Fixed bug mdev-6071.

The method JOIN_CACHE::init may fail (return 1) if some conditions on the
used join buffer is not satisfied. For example it fails if join_buffer_size
is greater than join_buffer_space_limit. The conditions should be checked
when running the EXPLAIN command for the query. That's why the method
JOIN_CACHE::init has to be called for EXPLAIN commands as well.

mysql-test/r/join_cache.result

@@ -5577,8 +5577,8 @@ EXPLAIN
 SELECT * FROM t1, t2 LEFT JOIN t3 ON t2.b=t3.b WHERE t1.a=t2.a;
 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	ALL	NULL	NULL	NULL	NULL	12	Using where; Using join buffer (flat, BNL join)
+1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	12	Using where
-1	SIMPLE	t3	ALL	NULL	NULL	NULL	NULL	3	Using where; Using join buffer (incremental, BNL join)
+1	SIMPLE	t3	ALL	NULL	NULL	NULL	NULL	3	Using where
 SELECT * FROM t1, t2 LEFT JOIN t3 ON t2.b=t3.b WHERE t1.a=t2.a;
 a	a	b	b	c
 3	3	30	30	300
@@ -5703,4 +5703,27 @@ select @counter;
 2
 drop table t1,t2,t3;
 set expensive_subquery_limit=default;
+# 
+# mdev-6071: EXPLAIN chooses to use join buffer while execution turns it down
+# 
+create table t1 (a int);
+insert into t1 values
+(7), (9), (1), (4), (2), (3), (5), (8), (11), (6), (10);
+explain select count(*) from t1, t1 t2 where t1.a=t2.a;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	11	
+1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	11	Using where; Using join buffer (flat, BNL join)
+set join_buffer_space_limit=1024*8;
+select @@join_buffer_space_limit;
+@@join_buffer_space_limit
+8192
+select @@join_buffer_size;
+@@join_buffer_size
+131072
+explain select count(*) from t1, t1 t2 where t1.a=t2.a;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	11	
+1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	11	Using where
+set join_buffer_space_limit=default;
+drop table t1;
 set @@optimizer_switch=@save_optimizer_switch;

mysql-test/r/subselect_sj.result

@@ -94,6 +94,7 @@ id	select_type	tABle	type	possiBle_keys	key	key_len	ref	rows	filtered	ExtrA
 2	MATERIALIZED	t10	index	PRIMARY	PRIMARY	4	NULL	10	100.00	Using index
 Warnings:
 Note	1003	select `test`.`t1`.`A` AS `A`,`test`.`t1`.`B` AS `B`,`test`.`t2`.`A` AS `A`,`test`.`t2`.`B` AS `B` from `test`.`t1` left join `test`.`t2` on(((`test`.`t2`.`A` = `test`.`t1`.`A`) And <in_optimizer>(`test`.`t1`.`A`,`test`.`t1`.`A` in ( <mAteriAlize> (select `test`.`t10`.`pk` from `test`.`t10` ), <primAry_index_lookup>(`test`.`t1`.`A` in <temporAry tABle> on distinct_key where ((`test`.`t1`.`A` = `<suBquery2>`.`pk`))))))) where 1
+set join_buffer_size=8*1024;
 we shouldn't flatten if we're going to get a join of > MAX_TABLES.
 explain select * from 
 t1 s00, t1 s01,  t1 s02, t1 s03, t1 s04,t1 s05,t1 s06,t1 s07,t1 s08,t1 s09,
@@ -185,6 +186,7 @@ a	b	a	b
 0	0	0	0
 1	1	1	1
 2	2	2	2
+set join_buffer_size=default;
 prepare s1 from
 ' select * from
     t1 left join t2 on (t2.a= t1.a and t2.a in (select pk from t10))

mysql-test/r/subselect_sj_jcl6.result

@@ -107,6 +107,7 @@ id	select_type	tABle	type	possiBle_keys	key	key_len	ref	rows	filtered	ExtrA
 2	MATERIALIZED	t10	index	PRIMARY	PRIMARY	4	NULL	10	100.00	Using index
 Warnings:
 Note	1003	select `test`.`t1`.`A` AS `A`,`test`.`t1`.`B` AS `B`,`test`.`t2`.`A` AS `A`,`test`.`t2`.`B` AS `B` from `test`.`t1` left join `test`.`t2` on(((`test`.`t2`.`A` = `test`.`t1`.`A`) And <in_optimizer>(`test`.`t1`.`A`,`test`.`t1`.`A` in ( <mAteriAlize> (select `test`.`t10`.`pk` from `test`.`t10` ), <primAry_index_lookup>(`test`.`t1`.`A` in <temporAry tABle> on distinct_key where ((`test`.`t1`.`A` = `<suBquery2>`.`pk`))))))) where 1
+set join_buffer_size=8*1024;
 we shouldn't flatten if we're going to get a join of > MAX_TABLES.
 explain select * from 
 t1 s00, t1 s01,  t1 s02, t1 s03, t1 s04,t1 s05,t1 s06,t1 s07,t1 s08,t1 s09,
@@ -198,6 +199,7 @@ a	b	a	b
 0	0	0	0
 1	1	1	1
 2	2	2	2
+set join_buffer_size=default;
 prepare s1 from
 ' select * from
     t1 left join t2 on (t2.a= t1.a and t2.a in (select pk from t10))

mysql-test/t/join_cache.test

@@ -3700,5 +3700,26 @@ select @counter;
 drop table t1,t2,t3;
 set expensive_subquery_limit=default;
 
+--echo # 
+--echo # mdev-6071: EXPLAIN chooses to use join buffer while execution turns it down
+--echo # 
+
+create table t1 (a int);
+insert into t1 values
+(7), (9), (1), (4), (2), (3), (5), (8), (11), (6), (10);
+
+explain select count(*) from t1, t1 t2 where t1.a=t2.a;
+
+set join_buffer_space_limit=1024*8;
+select @@join_buffer_space_limit;
+select @@join_buffer_size;
+
+explain select count(*) from t1, t1 t2 where t1.a=t2.a;
+
+set join_buffer_space_limit=default;
+
+drop table t1;
+
+# The following command must be the last one the file 
 # this must be the last command in the file
 set @@optimizer_switch=@save_optimizer_switch;

mysql-test/t/subselect_sj.test

@@ -79,6 +79,8 @@ select * from t1 left join (t2 A, t2 B) on ( A.a= t1.a and B.a in (select pk fro
 explain extended
 select * from t1 left join t2 on (t2.a= t1.a and t2.a in (select pk from t10));
 
+set join_buffer_size=8*1024;
+
 --echo we shouldn't flatten if we're going to get a join of > MAX_TABLES.
 explain select * from 
   t1 s00, t1 s01,  t1 s02, t1 s03, t1 s04,t1 s05,t1 s06,t1 s07,t1 s08,t1 s09,
@@ -97,6 +99,8 @@ select * from
   t1 left join t2 on (t2.a= t1.a and t2.a in (select pk from t10)) 
 where t1.a < 5;
 
+set join_buffer_size=default;
+
 # 
 # Prepared statements
 #

sql/sql_join_cache.cc

@@ -930,6 +930,9 @@ int JOIN_CACHE::alloc_buffer()
                                   join_buff_space_limit))))
     goto fail;
 
+  if (for_explain_only)
+    return 0;
+                               
   for (ulong buff_size_decr= (buff_size-min_buff_size)/4 + 1; ; )
   {
     ulong next_buff_size;
@@ -1023,6 +1026,7 @@ int JOIN_CACHE::realloc_buffer()
 
   SYNOPSIS
     init()
+      for_explain       join buffer is initialized for explain only
 
   DESCRIPTION
     The function initializes the join cache structure. It supposed to be called
@@ -1044,10 +1048,12 @@ int JOIN_CACHE::realloc_buffer()
     1   otherwise
 */
 
-int JOIN_CACHE::init()
+int JOIN_CACHE::init(bool for_explain)
 {
   DBUG_ENTER("JOIN_CACHE::init");
 
+  for_explain_only= for_explain; 
+
   calc_record_fields();
 
   collect_info_on_key_args();
@@ -2632,6 +2638,7 @@ void JOIN_CACHE_BKAH::save_explain_data(struct st_explain_bka_type *explain)
 
   SYNOPSIS
     init()
+      for_explain       join buffer is initialized for explain only
 
   DESCRIPTION
     The function initializes the cache structure with a hash table in it.
@@ -2651,7 +2658,7 @@ void JOIN_CACHE_BKAH::save_explain_data(struct st_explain_bka_type *explain)
     1   otherwise
 */
 
-int JOIN_CACHE_HASHED::init()
+int JOIN_CACHE_HASHED::init(bool for_explain)
 {
   int rc= 0;
   TABLE_REF *ref= &join_tab->ref;
@@ -2663,7 +2670,7 @@ int JOIN_CACHE_HASHED::init()
 
   key_length= ref->key_length;
 
-  if ((rc= JOIN_CACHE::init()))
+  if ((rc= JOIN_CACHE::init(for_explain)) || for_explain)
     DBUG_RETURN (rc); 
 
   if (!(key_buff= (uchar*) sql_alloc(key_length)))
@@ -3572,6 +3579,7 @@ void JOIN_CACHE_BNL::read_next_candidate_for_match(uchar *rec_ptr)
 
   SYNOPSIS
     init
+      for_explain       join buffer is initialized for explain only
 
   DESCRIPTION
     The function initializes the cache structure. It is supposed to be called
@@ -3586,14 +3594,14 @@ void JOIN_CACHE_BNL::read_next_candidate_for_match(uchar *rec_ptr)
     1   otherwise
 */
 
-int JOIN_CACHE_BNL::init()
+int JOIN_CACHE_BNL::init(bool for_explain)
 {
   DBUG_ENTER("JOIN_CACHE_BNL::init");
 
   if (!(join_tab_scan= new JOIN_TAB_SCAN(join, join_tab)))
     DBUG_RETURN(1);
 
-  DBUG_RETURN(JOIN_CACHE::init());
+  DBUG_RETURN(JOIN_CACHE::init(for_explain));
 }
 
 
@@ -3758,6 +3766,7 @@ void JOIN_CACHE_BNLH::read_next_candidate_for_match(uchar *rec_ptr)
 
   SYNOPSIS
     init
+      for_explain       join buffer is initialized for explain only
 
   DESCRIPTION
     The function initializes the cache structure. It is supposed to be called
@@ -3772,14 +3781,14 @@ void JOIN_CACHE_BNLH::read_next_candidate_for_match(uchar *rec_ptr)
     1   otherwise
 */
 
-int JOIN_CACHE_BNLH::init()
+int JOIN_CACHE_BNLH::init(bool for_explain)
 {
   DBUG_ENTER("JOIN_CACHE_BNLH::init");
 
   if (!(join_tab_scan= new JOIN_TAB_SCAN(join, join_tab)))
     DBUG_RETURN(1);
 
-  DBUG_RETURN(JOIN_CACHE_HASHED::init());
+  DBUG_RETURN(JOIN_CACHE_HASHED::init(for_explain));
 }
 
 
@@ -4176,6 +4185,8 @@ Initialize the BKA join cache
 
 SYNOPSIS
   init
+    for_explain       join buffer is initialized for explain only
+
 
 DESCRIPTION
   The function initializes the cache structure. It is supposed to be called
@@ -4190,7 +4201,7 @@ RETURN VALUE
   1   otherwise
 */
 
-int JOIN_CACHE_BKA::init()
+int JOIN_CACHE_BKA::init(bool for_explain)
 {
 int res;
 bool check_only_first_match= join_tab->check_only_first_match();
@@ -4209,7 +4220,7 @@ if (!(join_tab_scan= jsm= new JOIN_TAB_SCAN_MRR(join, join_tab,
                                                 mrr_mode, rs_funcs)))
   DBUG_RETURN(1);
 
-if ((res= JOIN_CACHE::init()))
+if ((res= JOIN_CACHE::init(for_explain)))
   DBUG_RETURN(res);
 
 if (use_emb_key)
@@ -4570,6 +4581,7 @@ if (no_association &&
 
   SYNOPSIS
     init
+      for_explain       join buffer is initialized for explain only
 
   DESCRIPTION
     The function initializes the cache structure. It is supposed to be called
@@ -4584,7 +4596,7 @@ if (no_association &&
     1   otherwise
 */
 
-int JOIN_CACHE_BKAH::init()
+int JOIN_CACHE_BKAH::init(bool for_explain)
 {
   bool check_only_first_match= join_tab->check_only_first_match();
 
@@ -4603,7 +4615,7 @@ int JOIN_CACHE_BKAH::init()
                                              mrr_mode, rs_funcs)))
     DBUG_RETURN(1);
 
-  DBUG_RETURN(JOIN_CACHE_HASHED::init());
+  DBUG_RETURN(JOIN_CACHE_HASHED::init(for_explain));
 }
 
 

sql/sql_join_cache.h

@@ -99,6 +99,9 @@ private:
   /* Size of the offset of a field within a record in the cache */   
   uint size_of_fld_ofs;
 
+  /* This structure is used only for explain, not for execution */
+  bool for_explain_only;
+
 protected:
        
   /* 3 functions below actually do not use the hidden parameter 'this' */ 
@@ -595,7 +598,7 @@ public:
   JOIN_CACHE *next_cache;
 
   /* Shall initialize the join cache structure */ 
-  virtual int init();
+  virtual int init(bool for_explain);
 
   /* Get the current size of the cache join buffer */ 
   size_t get_join_buffer_size() { return buff_size; }
@@ -991,7 +994,7 @@ protected:
 public:
 
   /* Initialize a hashed join cache */       
-  int init();
+  int init(bool for_explain);
 
   /* Reset the buffer of a hashed join cache for reading/writing */
   void reset(bool for_writing);
@@ -1127,7 +1130,7 @@ public:
     :JOIN_CACHE(j, tab, prev) {}
 
   /* Initialize the BNL cache */       
-  int init();
+  int init(bool for_explain);
 
   enum Join_algorithm get_join_alg() { return BNL_JOIN_ALG; }
 
@@ -1194,7 +1197,7 @@ public:
     : JOIN_CACHE_HASHED(j, tab, prev) {}
 
   /* Initialize the BNLH cache */       
-  int init();
+  int init(bool for_explain);
 
   enum Join_algorithm get_join_alg() { return BNLH_JOIN_ALG; }
 
@@ -1325,7 +1328,7 @@ public:
   uchar **get_curr_association_ptr() { return &curr_association; }
 
   /* Initialize the BKA cache */       
-  int init();
+  int init(bool for_explain);
 
   enum Join_algorithm get_join_alg() { return BKA_JOIN_ALG; }
 
@@ -1421,7 +1424,7 @@ public:
   uchar **get_curr_association_ptr() { return &curr_matching_chain; }
 
   /* Initialize the BKAH cache */       
-  int init();
+  int init(bool for_explain);
 
   enum Join_algorithm get_join_alg() { return BKAH_JOIN_ALG; }
 

sql/sql_select.cc

@@ -10541,7 +10541,7 @@ uint check_join_cache_usage(JOIN_TAB *tab,
     if (cache_level == 1)
       prev_cache= 0;
     if ((tab->cache= new JOIN_CACHE_BNL(join, tab, prev_cache)) &&
-        ((options & SELECT_DESCRIBE) || !tab->cache->init()))
+         !tab->cache->init(options & SELECT_DESCRIBE))
     {
       tab->icp_other_tables_ok= FALSE;
       return (2 - MY_TEST(!prev_cache));
@@ -10576,7 +10576,7 @@ uint check_join_cache_usage(JOIN_TAB *tab,
       if (cache_level == 3)
         prev_cache= 0;
       if ((tab->cache= new JOIN_CACHE_BNLH(join, tab, prev_cache)) &&
-          ((options & SELECT_DESCRIBE) || !tab->cache->init()))
+          !tab->cache->init(options & SELECT_DESCRIBE))
       {
         tab->icp_other_tables_ok= FALSE;        
         return (4 - MY_TEST(!prev_cache));
@@ -10597,7 +10597,7 @@ uint check_join_cache_usage(JOIN_TAB *tab,
         if (cache_level == 5)
           prev_cache= 0;
         if ((tab->cache= new JOIN_CACHE_BKA(join, tab, flags, prev_cache)) &&
-            ((options & SELECT_DESCRIBE) || !tab->cache->init()))
+            !tab->cache->init(options & SELECT_DESCRIBE))
           return (6 - MY_TEST(!prev_cache));
         goto no_join_cache;
       }
@@ -10606,7 +10606,7 @@ uint check_join_cache_usage(JOIN_TAB *tab,
         if (cache_level == 7)
           prev_cache= 0;
         if ((tab->cache= new JOIN_CACHE_BKAH(join, tab, flags, prev_cache)) &&
-            ((options & SELECT_DESCRIBE) || !tab->cache->init()))
+            !tab->cache->init(options & SELECT_DESCRIBE))
 	{
          tab->idx_cond_fact_out= FALSE;
           return (8 - MY_TEST(!prev_cache));