Fix bug#14186 select datefield is null not updated

Date field was declared as not null, thus expression 'datefield is null'
was always false. For SELECT special handling of such cases is used. 
There 'datefield is null' converted to 'datefield eq "0000-00-00"'.

In mysql_update() before creation of select added remove_eq_conds() call.
It makes some optimization of conds and in particular performs conversion
from 'is null' to 'eq'. 
Also remove_eq_conds() makes some evaluation of conds and if it founds that
conds is always false then update statement is not processed further.
All this allows to perform some update statements process faster due to
optimized conds, and not wasting resources if conds known to be false. 


sql/sql_select.cc:
  Fix bug#14186  select datefield is null not updated
  Remove static from remove_eq_conds()
sql/sql_select.h:
   Fix bug#14186  select datefield is null not updated
  Added remove_eq_conds() prototype.
mysql-test/r/update.result:
  Test case for  bug#14186  select datefield is null not updated
mysql-test/t/update.test:
  Test case for  bug#14186  select datefield is null not updated
sql/sql_update.cc:
  Fix bug#14186  select datefield is null not updated
  To mysql_update() added call to remove_eq_conds() to optimize conds and convert 'datefield is null' to 'datefield eq 0000-00-00'

3 files changed, 14 insertions, 7 deletions

diff --git a/sql/sql_select.cc b/sql/sql_select.cc
index c485d6cd04e..fe5b3d453de 100644
--- a/sql/sql_select.cc
+++ b/sql/sql_select.cc
@@ -69,8 +69,6 @@ static int return_zero_rows(JOIN *join, select_result *res,TABLE_LIST *tables,
 			    SELECT_LEX_UNIT *unit);
 static COND *optimize_cond(THD *thd, COND *conds,
 			   Item::cond_result *cond_value);
-static COND *remove_eq_conds(THD *thd, COND *cond, 
-			     Item::cond_result *cond_value);
 static bool const_expression_in_where(COND *conds,Item *item, Item **comp_item);
 static bool open_tmp_table(TABLE *table);
 static bool create_myisam_tmp_table(TABLE *table,TMP_TABLE_PARAM *param,
@@ -4611,7 +4609,7 @@ optimize_cond(THD *thd, COND *conds, Item::cond_result *cond_value)
   COND_FALSE always false	( 1 = 2 )
 */
 
-static COND *
+COND *
 remove_eq_conds(THD *thd, COND *cond, Item::cond_result *cond_value)
 {
   if (cond->type() == Item::COND_ITEM)
diff --git a/sql/sql_select.h b/sql/sql_select.h
index c7440fe4c3a..b92576587fe 100644
--- a/sql/sql_select.h
+++ b/sql/sql_select.h
@@ -456,3 +456,4 @@ bool cp_buffer_from_ref(THD *thd, TABLE_REF *ref);
 bool error_if_full_join(JOIN *join);
 int report_error(TABLE *table, int error);
 int safe_index_read(JOIN_TAB *tab);
+COND *remove_eq_conds(THD *thd, COND *cond, Item::cond_result *cond_value);
diff --git a/sql/sql_update.cc b/sql/sql_update.cc
index b6bce800b0e..cb8064bef87 100644
--- a/sql/sql_update.cc
+++ b/sql/sql_update.cc
@@ -70,7 +70,7 @@ int mysql_update(THD *thd,
   ha_rows	updated, found;
   key_map	old_used_keys;
   TABLE		*table;
-  SQL_SELECT	*select;
+  SQL_SELECT	*select= 0;
   READ_RECORD	info;
   TABLE_LIST    *update_table_list= ((TABLE_LIST*)
 				     thd->lex->select_lex.table_list.first);
@@ -131,11 +131,19 @@ int mysql_update(THD *thd,
     DBUG_RETURN(-1);				/* purecov: inspected */
   }
 
+  if (conds)
+  {
+    Item::cond_result cond_value;
+    conds= remove_eq_conds(thd, conds, &cond_value);
+    if (cond_value == Item::COND_FALSE)
+      limit= 0;                                   // Impossible WHERE
+  }
   // Don't count on usage of 'only index' when calculating which key to use
   table->used_keys.clear_all();
-  select=make_select(table,0,0,conds,&error);
-  if (error ||
-      (select && select->check_quick(thd, safe_update, limit)) || !limit)
+  if (limit)
+    select=make_select(table,0,0,conds,&error);
+  if (error || !limit ||
+      (select && select->check_quick(thd, safe_update, limit)))
   {
     delete select;
     free_underlaid_joins(thd, &thd->lex->select_lex);