- Testing: add DBUG_EXECUTE_IF("show_explain_probe_2"... which fires

only for selects with given select_id.
- Steps towards making SHOW EXPLAIN work for UNIONs.

mysql-test/r/show_explain.result

@@ -14,6 +14,7 @@ show explain for $thr2;
 ERROR HY000: Error when executing command SHOW EXPLAIN: Target is not running EXPLAINable command
 show explain for $thr1;
 ERROR HY000: Error when executing command SHOW EXPLAIN: Target is not running EXPLAINable command
+set @show_explain_probe_select_id=1;
 set debug='d,show_explain_probe_1';
 select count(*) from t1 where a < 100000;
 show explain for $thr2;
@@ -21,7 +22,6 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	index	a	a	5	NULL	1000	Using where; Using index
 count(*)
 1000
-set debug='d,show_explain_probe_1';
 select max(c) from t1 where a < 10;
 show explain for $thr2;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
@@ -29,7 +29,6 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 max(c)
 9
 set optimizer_switch='index_condition_pushdown=on,mrr=on,mrr_sort_keys=on';
-set debug='d,show_explain_probe_1';
 explain select max(c) from t1 where a < 10;
 show explain for $thr2;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra

mysql-test/t/show_explain.test

@@ -1,6 +1,8 @@
 #
 # Tests for SHOW EXPLAIN FOR functionality
 #
+--source include/have_debug.inc
+
 --disable_warnings
 drop table if exists t0, t1;
 --enable_warnings
@@ -45,6 +47,7 @@ let $wait_condition= select State='show_explain_trap' from information_schema.pr
 # Test SHOW EXPLAIN for simple queries
 #
 connection con1;
+set @show_explain_probe_select_id=1;
 set debug='d,show_explain_probe_1';
 send select count(*) from t1 where a < 100000;
 
@@ -55,7 +58,6 @@ connection con1;
 reap;
 
 
-set debug='d,show_explain_probe_1';
 send select max(c) from t1 where a < 10;
 connection default;
 --source include/wait_condition.inc
@@ -65,7 +67,6 @@ reap;
 
 # We can catch EXPLAIN, too.
 set optimizer_switch='index_condition_pushdown=on,mrr=on,mrr_sort_keys=on';
-set debug='d,show_explain_probe_1';
 send explain select max(c) from t1 where a < 10;
 connection default;
 --source include/wait_condition.inc
@@ -73,6 +74,8 @@ evalp show explain for $thr2;
 connection con1;
 reap;
 
+# Let's try with a subquery
+
 
 ## TODO: Test this: multiple SHOW EXPLAIN calls in course of running of one select
 ## 

sql/item_func.cc

@@ -3871,7 +3871,7 @@ longlong Item_func_sleep::val_int()
 
 #define extra_size sizeof(double)
 
-static user_var_entry *get_variable(HASH *hash, LEX_STRING &name,
+user_var_entry *get_variable(HASH *hash, LEX_STRING &name,
 				    bool create_if_not_exists)
 {
   user_var_entry *entry;

sql/sql_class.h

@@ -3474,6 +3474,8 @@ class user_var_entry
   DTCollation collation;
 };
 
+user_var_entry *get_variable(HASH *hash, LEX_STRING &name,
+				    bool create_if_not_exists);
 
 /*
    Unique -- class for unique (removing of duplicates).

sql/sql_lex.cc

@@ -3529,6 +3529,7 @@ void st_select_lex::set_explain_type(bool on_the_fly)
   if (this == master_unit()->fake_select_lex)
     type= "UNION RESULT";
 
+  if (!on_the_fly)
     options|= SELECT_DESCRIBE;
 }
 

sql/sql_select.cc

@@ -262,8 +262,34 @@ void dbug_serve_apcs(THD *thd, int n_calls)
   }
   thd_proc_info(thd, save_proc_info);
 }
+
+
+/*
+  Usage
+  
+  DBUG_EXECUTE_IF("show_explain_probe_2", 
+                     if (dbug_user_var_equals_int(thd, "select_id", select_id)) 
+                        dbug_serve_apcs(thd, 1);
+                 );
+
+*/
+
+bool dbug_user_var_equals_int(THD *thd, const char *name, int value)
+{
+  user_var_entry *var;
+  LEX_STRING varname= {(char*)name, strlen(name)};
+  if ((var= get_variable(&thd->user_vars, varname, FALSE)))
+  {
+    bool null_value;
+    longlong var_value= var->val_int(&null_value);
+    if (!null_value && var_value == value)
+      return TRUE;
+  }
+  return FALSE;
+}
 #endif 
 
+
 /**
   This handles SELECT with and without UNION.
 */
@@ -2067,7 +2093,13 @@ void JOIN::exec_inner()
   int      tmp_error;
   DBUG_ENTER("JOIN::exec");
   
-  DBUG_EXECUTE_IF("show_explain_probe_1", dbug_serve_apcs(thd, 1););
+  DBUG_EXECUTE_IF("show_explain_probe_2", dbug_serve_apcs(thd, 1););
+  DBUG_EXECUTE_IF("show_explain_probe_1", 
+                  if (dbug_user_var_equals_int(thd, 
+                                               "show_explain_probe_select_id", 
+                                               select_lex->select_number))
+                        dbug_serve_apcs(thd, 1);
+                 );
 
   thd_proc_info(thd, "executing");
   error= 0;
@@ -20397,8 +20429,8 @@ void JOIN::clear()
 /**
   EXPLAIN handling.
 
-  Send a description about what how the select will be done to stdout.
-
+  Produce lines explaining execution of *this* select (not including children
+  selects)
   @param on_the_fly TRUE <=> we're being executed on-the-fly, so don't make 
                     modifications to any select's data structures
 */