背景：
当数据库的建库字符集和表不一样时，在库下针对表创建存储过程可能导致全表扫描
如下例：
drop database if exists xx1; drop database if exists xx2; create database xx1 character set utf8; create database xx2 character set gbk;
然后分别在xx1 和 xx2下执行：
CREATE TABLE t1 ( `col1` varchar(10) NOT NULL, `col2` bigint(11) NOT NULL DEFAULT '0', PRIMARY KEY (`col1`)) ENGINE=InnoDB DEFAULT CHARSET=gbk;
insert into t1 values ('ac',2),('del',3),('sp',5),('tr',12);
DELIMITER //
CREATE PROCEDURE proc1(IN isn CHAR(20))
BEGIN
START TRANSACTION;
SELECT col2 FROM t1 WHERE col1= isn FOR UPDATE;
COMMIT;
END //
DELIMITER ;
在5.5版本中通过show processlist可以看到SQL进行了转换(简单的锁住行记录，执行存储过程，再show processlist)：
use xx1;
call proc1(‘tr’); 参数isn会被展开成：NAME_CONST(‘isn’,_utf8 0x7472 COLLATE ‘utf8_general_ci’)
use xx2;
call proc1(‘tr’); 参数isn会被展开成：NAME_CONST(‘isn’,_gbk 0x7472 COLLATE ‘gbk_chinese_ci’)
在5.6版本里使用ROW模式时，不会展开sql，但内部采取的是同样的策略。
由于name_const中引用的的字符集和表定义不同，因此xx1.proc1()每次调用都会产生两次全表扫描。但奇怪的是，如果单独执行上述SQL（而不是调用存储过程），就不会全表扫描。
还好MySQL5.6开始有了optimizer_trace这个利器，打开optimizer trace就一目了然了，对比单独执行SQL和执行存储过程：
| call xx1.proc1(‘tr’) |
| | SELECT col2 FROM t1 WHERE col1= NAME_CONST(‘isn’,_utf8’tr’ COLLATE ‘utf8_general_ci’) FOR UPDATE | { “steps”: [ { “join_preparation”: { “select#”: 1, “steps”: [ { “expanded_query”: “/* select#1 */ select `t1`.`col2` AS `col2` from `t1` where (convert(`t1`.`col1` using utf8) = isn@0)” } ] } }, { “join_optimization”: { “select#”: 1, “steps”: [ { “condition_processing”: { “condition”: “WHERE”, “original_condition”: “(convert(`t1`.`col1` using utf8) = isn@0)”, “steps”: [ { “transformation”: “equality_propagation”, “resulting_condition”: “(convert(`t1`.`col1` using utf8) = isn@0)” }, { “transformation”: “constant_propagation”, “resulting_condition”: “(convert(`t1`.`col1` using utf8) = isn@0)” }, { “transformation”: “trivial_condition_removal”, “resulting_condition”: “(convert(`t1`.`col1` using utf8) = isn@0)” } ] } }, { “table_dependencies”: [ { “table”: “`t1`”, “row_may_be_null”: false, “map_bit”: 0, “depends_on_map_bits”: [ ] } ] }, { “ref_optimizer_key_uses”: [ ] }, { “rows_estimation”: [ { “table”: “`t1`”, “table_scan”: { “rows”: 4, “cost”: 1 } } ] }, { “considered_execution_plans”: [ { “plan_prefix”: [ ], “table”: “`t1`”, “best_access_path”: { “considered_access_paths”: [ { “access_type”: “scan”, “rows”: 4, “cost”: 1.8, “chosen”: true } ] }, “cost_for_plan”: 1.8, “rows_for_plan”: 4, “chosen”: true } ] }, { “attaching_conditions_to_tables”: { “original_condition”: “(convert(`t1`.`col1` using utf8) = isn@0)”, “attached_conditions_computation”: [ ], “attached_conditions_summary”: [ { “table”: “`t1`”, “attached”: “(convert(`t1`.`col1` using utf8) = isn@0)” } ] } }, { “refine_plan”: [ { “table”: “`t1`”, “access_type”: “table_scan” } ] } ] } }, { “join_execution”: { “select#”: 1, “steps”: [ ] } } ] } |
| call xx2.proc1(‘tr’) |
| SELECT col2 FROM t1 WHERE col1= NAME_CONST(‘isn’,_gbk’tr’ COLLATE ‘gbk_chinese_ci’) FOR UPDATE | { “steps”: [ { “join_preparation”: { “select#”: 1, “steps”: [ { “expanded_query”: “/* select#1 */ select `t1`.`col2` AS `col2` from `t1` where (`t1`.`col1` = isn@0)” } ] } }, { “join_optimization”: { “select#”: 1, “steps”: [ { “condition_processing”: { “condition”: “WHERE”, “original_condition”: “(`t1`.`col1` = isn@0)”, “steps”: [ { “transformation”: “equality_propagation”, “resulting_condition”: “multiple equal(isn@0, `t1`.`col1`)” }, { “transformation”: “constant_propagation”, “resulting_condition”: “multiple equal(isn@0, `t1`.`col1`)” }, { “transformation”: “trivial_condition_removal”, “resulting_condition”: “multiple equal(isn@0, `t1`.`col1`)” } ] } }, { “table_dependencies”: [ { “table”: “`t1`”, “row_may_be_null”: false, “map_bit”: 0, “depends_on_map_bits”: [ ] } ] }, { “ref_optimizer_key_uses”: [ { “table”: “`t1`”, “field”: “col1″, “equals”: “isn@0″, “null_rejecting”: false } ] }, { “rows_estimation”: [ { “table”: “`t1`”, “rows”: 1, “cost”: 1, “table_type”: “const”, “empty”: false } ] }, { “condition_on_constant_tables”: “1”, “condition_value”: true }, { “attaching_conditions_to_tables”: { “original_condition”: “1”, “attached_conditions_computation”: [ ], “attached_conditions_summary”: [ ] } }, { “refine_plan”: [ ] } ] } }, { “join_execution”: { “select#”: 1, “steps”: [ ] } } ] } |
| use xx1;
SELECT col2 FROM t1 WHERE col1=
NAME_CONST
(‘isn’,_utf8’tr’ COLLATE ‘utf8_general_ci’)
FOR UPDATE
|
| SELECT col2 FROM t1 WHERE col1= NAME_CONST(‘isn’,_utf8’tr’ COLLATE ‘utf8_general_ci’) FOR UPDATE | { “steps”: [ { “join_preparation”: { “select#”: 1, “steps”: [ { “expanded_query”: “/* select#1 */ select `t1`.`col2` AS `col2` from `t1` where (`t1`.`col1` = convert(NAME_CONST(‘isn’,(_utf8’tr’ collate utf8_general_ci)) using gbk))” } ] } }, { “join_optimization”: { “select#”: 1, “steps”: [ { “condition_processing”: { “condition”: “WHERE”, “original_condition”: “(`t1`.`col1` = convert(NAME_CONST(‘isn’,(_utf8’tr’ collate utf8_general_ci)) using gbk))”, “steps”: [ { “transformation”: “equality_propagation”, “resulting_condition”: “multiple equal(convert(NAME_CONST(‘isn’,(_utf8’tr’ collate utf8_general_ci)) using gbk), `t1`.`col1`)” }, { “transformation”: “constant_propagation”, “resulting_condition”: “multiple equal(convert(NAME_CONST(‘isn’,(_utf8’tr’ collate utf8_general_ci)) using gbk), `t1`.`col1`)” }, { “transformation”: “trivial_condition_removal”, “resulting_condition”: “multiple equal(convert(NAME_CONST(‘isn’,(_utf8’tr’ collate utf8_general_ci)) using gbk), `t1`.`col1`)” } ] } }, { “table_dependencies”: [ { “table”: “`t1`”, “row_may_be_null”: false, “map_bit”: 0, “depends_on_map_bits”: [ ] } ] }, { “ref_optimizer_key_uses”: [ { “table”: “`t1`”, “field”: “col1″, “equals”: “convert(NAME_CONST(‘isn’,(_utf8’tr’ collate utf8_general_ci)) using gbk)”, “null_rejecting”: false } ] }, { “rows_estimation”: [ { “table”: “`t1`”, “rows”: 1, “cost”: 1, “table_type”: “const”, “empty”: false } ] }, { “condition_on_constant_tables”: “1”, “condition_value”: true }, { “attaching_conditions_to_tables”: { “original_condition”: “1”, “attached_conditions_computation”: [ ], “attached_conditions_summary”: [ ] } }, { “refine_plan”: [ ] } ] } }, { “join_execution”: { “select#”: 1, “steps”: [ ] } } ] } |
注意这expanded_query一项，对于这种方式的查询，需要读取每一条记录，进行字符集转换再比较：
expanded_query”: “/* select#1 */ select `t1`.`col2` AS `col2` from `t1` where (convert(`t1`.`col1` using utf8) = isn@0)
由于本人对优化器模块了解不深，也不确定是否是Bug，先Report到了一个小bug到Bug list，ref:http://bugs.mysql.com/bug.php?id=76002
Updated @2015/2/25, 存储过程的输入列需要显式的定义输入字符集，否则其输入列的字符集将继承自库的字符集。参考bug中的回复。
