !gotRow {
+				err = errors.New("blob not found")
+				return
+			}
+			if n < len(p) {
+				err = io.EOF
+			}
+		}
 	})
 	return
 }