fix(cypher): apply distinct before return limits

Signed-off-by: Pcristin <xxxokzxxx@protonmail.com>

Author: pcristin

src/cypher/cypher.c

@@ -4115,7 +4115,7 @@ static void build_return_columns(result_builder_t *rb, cbm_return_clause_t *ret)
 static void execute_return_simple(cbm_return_clause_t *ret, binding_t *bindings, int bind_count,
                                   int max_rows, result_builder_t *rb) {
     int proj_cap = max_rows;
-    if (ret->limit > 0 && !ret->order_by && ret->skip <= 0) {
+    if (ret->limit > 0 && !ret->distinct && !ret->order_by && ret->skip <= 0) {
         proj_cap = ret->limit;
     }
     for (int bi = 0; bi < bind_count && rb->row_count < proj_cap; bi++) {
@@ -4397,11 +4397,11 @@ static void execute_return_clause(cbm_query_t *q, cbm_return_clause_t *ret, bind
         }
     }
 
-    rb_apply_order_by(rb, ret);
-    rb_apply_skip_limit(rb, ret->skip, ret->limit >= 0 ? ret->limit : max_rows);
     if (ret->distinct) {
         rb_apply_distinct(rb);
     }
+    rb_apply_order_by(rb, ret);
+    rb_apply_skip_limit(rb, ret->skip, ret->limit >= 0 ? ret->limit : max_rows);
 }
 
 static int execute_single(cbm_store_t *store, cbm_query_t *q, const char *project, int max_rows,

tests/test_cypher.c

@@ -2528,6 +2528,52 @@ TEST(cypher_issue237_distinct_order_limit) {
     PASS();
 }
 
+/* #873: duplicate projected rows must be deduped before ORDER BY + LIMIT */
+TEST(cypher_issue873_distinct_order_limit_dedupes_before_limit) {
+    cbm_store_t *s = setup_cypher_store();
+    cbm_cypher_result_t r = {0};
+    int rc = cbm_cypher_execute(
+        s, "MATCH (n) RETURN DISTINCT n.label AS label ORDER BY label LIMIT 2", "test", 0, &r);
+    ASSERT_EQ(rc, 0);
+    ASSERT_NULL(r.error);
+    ASSERT_EQ(r.row_count, 2);
+    ASSERT_STR_EQ(r.rows[0][0], "Function");
+    ASSERT_STR_EQ(r.rows[1][0], "Module");
+    cbm_cypher_result_free(&r);
+    cbm_store_close(s);
+    PASS();
+}
+
+/* #873: early LIMIT must not truncate rows before DISTINCT for simple RETURN */
+TEST(cypher_issue873_distinct_limit_dedupes_before_limit) {
+    cbm_store_t *s = setup_cypher_store();
+    cbm_cypher_result_t r = {0};
+    int rc = cbm_cypher_execute(
+        s, "MATCH (n) RETURN DISTINCT n.label AS label LIMIT 2", "test", 0, &r);
+    ASSERT_EQ(rc, 0);
+    ASSERT_NULL(r.error);
+    ASSERT_EQ(r.row_count, 2);
+    cbm_cypher_result_free(&r);
+    cbm_store_close(s);
+    PASS();
+}
+
+/* #873: SKIP is applied after DISTINCT and ORDER BY, not before dedupe */
+TEST(cypher_issue873_distinct_order_skip_limit_dedupes_before_skip) {
+    cbm_store_t *s = setup_cypher_store();
+    cbm_cypher_result_t r = {0};
+    int rc = cbm_cypher_execute(
+        s, "MATCH (n) RETURN DISTINCT n.label AS label ORDER BY label SKIP 1 LIMIT 1", "test",
+        0, &r);
+    ASSERT_EQ(rc, 0);
+    ASSERT_NULL(r.error);
+    ASSERT_EQ(r.row_count, 1);
+    ASSERT_STR_EQ(r.rows[0][0], "Module");
+    cbm_cypher_result_free(&r);
+    cbm_store_close(s);
+    PASS();
+}
+
 /* #252: toInteger() */
 TEST(cypher_issue252_tointeger) {
     cbm_store_t *s = setup_cypher_store();
@@ -2669,6 +2715,9 @@ SUITE(cypher) {
     RUN_TEST(cypher_exec_match_all_functions);
     RUN_TEST(cypher_issue240_labels_function);
     RUN_TEST(cypher_issue237_distinct_order_limit);
+    RUN_TEST(cypher_issue873_distinct_order_limit_dedupes_before_limit);
+    RUN_TEST(cypher_issue873_distinct_limit_dedupes_before_limit);
+    RUN_TEST(cypher_issue873_distinct_order_skip_limit_dedupes_before_skip);
     RUN_TEST(cypher_issue252_tointeger);
     RUN_TEST(cypher_issue305_count_star_alias);
     RUN_TEST(cypher_exec_where_eq);