问题描述：MySQL5.5.15原sql如下：selectconstraint_schema,table_name,constraint_name,constraint_typefrominformation_schema.table_constraintswheretable_s
./storage/innobase/row/row0mysql.c
/*********************************************************************//**
Updates the table modification counter and calculates new estimates
for table and index statistics if necessary. */
UNIV_INLINE
void
row_update_statistics_if_needed(
/*============================*/
dict_table_t* table) /*!stat_modified_counter;
table->stat_modified_counter = counter + 1;
/* Calculate new statistics if 1 / 16 of table has been modified
since the last time a statistics batch was run, or if
stat_modified_counter > 2 000 000 000 (to avoid wrap-around).
We calculate statistics at most every 16th round, since we may have
a counter table which is very small and updated very often. */
if (counter > 2000000000
|| ((ib_int64_t)counter > 16 + table->stat_n_rows / 16)) {
dict_update_statistics(table, FALSE /* update even if stats
are initialized */);
}
}
---------------------------------------------------------------------------
#通过更新统计信息stat_modified_counter，每个表都有这个表里来维护：
./storage/innobase/row/row0mysql.c
/*********************************************************************//**
Updates the table modification counter and calculates new estimates
for table and index statistics if necessary. */
UNIV_INLINE
void
row_update_statistics_if_needed(
/*============================*/
dict_table_t* table) /*!stat_modified_counter;
table->stat_modified_counter = counter + 1;
if (DICT_TABLE_CHANGED_TOO_MUCH(table)) {
dict_update_statistics(
table,
FALSE, /* update even if stats are initialized */
TRUE /* only update if stats changed too much */);
}
}
/*********************************************************************/
规则：每一次DML操作导致1 行更新,stat_modified_counter加1，直到满足更新统计信息的条件，stat_modified_counter的值自动重置为0。
#更新统计信息的条件：（有超过1/16的row被更改过会更新表的条件信息）
./storage/innobase/include/dict0dict.h
/** Calculate new statistics if 1 / 16 of table has been modified
since the last time a statistics batch was run.
We calculate statistics at most every 16th round, since we may have
a counter table which is very small and updated very often.
@param t table
@return true if the table has changed too much and stats need to be
recalculated
*/
#define DICT_TABLE_CHANGED_TOO_MUCH(t) \
((ib_int64_t) (t)->stat_modified_counter > 16 + (t)->stat_n_rows / 16)
/*********************************************************************/
* 这样有个性能问题，若有多个线程同时检测到阈值，也即是并发调用会多次，，会导致dict_update_statistics函数多次的调用，浪费了系统资源。
解决方法：在dict_update_statistics{}函数对stat_modified_counter加锁，避免并发执行。
#统计新跟更新函数：dict_update_statistics
