MySQL派生表查询崩溃根源剖析与解决策略
一、问题的发现
在使用MySQL 8.0.32时，发现执行包含派生表的以下SQL会导致MySQL崩溃，其中sequence_table(2)替换为任意非常量表都会出现此情况：
仅MySQL 8.0.32版本受影响。
EXPLAIN FORMAT=TREE
select
trim(ref_15.c_ogj),
0<>0 as c_lrcm63eani
from
(select
0<>0 as c_ogj
from
sequence_table(2) t1
where 0<>0
order by c_ogj asc) as ref_15;
崩溃时的堆栈信息如下：
Thread 55 "mysqld" received signal SIGSEGV, Segmentation fault.
Item_view_ref::used_tables (this=0x7fff2418f410)
at sql/item.h:6670
6670 table_map inner_map = ref_item()->used_tables(); ==> ref_item()为空指针，因此crash了
(gdb) bt
#0 Item_view_ref::used_tables (this=0x7fff2418f410)
at sql/item.h:6670
#1 0x0000555558e978d1 in Item::const_item (this=0x7fff2418f410)
at sql/item.h:2342
#2 0x0000555558ecc765 in Item_ref::print (this=0x7fff2418f410, thd=0x7fff24001050,
str=0x7fffc83ee7e0, query_type=(QT_TO_SYSTEM_CHARSET | QT_SHOW_SELECT_NUMBER))
at sql/item.cc:9993
#3 0x000055555903b839 in Item_func_trim::print (this=0x7fff24120d20, thd=0x7fff24001050,
str=0x7fffc83ee7e0, query_type=(QT_TO_SYSTEM_CHARSET | QT_SHOW_SELECT_NUMBER))
at sql/item_strfunc.cc:3244
#4 0x0000555558ea7fc5 in Item::print_item_w_name (this=0x7fff24120d20, thd=0x7fff24001050,
str=0x7fffc83ee7e0, query_type=(QT_TO_SYSTEM_CHARSET | QT_SHOW_SELECT_NUMBER))
at sql/item.cc:727
#5 0x00005555593f18c0 in Query_block::print_item_list (this=0x7fff24120768, thd=0x7fff24001050,
str=0x7fffc83ee7e0, query_type=(QT_TO_SYSTEM_CHARSET | QT_SHOW_SELECT_NUMBER))
at sql/sql_lex.cc:4041
#6 0x00005555593efb50 in Query_block::print_query_block (this=0x7fff24120768,
thd=0x7fff24001050, str=0x7fffc83ee7e0,
query_type=(QT_TO_SYSTEM_CHARSET | QT_SHOW_SELECT_NUMBER))
at sql/sql_lex.cc:3614
#7 0x00005555593efa3d in Query_block::print (this=0x7fff24120768, thd=0x7fff24001050,
str=0x7fffc83ee7e0, query_type=(QT_TO_SYSTEM_CHARSET | QT_SHOW_SELECT_NUMBER))
at sql/sql_lex.cc:3598
#8 0x00005555593ee556 in Query_expression::print (this=0x7fff24120670, thd=0x7fff24001050,
str=0x7fffc83ee7e0, query_type=(QT_TO_SYSTEM_CHARSET | QT_SHOW_SELECT_NUMBER))
at sql/sql_lex.cc:3232
#9 0x0000555559a89c2c in print_query_for_explain (query_thd=0x7fff24001050,
unit=0x7fff24120670, str=0x7fffc83ee7e0)
at sql/opt_explain.cc:2288
#10 0x0000555559a10b11 in PrintQueryPlan[abi:cxx11](THD*, THD const*, Query_expression*) (
ethd=0x7fff24001050, query_thd=0x7fff24001050, unit=0x7fff24120670)
at sql/join_optimizer/explain_access_path.cc:1894
#11 0x0000555559a8985a in ExplainIterator (ethd=0x7fff24001050, query_thd=0x7fff24001050,
unit=0x7fff24120670) at sql/opt_explain.cc:2205
#12 0x0000555559a89e91 in explain_query (explain_thd=0x7fff24001050, query_thd=0x7fff24001050,
unit=0x7fff24120670) at sql/opt_explain.cc:2359
#13 0x000055555955cd46 in Sql_cmd_dml::execute_inner (this=0x7fff24165630, thd=0x7fff24001050)
二、问题调查过程
对执行该SQL的优化过程进行调查，发现该SQL的SQL变换情况如下：
在trim(ref_15.c_ogj)执行完find_order_in_list后，Item_func_trim的args[0]->m_ref_item[0]变为0<>0 as c_lrcm63eani，而非0<>0 as c_ogj，这是因为c_lrcm63eani和c_ogj名字相同，在find_order_in_list函数中因名字相同导致内层字段被外层替代。随后在Item::clean_up_after_removal执行时，Item_func_ne即c_lrcm63eani因出现2次，执行了2次decrement_ref_count()，但在Query_block::delete_unused_merged_columns函数中，0<>0 as c_lrcm63eani的Item被置为空，因为此时item->decrement_ref_count()后ref_count()为0，继续执行Item::clean_up_after_removal。
EXPLAIN FORMAT=TREE
select
trim(ref_15.c_ogj),
0<>0 as c_lrcm63eani
from
(select
0<>0 as c_ogj
from
sequence_table(2) t1
where 0<>0
order by c_ogj asc) as ref_15;
查看函数调用过程发现，Query_block在prepare时执行了delete_unused_merged_columns，相关函数调用过程如下：
-- 函数调用过程： Query_block::prepare -> Query_block::apply_local_transforms -> Query_block::delete_unused_merged_columns
bool find_order_in_list() {
if (select_item != not_found_item) {
if ((*order->item)->real_item() != (*select_item)->real_item()) {
Item::Cleanup_after_removal_context ctx(
thd->lex->current_query_block());
(*order->item)
->walk(&Item::clean_up_after_removal, walk_options, ==>Item_func_ne执行了2次，也执行了2次decrement_ref_count()
pointer_cast<uchar *>(&ctx));
}
}
}
bool Query_block::apply_local_transforms(THD *thd, bool prune) {
DBUG_TRACE;
assert(first_execution);
-- 这个函数把((Item_func *)&fields[0][0])->args[0]->m_ref_item[0]给删了
if (derived_table_count) delete_unused_merged_columns(&m_table_nest);
}
void Query_block::delete_unused_merged_columns(
mem_root_deque<Table_ref *> *tables) {
DBUG_TRACE;
for (Table_ref *tl : *tables) {
if (tl->nested_join == nullptr) continue;
if (tl->is_merged()) {
for (Field_translator *transl = tl->field_translation;
transl < tl->field_translation_end; transl++) {
Item *const item = transl->item;
// Decrement the ref count as its no more used in
// select list.
if (item->decrement_ref_count()) continue; -- 因为执行完decrement_ref_count()以后返回的m_ref_count=0因此不会跳出这个循环
// Cleanup the item since its not referenced from
// anywhere.
assert(item->fixed);
Item::Cleanup_after_removal_context ctx(this);
item->walk(&Item::clean_up_after_removal, walk_options,
pointer_cast<uchar *>(&ctx));
transl->item = nullptr; -- 这个地方把Item_view_ref引用的Item_func_ne对象置为空了，即把trim函数参数的c_lrcm63eani列删除了
}
}
delete_unused_merged_columns(&tl->nested_join->m_tables);
}
}
三、解决方案
经分析，问题源于多执行了一次Item::clean_up_after_removal。在MySQL最新代码中，该BUG已被修复，相关修复代码如下：
相关commit ID号为： 2171a1260e2cdbbd379646be8ff6413a92fd48f4
-- 相关修复代码如下：
@@ -7575,7 +7865,6 @@ bool Item::clean_up_after_removal(uchar *arg) {
if (reference_count() > 1) {
(void)decrement_ref_count();
+ ctx->stop_at(this);
}
return false;
}
修改后查看函数堆栈信息：
#0 Item::clean_up_after_removal (this=0x2,
arg=0x41 <error: Cannot access memory at address 0x41>)
at sql/item.cc:9236
#1 0x0000555558fea5a8 in Item::walk (this=0x7fff2c338db8, processor=&virtual table offset 864,
walk=7, arg=0x7fffc83ee4b0 "") at sql/item.h:2543
#2 0x00005555596cc6f2 in find_order_in_list (thd=0x7fff2c001070, ref_item_array=...,
tables=0x7fff2c330b90, order=0x7fff2c32eae8, fields=0x7fff2c32fb20, is_group_field=false,
is_window_order=false) at sql/sql_resolver.cc:4625
#3 0x00005555596cd0ae in setup_order (thd=0x7fff2c001070, ref_item_array=...,
tables=0x7fff2c330b90, fields=0x7fff2c32fb20, order=0x7fff2c32eae8)
at sql/sql_resolver.cc:4811
#4 0x00005555596bf528 in Query_block::prepare (this=0x7fff2c32fae0, thd=0x7fff2c001070,
insert_field_list=0x0) at sql/sql_resolver.cc:400
#5 0x00005555597d035d in Query_expression::prepare (this=0x7fff2c32f9e8, thd=0x7fff2c001070,
sel_result=0x7fff2c33b2a8, insert_field_list=0x0, added_options=0, removed_options=0)
at sql/sql_union.cc:758
#6 0x0000555559590772 in Table_ref::resolve_derived (this=0x7fff2c339790, thd=0x7fff2c001070,
apply_semijoin=true) at sql/sql_derived.cc:451
#7 0x00005555596c2a80 in Query_block::resolve_placeholder_tables (this=0x7fff2c333f08,
thd=0x7fff2c001070, apply_semijoin=true)
at sql/sql_resolver.cc:1408
#8 0x00005555596bea62 in Query_block::prepare (this=0x7fff2c333f08, thd=0x7fff2c001070,
insert_field_list=0x0) at sql/sql_resolver.cc:265
对于0<>0 as c_lrcm63eani这个Item_func_ne对象，执行Item::clean_up_after_removal时，因reference_count() > 1会执行ctx->stop_at(this)，下次执行该函数时，因ctx->is_stopped(this)直接返回，不再执行decrement_ref_count()，避免了transl->item = nullptr的执行。
bool find_order_in_list() {
if (select_item != not_found_item) {
if ((*order->item)->real_item() != (*select_item)->real_item()) {
Item::Cleanup_after_removal_context ctx(
thd->lex->current_query_block());
(*order->item)
->walk(&Item::clean_up_after_removal, walk_options, -- Item_func_ne执行了2次，而只执行了一次decrement_ref_count()
pointer_cast<uchar *>(&ctx));
}
}
}
void Query_block::delete_unused_merged_columns(
mem_root_deque<Table_ref *> *tables) {
DBUG_TRACE;
for (Table_ref *tl : *tables) {
if (tl->nested_join == nullptr) continue;
if (tl->is_merged()) {
for (Field_translator *transl = tl->field_translation;
transl < tl->field_translation_end; transl++) {
Item *const item = transl->item;
// Decrement the ref count as its no more used in
// select list.
if (item->decrement_ref_count()) continue; 因为执行完decrement_ref_count()以后返回的m_ref_count=1因此不会继续执行后面的置空设置
// Cleanup the item since its not referenced from
// anywhere.
assert(item->fixed);
Item::Cleanup_after_removal_context ctx(this);
item->walk(&Item::clean_up_after_removal, walk_options,
pointer_cast<uchar *>(&ctx));
transl->item = nullptr; ==>这个地方不会运行到
}
}
delete_unused_merged_columns(&tl->nested_join->m_tables);
}
}
四、问题总结
通过分析可知，复杂SQL的Item变换和不需要的Item删除过程易引发Crash。分析此类Crash问题需熟悉代码运行流程，具备复杂问题解决经验，才能有效应对。
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