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.

Move the functionality gate for FULL JOIN further into the codebase: convert
LEX::has_full_outer_join to a counter so we can see how many FULL JOINs
remain which makes the gate work correctly after simplify_joins and
eliminate_tables are called.

Fixes an old bug where, when running the server as a debug build and in
debug mode, a null pointer deference in
Dep_analysis_context::dbug_print_deps would cause a crash.

Author: DaveGosselin-MariaDB

mysql-test/main/table_elim.result

@@ -1112,3 +1112,35 @@ 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);
+create table t3 (a int primary key, b int)
+as select a, a as b from t1 where a in (1,3);
+# These will not be eliminated because contains 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`))
+explain extended select t1.a from t1 full join (t2 full join t3 on t2.b=t3.b) on t2.a=t1.a and t3.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	
+1	SIMPLE	t3	UNKNOWN	PRIMARY	NULL	NULL	NULL	0	0.00	
+Warnings:
+Note	1003	select `test`.`t1`.`a` AS `a` from `test`.`t1` full join (`test`.`t3` join `test`.`t2`) on(multiple equal(`test`.`t2`.`a`, `test`.`t1`.`a`, `test`.`t3`.`a`) and multiple equal(`test`.`t2`.`b`, `test`.`t3`.`b`))
+explain extended select t1.a from t1 full join (t2 left join t3 on t2.b=t3.b) on t2.a=t1.a and t3.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	
+1	SIMPLE	t3	UNKNOWN	PRIMARY	NULL	NULL	NULL	0	0.00	
+Warnings:
+Note	1003	select `test`.`t1`.`a` AS `a` from `test`.`t1` full join (`test`.`t2` join `test`.`t3`) on(multiple equal(`test`.`t2`.`a`, `test`.`t1`.`a`, `test`.`t3`.`a`) and multiple equal(`test`.`t2`.`b`, `test`.`t3`.`b`))
+drop table t1, t2, t3;
+# End of 12.3  tests

mysql-test/main/table_elim.test

@@ -844,3 +844,24 @@ 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);
+
+create table t3 (a int primary key, b int)
+  as select a, a as b from t1 where a in (1,3);
+
+--echo # These will not be eliminated because contains a FULL OUTER JOIN.
+explain extended select t1.a from t1 full join t2 on t2.a=t1.a;
+explain extended select t1.a from t1 full join (t2 full join t3 on t2.b=t3.b) on t2.a=t1.a and t3.a=t1.a;
+explain extended select t1.a from t1 full join (t2 left join t3 on t2.b=t3.b) on t2.a=t1.a and t3.a=t1.a;
+
+drop table t1, t2, t3;
+
+--echo # End of 12.3  tests

sql/opt_table_elimination.cc

@@ -825,7 +825,8 @@ eliminate_tables_for_list(JOIN *join, List<TABLE_LIST> *join_list,
       if (tbl->nested_join)
       {
         /* This is  "... LEFT JOIN (join_nest) ON cond" */
-        if (eliminate_tables_for_list(join,
+        if (!(tbl->outer_join & JOIN_TYPE_FULL) &&
+            eliminate_tables_for_list(join,
                                       &tbl->nested_join->join_list, 
                                       tbl->nested_join->used_tables, 
                                       tbl->on_expr,
@@ -840,7 +841,8 @@ eliminate_tables_for_list(JOIN *join, List<TABLE_LIST> *join_list,
       else
       {
         /* This is  "... LEFT JOIN tbl ON cond" */
-        if (!(tbl->table->map & outside_used_tables) &&
+        if (!(tbl->outer_join & JOIN_TYPE_FULL) &&
+            !(tbl->table->map & outside_used_tables) &&
             check_func_dependency(join, tbl->table->map, NULL, tbl, 
                                   tbl->on_expr))
         {
@@ -2105,7 +2107,8 @@ void Dep_analysis_context::dbug_print_deps()
     char buf[128];
     String str(buf, sizeof(buf), &my_charset_bin);
     str.length(0);
-    eq_mod->expr->print(&str, QT_ORDINARY);
+    if (eq_mod->expr)
+      eq_mod->expr->print(&str, QT_ORDINARY);
     if (eq_mod->field)
     {
       fprintf(DBUG_FILE, "  equality%ld: %s -> %s.%s\n", 

sql/sql_lex.cc

@@ -1343,7 +1343,7 @@ void LEX::start(THD *thd_arg)
 
   memset(&trg_chistics, 0, sizeof(trg_chistics));
   selects_for_hint_resolution.empty();
-  has_full_outer_join= false;
+  full_join_count= 0;
   DBUG_VOID_RETURN;
 }
 

sql/sql_lex.h

@@ -3593,8 +3593,8 @@ struct LEX: public Query_tables_list
   vers_select_conds_t vers_conditions;
   vers_select_conds_t period_conditions;
 
-  /* False by default, this will be true if the query has a full join. */
-  bool has_full_outer_join;
+  /* Zero by default, this counts the number of full joins in the query. */
+  uint full_join_count;
 
   inline void free_set_stmt_mem_root()
   {

sql/sql_select.cc

@@ -2773,7 +2773,7 @@ JOIN::optimize_inner()
     mature enough to correctly execute the queries.  But for now
     this allows for some EXPLAIN EXTENDED support.
   */
-  else if (!thd->lex->has_full_outer_join)
+  else if (!thd->lex->full_join_count)
   {
     if (optimize_stage2())
       DBUG_RETURN(1);
@@ -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 (thd->lex->full_join_count && !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;
@@ -20353,23 +20375,10 @@ static COND *rewrite_full_outer_joins(JOIN *join,
     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
@@ -20420,7 +20429,7 @@ static COND *rewrite_full_outer_joins(JOIN *join,
       RIGHT JOINs don't actually exist in MariaDB!
     */
     *table_ptr= li.swap_next();
-    join->thd->lex->has_full_outer_join= false;
+    --join->thd->lex->full_join_count;
   }
   else
   {
@@ -20457,7 +20466,7 @@ static COND *rewrite_full_outer_joins(JOIN *join,
     if (peeked_map & not_null_tables)
     {
       rewrite_full_to_left(left_table, right_table);
-      join->thd->lex->has_full_outer_join= false;
+      --join->thd->lex->full_join_count;
     }
     // else the FULL JOIN cannot be rewritten, pass it along.
   }

sql/sql_view.cc

@@ -1909,7 +1909,7 @@ bool mysql_make_view(THD *thd, TABLE_SHARE *share, TABLE_LIST *view_table_alias,
   DBUG_ASSERT(view_query_lex == thd->lex);
   thd->lex= parent_query_lex;                            // Needed for prepare_security
   result= !view_table_alias->prelocking_placeholder && view_table_alias->prepare_security(thd);
-  parent_query_lex->has_full_outer_join= view_query_lex->has_full_outer_join;
+  parent_query_lex->full_join_count= view_query_lex->full_join_count;
 
   lex_end(view_query_lex);
 end:

sql/sql_yacc.yy

@@ -12392,7 +12392,7 @@ join_table:
             $5->on_context= $1->on_context;
             Select->parsing_place= NO_MATTER;
             $$= $1;
-            Lex->has_full_outer_join= true;
+            ++Lex->full_join_count;
           }
         | table_ref FULL opt_outer JOIN_SYM table_factor
           {
@@ -12408,7 +12408,7 @@ join_table:
           USING '(' using_list ')'
           {
             add_join_natural($1,$5,$9,Select);
-            Lex->has_full_outer_join= true;
+            ++Lex->full_join_count;
           }
         | table_ref NATURAL FULL opt_outer JOIN_SYM table_factor
           {
@@ -12425,7 +12425,7 @@ join_table:
                               JOIN_TYPE_NATURAL);
 
             add_join_natural($6,$1,NULL,Select);
-            Lex->has_full_outer_join= true;
+            ++Lex->full_join_count;
           }
         ;