MDEV-36215: Avoid wrong result due to table elimination

There was a Wrong result due to Table Elimination
when 'unique_col IS NULL' condition is supplied in the left-join query.

An index on an unique_col was created for the table, and it
is being used in the plan.
As the query was a left join, no fields from the right table were
projected. The right table was getting wrongly eliminated although
multiple rows from the right could be matched to a single row from the
left with the condition `unique_col IS NULL'.

This PR addresses the problem by including an additional check before
eliminating a table in check_equality() function of opt_table_elimination.cc

Author: bsrikanth-mariadb

mysql-test/main/table_elim.result

@@ -362,6 +362,7 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 explain select t1.a from t1 left join t3 on t3.pk1=t1.a and t3.pk2 IS NULL;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	4	
+1	SIMPLE	t3	ref	pk1	pk1	10	test.t1.a,const	2	Using where; Using index
 drop table t1,t2,t3;
 #
 # Multi-equality tests 
@@ -1031,3 +1032,59 @@ DROP VIEW v;
 #
 # End of 10.10 tests
 #
+#
+# MDEV-36215: Table elimination wrongly done
+#
+CREATE TABLE t1(null_col INT, notnull_col INT NOT NULL);
+CREATE TABLE t2(unique_col INT, UNIQUE(unique_col));
+INSERT INTO t1 VALUES (1,100), (NULL, 101);
+INSERT INTO t2 VALUES (NULL), (NULL);
+# Test with 'unique_col IS NULL'
+# Here, table t2 should not be eliminated:
+explain
+select t1.null_col from t1 left join t2 on (t2.unique_col is null);
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	2	
+1	SIMPLE	t2	ref	unique_col	unique_col	5	const	2	Using where; Using index
+select t1.null_col from t1 left join t2 on (t2.unique_col is null);
+null_col
+1
+1
+NULL
+NULL
+# Check without table elimination:
+set statement optimizer_switch='table_elimination=off' for
+select t1.null_col from t1 left join t2 on (t2.unique_col is null);
+null_col
+1
+1
+NULL
+NULL
+# Test with 'unique_col <=> nullable_col'
+# t2 must not be eliminated:
+explain
+select t1.null_col from t1 left join t2 on t1.null_col<=>t2.unique_col;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	2	
+1	SIMPLE	t2	ref	unique_col	unique_col	5	test.t1.null_col	2	Using where; Using index
+select t1.null_col from t1 left join t2 on t1.null_col<=>t2.unique_col;
+null_col
+1
+NULL
+NULL
+# Check without table elimination:
+set statement optimizer_switch='table_elimination=off' for
+select t1.null_col from t1 left join t2 on t1.null_col<=>t2.unique_col;
+null_col
+1
+NULL
+NULL
+# Table t2 will be eliminated:
+explain
+select t1.null_col from t1 left join t2 on (t2.unique_col<=>t1.notnull_col);
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	2	
+DROP TABLE t1, t2;
+#
+# End of 10.11 tests
+#

mysql-test/main/table_elim.test

@@ -799,3 +799,44 @@ DROP VIEW v;
 --echo #
 --echo # End of 10.10 tests
 --echo #
+
+--echo #
+--echo # MDEV-36215: Table elimination wrongly done
+--echo #
+
+CREATE TABLE t1(null_col INT, notnull_col INT NOT NULL);
+CREATE TABLE t2(unique_col INT, UNIQUE(unique_col));
+INSERT INTO t1 VALUES (1,100), (NULL, 101);
+INSERT INTO t2 VALUES (NULL), (NULL);
+
+
+--echo # Test with 'unique_col IS NULL'
+--echo # Here, table t2 should not be eliminated:
+explain
+select t1.null_col from t1 left join t2 on (t2.unique_col is null);
+select t1.null_col from t1 left join t2 on (t2.unique_col is null);
+
+--echo # Check without table elimination:
+set statement optimizer_switch='table_elimination=off' for
+select t1.null_col from t1 left join t2 on (t2.unique_col is null);
+
+--echo # Test with 'unique_col <=> nullable_col'
+--echo # t2 must not be eliminated:
+explain
+select t1.null_col from t1 left join t2 on t1.null_col<=>t2.unique_col;
+select t1.null_col from t1 left join t2 on t1.null_col<=>t2.unique_col;
+
+--echo # Check without table elimination:
+set statement optimizer_switch='table_elimination=off' for
+select t1.null_col from t1 left join t2 on t1.null_col<=>t2.unique_col;
+
+--echo # Table t2 will be eliminated:
+explain
+select t1.null_col from t1 left join t2 on (t2.unique_col<=>t1.notnull_col);
+
+
+DROP TABLE t1, t2;
+
+--echo #
+--echo # End of 10.11 tests
+--echo #

sql/opt_table_elimination.cc

@@ -1596,6 +1596,13 @@ void check_equality(Dep_analysis_context *ctx, Dep_module_expr **eq_mod,
     if (field->can_optimize_outer_join_table_elimination(cond, right) !=
         Data_type_compatibility::OK)
       return;
+    /*
+      UNIQUE indexes over nullable columns may have duplicate NULL values.
+      This means, a condition like "field IS NULL" or "field <=> right_expr"
+      may match multiple rows. Dis-qualify such conditions.
+    */
+    if (field->real_maybe_null() && right->maybe_null())
+      return;
     Dep_value_field *field_val;
     if ((field_val= ctx->get_field_value(field)))
       add_module_expr(ctx, eq_mod, and_level, field_val, right, NULL);