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Fixed LP bug #794901.

Browse Source 
Also:
1. simplified the code of the function mysql_derived_merge_for_insert.
2. moved merge of views/dt for multi-update/delete to the prepare stage.
3. the list of the references to the candidates for semi-join now is
   allocated in the statement memory.
This commit is contained in:
Igor Babaev
2011-07-16 23:57:43 -07:00
parent 6e5413853e
commit d37465a9cc
13 changed files with 153 additions and 136 deletions
Download Patch File Download Diff File Expand all files Collapse all files

13
mysql-test/r/derived_view.result
View File

@ -1003,3 +1003,16 @@ Warnings:
Note 1003 select `test`.`t2`.`a` AS `a`,0 AS `a`,0 AS `b` from `test`.`t1` left join `test`.`t2` on((0 <> 0)) where <expr_cache><0>(<in_optimizer>(0,<exists>(select 0 from `test`.`t1` where (<cache>(0) = 0))))
DROP VIEW v1;
DROP TABLE t1,t2;
#
# LP bug #794901: insert into a multi-table view
#
CREATE TABLE t1 (a int);
CREATE TABLE t2 (a int);
CREATE TABLE t3 (a int);
CREATE VIEW v1 AS SELECT t1.a FROM t1,t2;
CREATE VIEW v2 AS SELECT a FROM t2 GROUP BY a;
CREATE VIEW v3 AS SELECT v1.a FROM v1,v2;
INSERT INTO v3(a) VALUES (1);
ERROR HY000: The target table v3 of the INSERT is not insertable-into
DROP VIEW v1,v2,v3;
DROP TABLE t1,t2,t3;

18
mysql-test/t/derived_view.test
View File

@ -582,3 +582,21 @@ SELECT * FROM t2 RIGHT JOIN v1 AS t ON t.a != 0
DROP VIEW v1;
DROP TABLE t1,t2;
--echo #
--echo # LP bug #794901: insert into a multi-table view
--echo #
CREATE TABLE t1 (a int);
CREATE TABLE t2 (a int);
CREATE TABLE t3 (a int);
CREATE VIEW v1 AS SELECT t1.a FROM t1,t2;
CREATE VIEW v2 AS SELECT a FROM t2 GROUP BY a;
CREATE VIEW v3 AS SELECT v1.a FROM v1,v2;
-- error ER_NON_INSERTABLE_TABLE
INSERT INTO v3(a) VALUES (1);
DROP VIEW v1,v2,v3;
DROP TABLE t1,t2,t3;

1
sql/item_subselect.cc
View File

@ -1147,6 +1147,7 @@ Item_in_subselect::Item_in_subselect(Item * left_exp,
Item_exists_subselect(), left_expr_cache(0), first_execution(TRUE),
optimizer(0), pushed_cond_guards(NULL), in_strategy(0),
is_jtbm_merged(FALSE), is_flattenable_semijoin(FALSE),
is_registered_semijoin(FALSE),
upper_item(0)
{
DBUG_ENTER("Item_in_subselect::Item_in_subselect");

5
sql/item_subselect.h
View File

@ -458,6 +458,11 @@ public:
*/
bool is_flattenable_semijoin;
/*
TRUE<=>registered in the list of semijoins in outer select
*/
bool is_registered_semijoin;
/*
Used to determine how this subselect item is represented in the item tree,
in case there is a need to locate it there and replace with something else.

127
sql/opt_subselect.cc
View File

@ -177,8 +177,8 @@ bool subquery_types_allow_materialization(Item_in_subselect *in_subs);
static bool replace_where_subcondition(JOIN *join, Item **expr,
Item *old_cond, Item *new_cond,
bool do_fix_fields);
static int subq_sj_candidate_cmp(Item_in_subselect* const *el1,
Item_in_subselect* const *el2);
static int subq_sj_candidate_cmp(Item_in_subselect* el1, Item_in_subselect* el2,
void *arg);
static bool convert_subq_to_sj(JOIN *parent_join, Item_in_subselect *subq_pred);
static bool convert_subq_to_jtbm(JOIN *parent_join,
Item_in_subselect *subq_pred, bool *remove);
@ -357,8 +357,15 @@ int check_and_do_in_subquery_rewrites(JOIN *join)
in_subs->is_flattenable_semijoin= TRUE;
/* Register the subquery for further processing in flatten_subqueries() */
select_lex->
outer_select()->join->sj_subselects.append(thd->mem_root, in_subs);
if (!in_subs->is_registered_semijoin)
{
Query_arena *arena, backup;
arena= thd->activate_stmt_arena_if_needed(&backup);
select_lex->outer_select()->sj_subselects.push_back(in_subs);
if (arena)
thd->restore_active_arena(arena, &backup);
in_subs->is_registered_semijoin= TRUE;
}
}
else
{
@ -405,7 +412,7 @@ int check_and_do_in_subquery_rewrites(JOIN *join)
condition also excludes multi-table update statements.
A note about prepared statements: we want the if-branch to be taken on
PREPARE and each EXECUTE. The rewrites are only done once, but we need
join->sj_subselects list to be populated for every EXECUTE.
select_lex->sj_subselects list to be populated for every EXECUTE.
*/
if (optimizer_flag(thd, OPTIMIZER_SWITCH_MATERIALIZATION) && // 0
@ -432,8 +439,15 @@ int check_and_do_in_subquery_rewrites(JOIN *join)
{
in_subs->emb_on_expr_nest= thd->thd_marker.emb_on_expr_nest;
in_subs->is_flattenable_semijoin= FALSE;
select_lex->outer_select()->
join->sj_subselects.append(thd->mem_root, in_subs);
if (!in_subs->is_registered_semijoin)
{
Query_arena *arena, backup;
arena= thd->activate_stmt_arena_if_needed(&backup);
select_lex->outer_select()->sj_subselects.push_back(in_subs);
if (arena)
thd->restore_active_arena(arena, &backup);
in_subs->is_registered_semijoin= TRUE;
}
}
}
@ -731,21 +745,19 @@ bool check_for_outer_joins(List<TABLE_LIST> *join_list)
bool convert_join_subqueries_to_semijoins(JOIN *join)
{
Query_arena *arena, backup;
Item_in_subselect **in_subq;
Item_in_subselect **in_subq_end;
Item_in_subselect *in_subq;
THD *thd= join->thd;
List_iterator<TABLE_LIST> ti(join->select_lex->leaf_tables);
DBUG_ENTER("convert_join_subqueries_to_semijoins");
if (join->sj_subselects.elements() == 0)
if (join->select_lex->sj_subselects.is_empty())
DBUG_RETURN(FALSE);
for (in_subq= join->sj_subselects.front(),
in_subq_end= join->sj_subselects.back();
in_subq != in_subq_end;
in_subq++)
List_iterator_fast<Item_in_subselect> li(join->select_lex->sj_subselects);
while ((in_subq= li++))
{
SELECT_LEX *subq_sel= (*in_subq)->get_select_lex();
SELECT_LEX *subq_sel= in_subq->get_select_lex();
if (subq_sel->handle_derived(thd->lex, DT_OPTIMIZE))
DBUG_RETURN(1);
if (subq_sel->handle_derived(thd->lex, DT_MERGE))
@ -753,13 +765,11 @@ bool convert_join_subqueries_to_semijoins(JOIN *join)
subq_sel->update_used_tables();
}
li.rewind();
/* First, convert child join's subqueries. We proceed bottom-up here */
for (in_subq= join->sj_subselects.front(),
in_subq_end= join->sj_subselects.back();
in_subq != in_subq_end;
in_subq++)
while ((in_subq= li++))
{
st_select_lex *child_select= (*in_subq)->get_select_lex();
st_select_lex *child_select= in_subq->get_select_lex();
JOIN *child_join= child_select->join;
child_join->outer_tables = child_join->table_count;
@ -773,9 +783,9 @@ bool convert_join_subqueries_to_semijoins(JOIN *join)
if (convert_join_subqueries_to_semijoins(child_join))
DBUG_RETURN(TRUE);
(*in_subq)->sj_convert_priority=
test((*in_subq)->emb_on_expr_nest != NO_JOIN_NEST) * MAX_TABLES * 2 +
(*in_subq)->is_correlated * MAX_TABLES + child_join->outer_tables;
in_subq->sj_convert_priority=
test(in_subq->emb_on_expr_nest != NO_JOIN_NEST) * MAX_TABLES * 2 +
in_subq->is_correlated * MAX_TABLES + child_join->outer_tables;
}
// Temporary measure: disable semi-joins when they are together with outer
@ -783,7 +793,7 @@ bool convert_join_subqueries_to_semijoins(JOIN *join)
#if 0
if (check_for_outer_joins(join->join_list))
{
in_subq= join->sj_subselects.front();
in_subq= join->select_lex->sj_subselects.head();
arena= thd->activate_stmt_arena_if_needed(&backup);
goto skip_conversion;
}
@ -795,41 +805,41 @@ bool convert_join_subqueries_to_semijoins(JOIN *join)
- prefer correlated subqueries over uncorrelated;
- prefer subqueries that have greater number of outer tables;
*/
join->sj_subselects.sort(subq_sj_candidate_cmp);
bubble_sort<Item_in_subselect>(&join->select_lex->sj_subselects,
subq_sj_candidate_cmp, NULL);
// #tables-in-parent-query + #tables-in-subquery < MAX_TABLES
/* Replace all subqueries to be flattened with Item_int(1) */
arena= thd->activate_stmt_arena_if_needed(&backup);
for (in_subq= join->sj_subselects.front();
in_subq != in_subq_end;
in_subq++)
li.rewind();
while ((in_subq= li++))
{
bool remove_item= TRUE;
/* Stop processing if we've reached a subquery that's attached to the ON clause */
if ((*in_subq)->emb_on_expr_nest != NO_JOIN_NEST)
if (in_subq->emb_on_expr_nest != NO_JOIN_NEST)
break;
if ((*in_subq)->is_flattenable_semijoin)
if (in_subq->is_flattenable_semijoin)
{
if (join->table_count +
(*in_subq)->unit->first_select()->join->table_count >= MAX_TABLES)
in_subq->unit->first_select()->join->table_count >= MAX_TABLES)
break;
if (convert_subq_to_sj(join, *in_subq))
if (convert_subq_to_sj(join, in_subq))
DBUG_RETURN(TRUE);
}
else
{
if (join->table_count + 1 >= MAX_TABLES)
break;
if (convert_subq_to_jtbm(join, *in_subq, &remove_item))
if (convert_subq_to_jtbm(join, in_subq, &remove_item))
DBUG_RETURN(TRUE);
}
if (remove_item)
{
Item **tree= ((*in_subq)->emb_on_expr_nest == NO_JOIN_NEST)?
&join->conds : &((*in_subq)->emb_on_expr_nest->on_expr);
Item *replace_me= (*in_subq)->original_item();
Item **tree= (in_subq->emb_on_expr_nest == NO_JOIN_NEST)?
&join->conds : &(in_subq->emb_on_expr_nest->on_expr);
Item *replace_me= in_subq->original_item();
if (replace_where_subcondition(join, tree, replace_me, new Item_int(1),
FALSE))
DBUG_RETURN(TRUE); /* purecov: inspected */
@ -840,34 +850,34 @@ bool convert_join_subqueries_to_semijoins(JOIN *join)
3. Finalize (perform IN->EXISTS rewrite) the subqueries that we didn't
convert:
*/
for (; in_subq!= in_subq_end; in_subq++)
while (in_subq)
{
JOIN *child_join= (*in_subq)->unit->first_select()->join;
(*in_subq)->changed= 0;
(*in_subq)->fixed= 0;
JOIN *child_join= in_subq->unit->first_select()->join;
in_subq->changed= 0;
in_subq->fixed= 0;
SELECT_LEX *save_select_lex= thd->lex->current_select;
thd->lex->current_select= (*in_subq)->unit->first_select();
thd->lex->current_select= in_subq->unit->first_select();
bool res= (*in_subq)->select_transformer(child_join);
bool res= in_subq->select_transformer(child_join);
thd->lex->current_select= save_select_lex;
if (res)
DBUG_RETURN(TRUE);
(*in_subq)->changed= 1;
(*in_subq)->fixed= 1;
in_subq->changed= 1;
in_subq->fixed= 1;
Item *substitute= (*in_subq)->substitution;
bool do_fix_fields= !(*in_subq)->substitution->fixed;
Item **tree= ((*in_subq)->emb_on_expr_nest == NO_JOIN_NEST)?
&join->conds : &((*in_subq)->emb_on_expr_nest->on_expr);
Item *replace_me= (*in_subq)->original_item();
Item *substitute= in_subq->substitution;
bool do_fix_fields= !in_subq->substitution->fixed;
Item **tree= (in_subq->emb_on_expr_nest == NO_JOIN_NEST)?
&join->conds : &(in_subq->emb_on_expr_nest->on_expr);
Item *replace_me= in_subq->original_item();
if (replace_where_subcondition(join, tree, replace_me, substitute,
do_fix_fields))
DBUG_RETURN(TRUE);
(*in_subq)->substitution= NULL;
in_subq->substitution= NULL;
#if 0
/*
Don't do the following, because the simplify_join() call is after this
@ -881,9 +891,9 @@ bool convert_join_subqueries_to_semijoins(JOIN *join)
*/
if (!thd->stmt_arena->is_conventional())
{
tree= ((*in_subq)->emb_on_expr_nest == NO_JOIN_NEST)?
tree= (in_subq->emb_on_expr_nest == NO_JOIN_NEST)?
&join->select_lex->prep_where :
&((*in_subq)->emb_on_expr_nest->prep_on_expr);
&(in_subq->emb_on_expr_nest->prep_on_expr);
if (replace_where_subcondition(join, tree, replace_me, substitute,
FALSE))
@ -898,12 +908,13 @@ bool convert_join_subqueries_to_semijoins(JOIN *join)
re-run the test for materialization that was done in
check_and_do_in_subquery_rewrites.
*/
(*in_subq)->in_strategy= SUBS_IN_TO_EXISTS;
in_subq->in_strategy= SUBS_IN_TO_EXISTS;
in_subq= li++;
}
if (arena)
thd->restore_active_arena(arena, &backup);
join->sj_subselects.clear();
join->select_lex->sj_subselects.empty();
DBUG_RETURN(FALSE);
}
@ -1004,11 +1015,11 @@ static bool replace_where_subcondition(JOIN *join, Item **expr,
return TRUE;
}
static int subq_sj_candidate_cmp(Item_in_subselect* const *el1,
Item_in_subselect* const *el2)
static int subq_sj_candidate_cmp(Item_in_subselect* el1, Item_in_subselect* el2,
void *arg)
{
return ((*el1)->sj_convert_priority < (*el2)->sj_convert_priority) ? 1 :
( ((*el1)->sj_convert_priority == (*el2)->sj_convert_priority)? 0 : -1);
return (el1->sj_convert_priority > el2->sj_convert_priority) ? 1 :
( (el1->sj_convert_priority == el2->sj_convert_priority)? 0 : -1);
}

4
sql/sql_delete.cc
View File

@ -603,9 +603,11 @@ int mysql_multi_delete_prepare(THD *thd)
&thd->lex->select_lex.top_join_list,
lex->query_tables,
lex->select_lex.leaf_tables, FALSE,
DELETE_ACL, SELECT_ACL, TRUE))
DELETE_ACL, SELECT_ACL, FALSE))
DBUG_RETURN(TRUE);
if (lex->select_lex.handle_derived(thd->lex, DT_MERGE))
DBUG_RETURN(TRUE);
/*
Multi-delete can't be constructed over-union => we always have

53
sql/sql_derived.cc
View File

@ -346,10 +346,17 @@ bool mysql_derived_merge(THD *thd, LEX *lex, TABLE_LIST *derived)
if (derived->merged)
return FALSE;
if (thd->lex->sql_command == SQLCOM_UPDATE_MULTI ||
thd->lex->sql_command == SQLCOM_DELETE_MULTI)
thd->save_prep_leaf_list= TRUE;
arena= thd->activate_stmt_arena_if_needed(&backup); // For easier test
derived->merged= TRUE;
if (!derived->merged_for_insert)
if (!derived->merged_for_insert ||
(derived->is_multitable() &&
(thd->lex->sql_command == SQLCOM_UPDATE_MULTI ||
thd->lex->sql_command == SQLCOM_DELETE_MULTI)))
{
/*
Check whether there is enough free bits in table map to merge subquery.
@ -392,7 +399,7 @@ bool mysql_derived_merge(THD *thd, LEX *lex, TABLE_LIST *derived)
}
/* Merge derived table's subquery in the parent select. */
if (parent_lex->merge_subquery(derived, dt_select, tablenr, map))
if (parent_lex->merge_subquery(thd, derived, dt_select, tablenr, map))
{
res= TRUE;
goto exit_merge;
@ -468,8 +475,6 @@ exit_merge:
bool mysql_derived_merge_for_insert(THD *thd, LEX *lex, TABLE_LIST *derived)
{
SELECT_LEX *dt_select= derived->get_single_select();
if (derived->merged_for_insert)
return FALSE;
if (derived->is_materialized_derived())
@ -482,43 +487,13 @@ bool mysql_derived_merge_for_insert(THD *thd, LEX *lex, TABLE_LIST *derived)
{
if (!derived->updatable)
return derived->create_field_translation(thd);
TABLE_LIST *tl=((TABLE_LIST*)dt_select->table_list.first);
TABLE *table= tl->table;
/* preserve old map & tablenr. */
if (!derived->merged_for_insert && derived->table)
table->set_table_map(derived->table->map, derived->table->tablenr);
derived->table= table;
derived->schema_table=
((TABLE_LIST*)dt_select->table_list.first)->schema_table;
if (!derived->merged)
if (derived->merge_underlying_list)
{
Query_arena *arena, backup;
arena= thd->activate_stmt_arena_if_needed(&backup); // For easier test
derived->select_lex->leaf_tables.push_back(tl);
derived->nested_join= (NESTED_JOIN*) thd->calloc(sizeof(NESTED_JOIN));
if (derived->nested_join)
{
derived->wrap_into_nested_join(tl->select_lex->top_join_list);
derived->get_unit()->exclude_level();
}
if (arena)
thd->restore_active_arena(arena, &backup);
derived->merged= TRUE;
if (!derived->nested_join)
return TRUE;
}
}
else
{
if (thd->lex->sql_command == SQLCOM_UPDATE_MULTI ||
thd->lex->sql_command == SQLCOM_DELETE_MULTI)
thd->save_prep_leaf_list= TRUE;
if (!derived->merged_for_insert && mysql_derived_merge(thd, lex, derived))
return TRUE;
}
derived->table= derived->merge_underlying_list->table;
derived->schema_table= derived->merge_underlying_list->schema_table;
derived->merged_for_insert= TRUE;
}
}
return FALSE;
}

2
sql/sql_insert.cc
View File

@ -135,7 +135,7 @@ bool check_view_single_update(List<Item> &fields, List<Item> *values,
A buffer for the insert values was allocated for the merged view.
Use it.
*/
//tbl->table->insert_values= view->table->insert_values;
tbl->table->insert_values= view->table->insert_values;
view->table= tbl->table;
*map= tables;

30
sql/sql_lex.cc
View File

@ -1647,6 +1647,7 @@ void st_select_lex::init_select()
{
st_select_lex_node::init_select();
sj_nests.empty();
sj_subselects.empty();
group_list.empty();
type= db= 0;
having= 0;
@ -3235,12 +3236,6 @@ bool st_select_lex::get_free_table_map(table_map *map, uint *tablenr)
*map= 0;
*tablenr= 0;
TABLE_LIST *tl;
if (!join)
{
(*map)= 1<<1;
(*tablenr)++;
return FALSE;
}
List_iterator<TABLE_LIST> ti(leaf_tables);
while ((tl= ti++))
{
@ -3397,7 +3392,8 @@ void st_select_lex::remap_tables(TABLE_LIST *derived, table_map map,
@return FALSE ok
*/
bool SELECT_LEX::merge_subquery(TABLE_LIST *derived, SELECT_LEX *subq_select,
bool SELECT_LEX::merge_subquery(THD *thd, TABLE_LIST *derived,
SELECT_LEX *subq_select,
uint table_no, table_map map)
{
derived->wrap_into_nested_join(subq_select->top_join_list);
@ -3405,19 +3401,17 @@ bool SELECT_LEX::merge_subquery(TABLE_LIST *derived, SELECT_LEX *subq_select,
leaf_tables.concat(&subq_select->leaf_tables);
ftfunc_list->concat(subq_select->ftfunc_list);
if (join)
if (join ||
thd->lex->sql_command == SQLCOM_UPDATE_MULTI ||
thd->lex->sql_command == SQLCOM_DELETE_MULTI)
{
Item_in_subselect **in_subq;
Item_in_subselect **in_subq_end;
for (in_subq= subq_select->join->sj_subselects.front(),
in_subq_end= subq_select->join->sj_subselects.back();
in_subq != in_subq_end;
in_subq++)
List_iterator_fast<Item_in_subselect> li(subq_select->sj_subselects);
Item_in_subselect *in_subq;
while ((in_subq= li++))
{
join->sj_subselects.append(join->thd->mem_root, *in_subq);
DBUG_ASSERT((*in_subq)->emb_on_expr_nest != NULL);
if ((*in_subq)->emb_on_expr_nest == NO_JOIN_NEST)
(*in_subq)->emb_on_expr_nest= derived;
sj_subselects.push_back(in_subq);
if (in_subq->emb_on_expr_nest == NO_JOIN_NEST)
in_subq->emb_on_expr_nest= derived;
}
}
/*

8
sql/sql_lex.h
View File

@ -637,6 +637,12 @@ public:
tables. Unlike 'next_local', this in this list views are *not*
leaves. Created in setup_tables() -> make_leaves_list().
*/
/*
Subqueries that will need to be converted to semi-join nests, including
those converted to jtbm nests. The list is emptied when conversion is done.
*/
List<Item_in_subselect> sj_subselects;
List<TABLE_LIST> leaf_tables;
List<TABLE_LIST> leaf_tables_exec;
List<TABLE_LIST> leaf_tables_prep;
@ -886,7 +892,7 @@ public:
void remove_table_from_list(TABLE_LIST *table);
void remap_tables(TABLE_LIST *derived, table_map map,
uint tablenr, st_select_lex *parent_lex);
bool merge_subquery(TABLE_LIST *derived, st_select_lex *subq_lex,
bool merge_subquery(THD *thd, TABLE_LIST *derived, st_select_lex *subq_lex,
uint tablenr, table_map map);
inline bool is_mergeable()
{

7
sql/sql_select.h
View File

@ -964,11 +964,6 @@ public:
bool optimized; ///< flag to avoid double optimization in EXPLAIN
bool initialized; ///< flag to avoid double init_execution calls
/*
Subqueries that will need to be converted to semi-join nests, including
those converted to jtbm nests. The list is emptied when conversion is done.
*/
Array<Item_in_subselect> sj_subselects;
/*
Additional WHERE and HAVING predicates to be considered for IN=>EXISTS
subquery transformation of a JOIN object.
@ -998,7 +993,7 @@ public:
JOIN(THD *thd_arg, List<Item> &fields_arg, ulonglong select_options_arg,
select_result *result_arg)
:fields_list(fields_arg), sj_subselects(thd_arg->mem_root, 4)
:fields_list(fields_arg)
{
init(thd_arg, fields_arg, select_options_arg, result_arg);
}

15
sql/sql_update.cc
View File

@ -248,6 +248,7 @@ int mysql_update(THD *thd,
DBUG_RETURN(1);
close_tables_for_reopen(thd, &table_list);
}
if (table_list->handle_derived(thd->lex, DT_MERGE_FOR_INSERT))
DBUG_RETURN(1);
if (table_list->handle_derived(thd->lex, DT_PREPARE))
@ -1037,10 +1038,10 @@ reopen_tables:
second time, but this call will do nothing (there are check for second
call in setup_tables()).
*/
//We need to merge for insert prior to prepare.
if (mysql_handle_derived(lex, DT_MERGE_FOR_INSERT))
DBUG_RETURN(TRUE);
if (mysql_handle_derived(lex, DT_PREPARE))
DBUG_RETURN(TRUE);
@ -1048,7 +1049,10 @@ reopen_tables:
&lex->select_lex.top_join_list,
table_list,
lex->select_lex.leaf_tables, FALSE,
UPDATE_ACL, SELECT_ACL, TRUE))
UPDATE_ACL, SELECT_ACL, FALSE))
DBUG_RETURN(TRUE);
if (lex->select_lex.handle_derived(thd->lex, DT_MERGE))
DBUG_RETURN(TRUE);
if (setup_fields_with_no_wrap(thd, 0, *fields, MARK_COLUMNS_WRITE, 0, 0))
@ -1334,13 +1338,6 @@ int multi_update::prepare(List<Item> &not_used_values,
thd->cuted_fields=0L;
thd_proc_info(thd, "updating main table");
SELECT_LEX *select_lex= lex_unit->first_select();
if (select_lex->first_cond_optimization)
{
if (select_lex->handle_derived(thd->lex, DT_MERGE))
DBUG_RETURN(TRUE);
}
tables_to_update= get_table_map(fields);
if (!tables_to_update)