select: improve behavior of select(nil) + after

Previously, the traverse always started from the beginning of the index,
now after the after key.

Author: oleg-jukovec

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 ### Added
 
 ### Changed
+* Optimize `crud.select()` without conditions and with `after`.
 
 ### Fixed
 * `crud.select()` if a condition is '<=' and it's value < `after`.

crud/select/executor.lua

@@ -43,18 +43,24 @@ local generate_value
 
 if has_keydef then
     generate_value = function(after_tuple, scan_value, index_parts, tarantool_iter)
-        local cmp_operator = select_comparators.get_cmp_operator(tarantool_iter)
         local key_def = keydef_lib.new(index_parts)
+        if #scan_value == 0 and after_tuple ~= nil then
+            return key_def:extract_key(after_tuple)
+        end
+        local cmp_operator = select_comparators.get_cmp_operator(tarantool_iter)
         local cmp = key_def:compare_with_key(after_tuple, scan_value)
         if (cmp_operator == '<' and cmp < 0) or (cmp_operator == '>' and cmp > 0) then
             return key_def:extract_key(after_tuple)
         end
     end
 else
     generate_value = function(after_tuple, scan_value, index_parts, tarantool_iter)
+        local after_tuple_key = utils.extract_key(after_tuple, index_parts)
+        if #scan_value == 0 and after_tuple ~= nil then
+            return after_tuple_key
+        end
         local cmp_operator = select_comparators.get_cmp_operator(tarantool_iter)
         local scan_comparator = select_comparators.gen_tuples_comparator(cmp_operator, index_parts)
-        local after_tuple_key = utils.extract_key(after_tuple, index_parts)
         if scan_comparator(after_tuple_key, scan_value) then
             return after_tuple_key
         end