MDEV-27382: OFFSET is ignored when combined with DISTINCT

A query in form

  SELECT DISTINCT expr_that_is_inferred_to_be_const LIMIT 0 OFFSET n

produces one row when it should produce none. The issue was in
JOIN_TAB::remove_duplicates() in the piece of logic that tried to
avoid duplicate removal for such cases but didn't account for possible
"LIMIT 0".

Fixed by making Select_limit_counters::set_limit() change OFFSET to 0
when LIMIT is 0.

mysql-test/main/distinct.result

@@ -1070,3 +1070,39 @@ UNION
 1
 drop table t1;
 End of 5.5 tests
+#
+# MDEV-27382: OFFSET is ignored when it is combined with the DISTINCT, IN() and JOIN
+#
+CREATE TABLE t1 (
+id int(7) NOT NULL AUTO_INCREMENT,
+name varchar(50) DEFAULT NULL,
+primary key (id)
+);
+INSERT INTO t1 VALUES (1, 'Reed'), (10, 'no-child');
+CREATE TABLE t2 (
+id int(11) NOT NULL AUTO_INCREMENT,
+parent_id int(7) NOT NULL,
+name varchar(100) DEFAULT NULL,
+primary key (id),
+key(parent_id)
+);
+INSERT INTO t2 VALUES (1, 1,'John'), (2, 2,'no-parent');
+SELECT DISTINCT p.id
+FROM t1 p LEFT JOIN t2 c ON p.id = c.parent_id
+WHERE p.id=1
+LIMIT 0;
+id
+SELECT DISTINCT p.id
+FROM t1 p LEFT JOIN t2 c ON p.id = c.parent_id
+WHERE p.id=1
+LIMIT 0 offset 5;
+id
+# Test the second part of the fix: just check that "LIMIT 0 OFFSET n" is
+# handled in the same way as "LIMIT 0"
+explain select * from t1 limit 0;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Zero limit
+explain select * from t1 limit 0 offset 10;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Zero limit
+drop table t1, t2;

mysql-test/main/distinct.test

@@ -818,3 +818,41 @@ UNION
 drop table t1;
 
 --echo End of 5.5 tests
+
+--echo #
+--echo # MDEV-27382: OFFSET is ignored when it is combined with the DISTINCT, IN() and JOIN
+--echo #
+CREATE TABLE t1 (
+  id int(7) NOT NULL AUTO_INCREMENT,
+  name varchar(50) DEFAULT NULL,
+  primary key (id)
+);
+INSERT INTO t1 VALUES (1, 'Reed'), (10, 'no-child');
+
+CREATE TABLE t2 (
+  id int(11) NOT NULL AUTO_INCREMENT,
+  parent_id int(7) NOT NULL,
+  name varchar(100) DEFAULT NULL,
+  primary key (id),
+  key(parent_id)
+);
+
+INSERT INTO t2 VALUES (1, 1,'John'), (2, 2,'no-parent');
+
+SELECT DISTINCT p.id
+FROM t1 p LEFT JOIN t2 c ON p.id = c.parent_id
+WHERE p.id=1
+LIMIT 0;
+
+SELECT DISTINCT p.id
+FROM t1 p LEFT JOIN t2 c ON p.id = c.parent_id
+WHERE p.id=1
+LIMIT 0 offset 5;
+
+--echo # Test the second part of the fix: just check that "LIMIT 0 OFFSET n" is
+--echo # handled in the same way as "LIMIT 0"
+
+explain select * from t1 limit 0;
+explain select * from t1 limit 0 offset 10;
+
+drop table t1, t2;

sql/sql_limit.h

@@ -35,6 +35,8 @@ class Select_limit_counters
 
    void set_limit(ha_rows limit, ha_rows offset)
    {
+      if (limit == 0)
+        offset= 0;
       offset_limit_cnt= offset;
       select_limit_cnt= limit;
       if (select_limit_cnt + offset_limit_cnt >=