MDEV-29662 Replace same values in 'IN' list with an equality

If all elements in the list of 'IN' or 'NOT IN' clause are equal
and there are no NULLs then clause
-  "a IN (e1,..,en)" can be converted to "a = e1"
-  "a NOT IN (e1,..,en)" can be converted to "a <> e1".
This means an object of Item_func_in can be replaced with an object
of Item_func_eq for IN (e1,..,en) clause and Item_func_ne for
NOT IN (e1,...,en). Such a replacement allows the optimizer to choose
a better execution plan

mysql-test/main/func_in.test

@@ -721,6 +721,123 @@ SELECT '0x' IN (0,1);
 SELECT ('0x',1) IN ((0,1));
 SELECT ('0x',1) IN ((0,1),(1,1));
 
+--echo #
+--echo # MDEV-29662 same values in `IN` set vs equal comparison produces
+--echo #            the different performance
+--echo #
+CREATE TABLE t1 (a INT, PRIMARY KEY(a));
+INSERT INTO t1 VALUES (1),(2),(3);
+SELECT * FROM t1 WHERE a IN (1,1);
+--echo # 'const' access since 'a IN (1,1)' is converted to equality 'a=1'
+EXPLAIN SELECT * FROM t1 WHERE a IN (1,1);
+EXPLAIN SELECT * FROM t1 WHERE a = 1;
+SELECT * FROM t1 WHERE a IN (1,1,2);
+--echo # Conversion to equality is impossible due to different values
+EXPLAIN SELECT * FROM t1 WHERE a IN (1,1,2);
+SELECT * FROM t1 WHERE a IN (1,NULL,1);
+--echo # Conversion to equality is impossible due to NULL in the IN list
+EXPLAIN SELECT * FROM t1 WHERE a IN (1,NULL,1);
+
+SELECT * FROM t1 WHERE a NOT IN (2,2,2,2,2,2);
+EXPLAIN SELECT * FROM t1 WHERE a NOT IN (2,2,2,2,2,2);
+EXPLAIN SELECT * FROM t1 WHERE a != 3;
+SELECT * FROM t1 WHERE a NOT IN (3,3,1,1);
+EXPLAIN SELECT * FROM t1 WHERE a NOT IN (3,3,1,1);
+SELECT * FROM t1 WHERE a NOT IN (1,2,NULL,NULL);
+
+--echo # No conversion is possible since elements are not constant
+SELECT * FROM t1 WHERE a IN ((SELECT MAX(a) FROM t1), (SELECT MAX(a) FROM t1));
+EXPLAIN SELECT * FROM t1 WHERE a IN 
+  ((SELECT MAX(a) FROM t1), (SELECT MAX(a) FROM t1));
+
+--echo # There must be no conversion here:
+SELECT * FROM t1 WHERE a IN (3,2,3,3,1,2,3);
+
+--echo # Prepared statement
+PREPARE stmt FROM "SELECT * FROM t1 WHERE a IN (3,3,3)";
+EXECUTE stmt;
+EXECUTE stmt;
+DEALLOCATE PREPARE stmt;
+
+--echo # Conversion to equality since SELECT 2 is evaluated as const
+--disable_warnings
+SELECT * FROM t1 WHERE a IN ((SELECT 2), (SELECT 2));
+EXPLAIN SELECT * FROM t1 WHERE a IN ((SELECT 2), (SELECT 2));
+--enable_warnings
+
+CREATE TABLE t2 (a INT, b VARCHAR(10), PRIMARY KEY(a,b));
+INSERT INTO t2 VALUES (1,'abc'),(2,'def'),(3,'ghi');
+SELECT * FROM t2 WHERE (a,b) IN ((1,'abc'),(1,'abc'));
+--echo # 'const' access due to conversion to equality
+EXPLAIN SELECT * FROM t2 WHERE (a,b) IN ((1,'abc'),(1,'abc'));
+SELECT * FROM t2 WHERE (a,b) IN ((2,'def'),(2,'def'),(2,'XYZ'));
+--echo # No conversion due to different values
+EXPLAIN SELECT * FROM t2 WHERE (a,b) IN ((2,'def'),(2,'def'),(2,'XYZ'));
+SELECT * FROM t2 WHERE (a,b) IN ((2,'def'),(2,'def'),(2,NULL));
+--echo # No conversion due to NULL
+EXPLAIN SELECT * FROM t2 WHERE (a,b) IN ((2,'def'),(2,'def'),(2,NULL));
+
+SELECT * FROM t2 WHERE (a,b) NOT IN ((2,'def'),(2,'def'),(2,NULL));
+
+SELECT * FROM t2 WHERE a IN (1,1,1,1);
+EXPLAIN SELECT * FROM t2 WHERE a IN (1,1,1,1);
+EXPLAIN SELECT * FROM t2 WHERE a = 1;
+
+SELECT * FROM t2 WHERE b NOT IN ('abc','abc');
+EXPLAIN SELECT * FROM t2 WHERE b NOT IN ('abc','abc');
+EXPLAIN SELECT * FROM t2 WHERE b != 'abc';
+
+--echo # Prepared statements
+PREPARE stmt FROM "EXPLAIN SELECT * FROM t2 WHERE (a,b) IN ((1,'abc'),(1,'abc'))";
+EXECUTE stmt;
+EXECUTE stmt;
+DEALLOCATE PREPARE stmt;
+
+CREATE TABLE t3(a INT, PRIMARY KEY(a));
+INSERT INTO t3 VALUES (1),(2),(3);
+PREPARE stmt FROM "EXPLAIN SELECT * FROM t3 WHERE a IN (?,?,?)";
+EXECUTE stmt USING 1,1,1;
+EXECUTE stmt USING 2,3,4;
+DEALLOCATE PREPARE stmt;
+DROP TABLE t1, t2, t3;
+
+--echo # Nested joins
+CREATE TABLE t1 (a INT, b VARCHAR(10), PRIMARY KEY(a));
+INSERT INTO t1 VALUES (1,'abc'),(2,'def'),(3,'ghi');
+CREATE TABLE t2 (a INT, b VARCHAR(20), PRIMARY KEY(a));
+INSERT INTO t2 (a) VALUES (2),(3);
+CREATE TABLE t3 (a INT, PRIMARY KEY(a));
+INSERT INTO t3 VALUES (1),(2),(3),(4);
+CREATE TABLE t4 (a INT);
+INSERT INTO t4 VALUES (2),(3);
+--echo # Conversion to equalities
+EXPLAIN SELECT * FROM (t1,t2) LEFT JOIN (t3,t4) ON t1.a IN (2,2,2)
+    AND t3.a IN (1,1,1);
+--echo # No conversion to equalities due to different values in IN()
+EXPLAIN SELECT * FROM (t1,t2) LEFT JOIN (t3,t4) ON t1.a IN (2,3)
+    AND t3.a IN (1,2);
+--echo # Conversion to equalities
+EXPLAIN SELECT * FROM t1 LEFT JOIN ((t2, t3) LEFT JOIN t4 ON t2.a = t4.a)
+    ON t1.a = t2.a WHERE t1.a IN (2,2,2);
+--echo # No conversion to equalities due to different values in IN()
+EXPLAIN SELECT * FROM t1 LEFT JOIN ((t2, t3) LEFT JOIN t4 ON t2.a = t4.a)
+    ON t1.a = t2.a WHERE t1.a IN (1,3);
+
+--echo # View
+CREATE VIEW v1 AS SELECT t1.*, t2.b AS t2_b FROM t1 LEFT JOIN t2
+    ON t1.a = t2.a;
+EXPLAIN SELECT * FROM v1 WHERE a IN (2,2,2);
+EXPLAIN SELECT * FROM v1 WHERE a IN (1,2,3);
+
+--echo # Stored procedures
+CREATE PROCEDURE p1(pa INT, pb INT)
+    EXPLAIN SELECT * FROM t1 WHERE a IN (pa, pb);
+CALL p1(1,1);
+CALL p1(2,1);
+
+DROP TABLE t1, t2, t3, t4;
+DROP VIEW v1;
+DROP PROCEDURE p1;
 
 --echo #
 --echo # End of 10.4 tests