mysql 5.6 即將發布， 5.6對優化器方面做了諸多優化。 我這次主要解釋mrr（multi-range-read）。
我用儲存過程解釋了這一過程的改變。大家細心體會去吧。
我們針對語句：
select log_time from person where nick_name = 'lucy';
表結構為：
create table `person` (`id` int(10) unsigned not null auto_increment,
`nick_name` varchar(40) not null,
`log_time` timestamp not null default current_timestamp,
primary key (`id`),
key `idx_nick_name` (`nick_name`)
) engine=innodb auto_increment=5 default charset=latin1
首先是mysql 5.5.
delimiter $$use `ytt`$$
drop procedure if exists `sp_range_scan5_5`$$
create definer=`admin`@`%` procedure `sp_range_scan5_5`()
begin
-- sample sql statement is below.
-- select log_time from person where nick_name = 'lucy';
declare i int unsigned default 0;
declare cnt int unsigned default 0;
set @result = '';
select count(1) into cnt from person where nick_name = 'lucy';
loop1:while i < cnt
doset @stmt = concat('select id into @v_id from person where nick_name = ''lucy'' order by nick_name asc limit ',i,',1');
prepare s1 from @stmt;
execute s1;
set @result = concat(@result,'select log_time from person where id = @v_id');
set @result = concat(@result,' union all ');
set i = i + 1;
end while loop1;
set @result = substr(@result,1,char_length(@result)-char_length(' union all '));
prepare s1 from @result;
execute s1;
drop prepare s1;
set @result = null;
end$$
delimiter ;
下來是mysql 5.6.
delimiter $$use `ytt`$$
drop procedure if exists `sp_range_scan5_6`$$
create definer=`admin`@`%` procedure `sp_range_scan5_6`()
begin
-- sample sql statement is below.
-- select log_time from person where nick_name = 'lucy';
declare i int unsigned default 0;
declare cnt int unsigned default 0;
declare ids text;
set ids = '';
select count(1) into cnt from person where nick_name = 'lucy';
loop1:while i < cnt
doset @stmt = concat('select id into @v_id from person where nick_name = ''lucy''
order by nick_name asc limit ',i,',1');
prepare s1 from @stmt;
execute s1;
set ids = concat(ids,@v_id,',');
set i = i + 1;
end while loop1;
set ids = concat('(',substr(ids,1,char_length(ids)-1),')');
set @result = concat('select log_time from person where id in',ids);
prepare s1 from @result;
execute s1;
drop prepare s1;
set @result = null;
end$$
delimiter ;
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