MDEV-38136: Prevent elimination of tables in a FULL OUTER JOIN

Prevent elimination of tables participating in a FULL OUTER JOIN during
eliminate_tables as part of phase one FULL OUTER JOIN development.

Author: DaveGosselin-MariaDB

mysql-test/main/table_elim.result

@@ -1112,3 +1112,19 @@ DROP TABLE t1, t2;
 #
 # End of 10.11 tests
 #
+#
+# MDEV-38136 Prevent elimination of tables in a FULL OUTER JOIN
+#
+create table t1 (a int);
+insert into t1 values (0),(1),(2),(3);
+create table t2 (a int primary key, b int)
+as select a, a as b from t1 where a in (1,2);
+# This will not be eliminated because it's a FULL OUTER JOIN.
+explain extended select t1.a from t1 full join t2 on t2.a=t1.a;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	SIMPLE	t1	UNKNOWN	NULL	NULL	NULL	NULL	0	0.00	
+1	SIMPLE	t2	UNKNOWN	PRIMARY	NULL	NULL	NULL	0	0.00	
+Warnings:
+Note	1003	select `test`.`t1`.`a` AS `a` from `test`.`t1` full join `test`.`t2` on(multiple equal(`test`.`t2`.`a`, `test`.`t1`.`a`))
+drop table t1, t2;
+# End of 12.3  tests

mysql-test/main/table_elim.test

@@ -844,3 +844,19 @@ DROP TABLE t1, t2;
 --echo #
 --echo # End of 10.11 tests
 --echo #
+
+--echo #
+--echo # MDEV-38136 Prevent elimination of tables in a FULL OUTER JOIN
+--echo #
+create table t1 (a int);
+insert into t1 values (0),(1),(2),(3);
+
+create table t2 (a int primary key, b int)
+  as select a, a as b from t1 where a in (1,2);
+
+--echo # This will not be eliminated because it's a FULL OUTER JOIN.
+explain extended select t1.a from t1 full join t2 on t2.a=t1.a;
+
+drop table t1, t2;
+
+--echo # End of 12.3  tests

sql/opt_table_elimination.cc

@@ -732,7 +732,10 @@ void eliminate_tables(JOIN *join)
     for (ORDER *cur_list= all_lists[i]; cur_list; cur_list= cur_list->next)
       used_tables |= (*(cur_list->item))->used_tables();
   }
-  
+
+  /* Add tables that participate in a FULL JOIN */
+  used_tables|= join->full_outer_tables();
+
   if (join->select_lex == thd->lex->first_select_lex())
   {
 

sql/sql_select.cc

@@ -5627,7 +5627,16 @@ make_join_statistics(JOIN *join, List<TABLE_LIST> &tables_list,
   TABLE **table_vector;
   JOIN_TAB *stat,*stat_end,*s,**stat_ref, **stat_vector;
   KEYUSE *keyuse,*start_keyuse;
+
+  /*
+    outer_join here does not have the same meaning as TABLE_LIST::outer_join.
+    Here, outer_join is the union of all table numbers representing tables
+    that participate in this join.  TABLE_LIST::outer_join marks how a
+    TABLE_LIST participates in a particular JOIN (as a right table, left table,
+    as part of a FULL JOIN, etc).
+  */
   table_map outer_join=0;
+
   table_map no_rows_const_tables= 0;
   SARGABLE_PARAM *sargables= 0;
   List_iterator<TABLE_LIST> ti(tables_list);
@@ -5841,6 +5850,19 @@ make_join_statistics(JOIN *join, List<TABLE_LIST> &tables_list,
   join->const_table_map= no_rows_const_tables;
   join->const_tables= const_count;
   eliminate_tables(join);
+
+  /*
+    Temporary gate.  As the FULL JOIN implementation matures, this keeps moving
+    deeper into the server until it's eventually eliminated.
+  */
+  if (join->full_outer_tables() && !thd->lex->describe)
+  {
+    my_error(ER_NOT_SUPPORTED_YET, MYF(0),
+             "FULL JOINs that cannot be converted to LEFT, RIGHT, or "
+             "INNER JOINs");
+    goto error;
+  }
+
   join->const_table_map &= ~no_rows_const_tables;
   const_count= join->const_tables;
   found_const_table_map= join->const_table_map;
@@ -6502,6 +6524,20 @@ void JOIN::restore_table_dependencies(table_map *orig_dep_array)
 }
 
 
+table_map JOIN::full_outer_tables(List<TABLE_LIST> *jl) const
+{
+  List_iterator<TABLE_LIST> itr(jl ? *jl : *join_list);
+  TABLE_LIST* tbl= nullptr;
+  table_map fo_tables= 0;
+  while ((tbl=itr++))
+    fo_tables|=
+      tbl->nested_join ? full_outer_tables(&tbl->nested_join->join_list)
+                       : (tbl->outer_join & JOIN_TYPE_FULL ? tbl->get_map()
+                                                           : 0);
+  return fo_tables;
+}
+
+
 /*****************************************************************************
   Check with keys are used and with tables references with tables
   Updates in stat:
@@ -20348,28 +20384,17 @@ static COND *rewrite_full_outer_joins(JOIN *join,
   if (!(right_table->outer_join & JOIN_TYPE_FULL))
     return conds;
 
+  DBUG_ASSERT(join->full_outer_tables());
+
   /*
     The join_list enumerates the tables from t_n, ..., t_0 so we always
     see the right table first.  If, on this call to rewrite_full_outer_joins,
     the current table is left member of the JOIN (e.g., left_member FULL JOIN
     ...) it means we couldn't rewrite the FULL JOIN as a LEFT, RIGHT, or
-    INNER JOIN, so emit an error (unless we're in an EXPLAIN EXTENDED, permit
-    that).
+    INNER JOIN, so pass along the unmodified FULL JOIN.
   */
   if (right_table->outer_join & JOIN_TYPE_LEFT)
-  {
-    if (join->thd->lex->describe)
-      return conds;
-
-    /*
-      We always see the RIGHT table before the LEFT table, so nothing to
-      do here for JOIN_TYPE_LEFT.
-    */
-    my_error(ER_NOT_SUPPORTED_YET, MYF(0),
-             "FULL JOINs that cannot be converted to LEFT, RIGHT, or "
-             "INNER JOINs");
-    return nullptr;
-  }
+    return conds;
 
   /*
     Must always see the right table before the left.  Down below, we deal

sql/sql_select.h

@@ -1961,6 +1961,17 @@ class JOIN :public Sql_alloc
   table_map *export_table_dependencies() const;
   void restore_table_dependencies(table_map *orig_dep_array);
 
+  /**
+    Generates a bitmap corresponding to the table numbers for
+    all tables participating in a FULL OUTER JOIN in the passed-in
+    join_list.  If no join_list is passed-in, then this->join_list
+    is used instead.
+
+    @param join_list the list of tables to check, default is this->join_list
+    @return table_map of tables that are part of a FULL JOIN.
+  */
+  table_map full_outer_tables(List<TABLE_LIST> *join_list=nullptr) const;
+
 private:
   /**
     Create a temporary table to be used for processing DISTINCT/ORDER