select: fix EQ with negative first results

EQ with after has always been replaced by LT. The idea was to start
traverse not from the after value, but from a next value. The
micro-optimization allowed to fetch one less tuple per time.

It worked well for unique indexes, but this led to wrong results for
non-unique indexes.

Author: oleg-jukovec

CHANGELOG.md

@@ -53,6 +53,40 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
   ...
 
 * `crud.select()` results order with negative `first`.
+* `crud.select()` if a condition is '=' or '==' with negative `first`.
+
+  Suppose we have a non-unique secondary index by the field `age` field and a
+  space:
+
+  tarantool> crud.select('developers', nil, {first = 10})
+  ---
+  - metadata: [{'name': 'id', 'type': 'unsigned'}, {'name': 'bucket_id', 'type': 'unsigned'},
+      {'name': 'name', 'type': 'string'}, {'name': 'surname', 'type': 'string'}, {'name': 'age',
+        'type': 'number'}]
+    rows:
+    - [1, 477, 'Alexey', 'Adams', 20]
+    - [2, 401, 'Sergey', 'Allred', 27]
+    - [3, 2804, 'Pavel', 'Adams', 27]
+    - [4, 1161, 'Mikhail', 'Liston', 27]
+    - [5, 1172, 'Dmitry', 'Jacobi', 27]
+    - [6, 1064, 'Alexey', 'Sidorov', 31]
+  - null
+  ...
+
+  Before this patch:
+
+  tarantool> crud.select('developers', {{'=', 'age', 27}}, {first = -10, after = rows[4]}).rows
+  ---
+  - []
+  ...
+
+  After this patch:
+
+  tarantool> crud.select('developers', {{'=', 'age', 27}}, {first = -10, after = rows[4]}).rows
+  ---
+  - - [2, 401, 'Sergey', 'Allred', 27]
+    - [3, 2804, 'Pavel', 'Adams', 27]
+  ...
 
 ## [0.11.2] - 23-05-22
 

crud/compare/plan.lua

@@ -269,7 +269,7 @@ function plan.new(space, conditions, opts)
         if scan_iter == box.index.EQ then
             scan_iter = box.index.GE
         elseif scan_iter == box.index.REQ then
-            scan_iter = box.index.LT
+            scan_iter = box.index.LE
         end
     end
 

test/integration/select_test.lua

@@ -579,7 +579,10 @@ pgroup.test_eq_condition_with_index = function(g)
         },{
             id = 7, name = "Jack", last_name = "Sparrow",
             age = 33, city = "Chicago",
-        },
+        }, {
+            id = 8, name = "Nick", last_name = "Smith",
+            age = 20, city = "London",
+        }
     })
 
     table.sort(customers, function(obj1, obj2) return obj1.id < obj2.id end)
@@ -602,6 +605,51 @@ pgroup.test_eq_condition_with_index = function(g)
     t.assert_equals(err, nil)
     local objects = crud.unflatten_rows(result.rows, result.metadata)
     t.assert_equals(objects, helpers.get_objects_by_idxs(customers, {5, 7})) -- in id order
+
+    -- after obj 5 with negative first
+    local after = crud_utils.flatten(customers[5], g.space_format)
+    local result, err = g.cluster.main_server.net_box:call(
+        'crud.select', {'customers', conditions, {after=after, first=-10}})
+
+    t.assert_equals(err, nil)
+    local objects = crud.unflatten_rows(result.rows, result.metadata)
+    t.assert_equals(objects, helpers.get_objects_by_idxs(customers, {1, 3})) -- in id order
+
+    -- after obj 8
+    local after = crud_utils.flatten(customers[8], g.space_format)
+    local result, err = g.cluster.main_server.net_box:call(
+        'crud.select', {'customers', conditions, {after=after, first=10}})
+
+    t.assert_equals(err, nil)
+    local objects = crud.unflatten_rows(result.rows, result.metadata)
+    t.assert_equals(objects, helpers.get_objects_by_idxs(customers, {1, 3, 5, 7})) -- in id order
+
+    -- after obj 8 with negative first
+    local after = crud_utils.flatten(customers[8], g.space_format)
+    local result, err = g.cluster.main_server.net_box:call(
+        'crud.select', {'customers', conditions, {after=after, first=-10}})
+
+    t.assert_equals(err, nil)
+    local objects = crud.unflatten_rows(result.rows, result.metadata)
+    t.assert_equals(objects, helpers.get_objects_by_idxs(customers, {}))
+
+    -- after obj 2
+    local after = crud_utils.flatten(customers[2], g.space_format)
+    local result, err = g.cluster.main_server.net_box:call(
+        'crud.select', {'customers', conditions, {after=after, first=10}})
+
+    t.assert_equals(err, nil)
+    local objects = crud.unflatten_rows(result.rows, result.metadata)
+    t.assert_equals(objects, helpers.get_objects_by_idxs(customers, {}))
+
+    -- after obj 2 with negative first
+    local after = crud_utils.flatten(customers[2], g.space_format)
+    local result, err = g.cluster.main_server.net_box:call(
+        'crud.select', {'customers', conditions, {after=after, first=-10}})
+
+    t.assert_equals(err, nil)
+    local objects = crud.unflatten_rows(result.rows, result.metadata)
+    t.assert_equals(objects, helpers.get_objects_by_idxs(customers, {1, 3, 5, 7})) -- in id order
 end
 
 pgroup.test_ge_condition_with_index = function(g)