2008-06-25
在Rails中关于size,length,count三个方法的区别
关键字: activerocord
count,size,length到底有什么区别呢？通过以下的测试结果可以发现问题：
Post.find_by_id(953125641).comments.length产生了如下的SQL语句
[4;35;1mPost Columns (0.000000)[0m [0mSHOW FIELDS FROM `posts`[0m
[4;36;1mPost Load (0.016000)[0m [0;1mSELECT * FROM `posts` WHERE (`posts`.`id` = 953125641) LIMIT 1[0m
[4;35;1mComment Load (0.016000)[0m [0mSELECT * FROM `comments` WHERE (`comments`.post_id = 953125641) [0m
Post.find_by_id(953125641).comments.size产生了如下的SQL语句
[4;36;1mPost Columns (0.000000)[0m [0;1mSHOW FIELDS FROM `posts`[0m
[4;35;1mPost Load (0.016000)[0m [0mSELECT * FROM `posts` WHERE (`posts`.`id` = 953125641) LIMIT 1[0m
[4;36;1mComment Columns (0.000000)[0m [0;1mSHOW FIELDS FROM `comments`[0m
[4;35;1mSQL (0.015000)[0m [0mSELECT count(*) AS count_all FROM `comments` WHERE (`comments`.post_id = 953125641) [0m
Post.find_by_id(953125641).comments.count产生了如下的SQL语句
[4;36;1mPost Columns (0.000000)[0m [0;1mSHOW FIELDS FROM `posts`[0m
[4;35;1mPost Load (0.015000)[0m [0mSELECT * FROM `posts` WHERE (`posts`.`id` = 953125641) LIMIT 1[0m
[4;36;1mComment Columns (0.000000)[0m [0;1mSHOW FIELDS FROM `comments`[0m
[4;35;1mSQL (0.016000)[0m [0mSELECT count(*) AS count_all FROM `comments` WHERE (`comments`.post_id = 953125641) [0m
@post = Post.find_by_id(953125641)
@length = @post.comments.length产生了如下的SQL语句
[4;35;1mPost Columns (0.000000)[0m [0mSHOW FIELDS FROM `posts`[0m
[4;36;1mPost Load (0.000000)[0m [0;1mSELECT * FROM `posts` WHERE (`posts`.`id` = 953125641) LIMIT 1[0m
[4;35;1mComment Load (0.000000)[0m [0mSELECT * FROM `comments` WHERE (`comments`.post_id = 953125641) [0m
@post = Post.find_by_id(953125641)
@length = @post.comments.size产生了如下的SQL语句
[4;36;1mPost Columns (0.015000)[0m [0;1mSHOW FIELDS FROM `posts`[0m
[4;35;1mPost Load (0.016000)[0m [0mSELECT * FROM `posts` WHERE (`posts`.`id` = 953125641) LIMIT 1[0m
[4;36;1mComment Columns (0.000000)[0m [0;1mSHOW FIELDS FROM `comments`[0m
[4;35;1mSQL (0.000000)[0m [0mSELECT count(*) AS count_all FROM `comments` WHERE (`comments`.post_id = 953125641) [0m
@post = Post.find_by_id(953125641)
@length = @post.comments.count产生了如下的SQL语句
[4;36;1mPost Columns (0.015000)[0m [0;1mSHOW FIELDS FROM `posts`[0m
[4;35;1mPost Load (0.000000)[0m [0mSELECT * FROM `posts` WHERE (`posts`.`id` = 953125641) LIMIT 1[0m
[4;36;1mComment Columns (0.000000)[0m [0;1mSHOW FIELDS FROM `comments`[0m
[4;35;1mSQL (0.000000)[0m [0mSELECT count(*) AS count_all FROM `comments` WHERE (`comments`.post_id = 953125641) [0m
@post = Post.find_by_id(953125641)
@comments = @post.comments
@length = @comments.count产生了如下的SQL语句
[4;36;1mPost Columns (0.000000)[0m [0;1mSHOW FIELDS FROM `posts`[0m
[4;35;1mPost Load (0.016000)[0m [0mSELECT * FROM `posts` WHERE (`posts`.`id` = 953125641) LIMIT 1[0m
[4;36;1mComment Columns (0.000000)[0m [0;1mSHOW FIELDS FROM `comments`[0m
[4;35;1mSQL (0.000000)[0m [0mSELECT count(*) AS count_all FROM `comments` WHERE (`comments`.post_id = 953125641) [0m
@post = Post.find_by_id(953125641)
@comments = @post.comments
@length = @comments.length产生了如下的SQL语句
[4;36;1mPost Columns (0.015000)[0m [0;1mSHOW FIELDS FROM `posts`[0m
[4;35;1mPost Load (0.000000)[0m [0mSELECT * FROM `posts` WHERE (`posts`.`id` = 953125641) LIMIT 1[0m
[4;36;1mComment Load (0.000000)[0m [0;1mSELECT * FROM `comments` WHERE (`comments`.post_id = 953125641) [0m
@post = Post.find_by_id(953125641)
@comments = @post.comments
@length = @comments.size产生了如下的SQL语句
[4;35;1mPost Columns (0.016000)[0m [0mSHOW FIELDS FROM `posts`[0m
[4;36;1mPost Load (0.000000)[0m [0;1mSELECT * FROM `posts` WHERE (`posts`.`id` = 953125641) LIMIT 1[0m
[4;35;1mComment Columns (0.015000)[0m [0mSHOW FIELDS FROM `comments`[0m
[4;36;1mSQL (0.000000)[0m [0;1mSELECT count(*) AS count_all FROM `comments` WHERE (`comments`.post_id = 953125641) [0m
@post = Post.find_by_id(953125641)
@comments = @post.comments.find(:all)
@length = @comments.size产生了如下的SQL语句
[4;35;1mPost Columns (0.015000)[0m [0mSHOW FIELDS FROM `posts`[0m
[4;36;1mPost Load (0.032000)[0m [0;1mSELECT * FROM `posts` WHERE (`posts`.`id` = 953125641) LIMIT 1[0m
[4;35;1mComment Load (0.000000)[0m [0mSELECT * FROM `comments` WHERE (`comments`.post_id = 953125641) [0m
由以上的测试数据表明： #count方法总是会产生count(*)的SQL语句去查询数据库，而#length方法总是取查询出的集合的个数，它总是不会产生count(*)的查询，#size方法就比较好了，如果目标集合还没有取出来，它会像#count方法一样，产生count(*)的查询，如果记录已经取出来了，它就像#length方法一样，直接读取集合的个数了。
基于以上的分析，结论：虽然以上三种方法都可以取得数据记录的个数，但是还是要根据根据实际所需要，调用相应的方法，以达到优化。
参考资料：
我发的帖子： http://www.railsforum.com/viewtopic.php?id=19593
文章：count vs length vs size http://blog.hasmanythrough.com/2008/2/27/count-length-size
Post.find_by_id(953125641).comments.length产生了如下的SQL语句
[4;35;1mPost Columns (0.000000)[0m [0mSHOW FIELDS FROM `posts`[0m
[4;36;1mPost Load (0.016000)[0m [0;1mSELECT * FROM `posts` WHERE (`posts`.`id` = 953125641) LIMIT 1[0m
[4;35;1mComment Load (0.016000)[0m [0mSELECT * FROM `comments` WHERE (`comments`.post_id = 953125641) [0m
Post.find_by_id(953125641).comments.size产生了如下的SQL语句
[4;36;1mPost Columns (0.000000)[0m [0;1mSHOW FIELDS FROM `posts`[0m
[4;35;1mPost Load (0.016000)[0m [0mSELECT * FROM `posts` WHERE (`posts`.`id` = 953125641) LIMIT 1[0m
[4;36;1mComment Columns (0.000000)[0m [0;1mSHOW FIELDS FROM `comments`[0m
[4;35;1mSQL (0.015000)[0m [0mSELECT count(*) AS count_all FROM `comments` WHERE (`comments`.post_id = 953125641) [0m
Post.find_by_id(953125641).comments.count产生了如下的SQL语句
[4;36;1mPost Columns (0.000000)[0m [0;1mSHOW FIELDS FROM `posts`[0m
[4;35;1mPost Load (0.015000)[0m [0mSELECT * FROM `posts` WHERE (`posts`.`id` = 953125641) LIMIT 1[0m
[4;36;1mComment Columns (0.000000)[0m [0;1mSHOW FIELDS FROM `comments`[0m
[4;35;1mSQL (0.016000)[0m [0mSELECT count(*) AS count_all FROM `comments` WHERE (`comments`.post_id = 953125641) [0m
@post = Post.find_by_id(953125641)
@length = @post.comments.length产生了如下的SQL语句
[4;35;1mPost Columns (0.000000)[0m [0mSHOW FIELDS FROM `posts`[0m
[4;36;1mPost Load (0.000000)[0m [0;1mSELECT * FROM `posts` WHERE (`posts`.`id` = 953125641) LIMIT 1[0m
[4;35;1mComment Load (0.000000)[0m [0mSELECT * FROM `comments` WHERE (`comments`.post_id = 953125641) [0m
@post = Post.find_by_id(953125641)
@length = @post.comments.size产生了如下的SQL语句
[4;36;1mPost Columns (0.015000)[0m [0;1mSHOW FIELDS FROM `posts`[0m
[4;35;1mPost Load (0.016000)[0m [0mSELECT * FROM `posts` WHERE (`posts`.`id` = 953125641) LIMIT 1[0m
[4;36;1mComment Columns (0.000000)[0m [0;1mSHOW FIELDS FROM `comments`[0m
[4;35;1mSQL (0.000000)[0m [0mSELECT count(*) AS count_all FROM `comments` WHERE (`comments`.post_id = 953125641) [0m
@post = Post.find_by_id(953125641)
@length = @post.comments.count产生了如下的SQL语句
[4;36;1mPost Columns (0.015000)[0m [0;1mSHOW FIELDS FROM `posts`[0m
[4;35;1mPost Load (0.000000)[0m [0mSELECT * FROM `posts` WHERE (`posts`.`id` = 953125641) LIMIT 1[0m
[4;36;1mComment Columns (0.000000)[0m [0;1mSHOW FIELDS FROM `comments`[0m
[4;35;1mSQL (0.000000)[0m [0mSELECT count(*) AS count_all FROM `comments` WHERE (`comments`.post_id = 953125641) [0m
@post = Post.find_by_id(953125641)
@comments = @post.comments
@length = @comments.count产生了如下的SQL语句
[4;36;1mPost Columns (0.000000)[0m [0;1mSHOW FIELDS FROM `posts`[0m
[4;35;1mPost Load (0.016000)[0m [0mSELECT * FROM `posts` WHERE (`posts`.`id` = 953125641) LIMIT 1[0m
[4;36;1mComment Columns (0.000000)[0m [0;1mSHOW FIELDS FROM `comments`[0m
[4;35;1mSQL (0.000000)[0m [0mSELECT count(*) AS count_all FROM `comments` WHERE (`comments`.post_id = 953125641) [0m
@post = Post.find_by_id(953125641)
@comments = @post.comments
@length = @comments.length产生了如下的SQL语句
[4;36;1mPost Columns (0.015000)[0m [0;1mSHOW FIELDS FROM `posts`[0m
[4;35;1mPost Load (0.000000)[0m [0mSELECT * FROM `posts` WHERE (`posts`.`id` = 953125641) LIMIT 1[0m
[4;36;1mComment Load (0.000000)[0m [0;1mSELECT * FROM `comments` WHERE (`comments`.post_id = 953125641) [0m
@post = Post.find_by_id(953125641)
@comments = @post.comments
@length = @comments.size产生了如下的SQL语句
[4;35;1mPost Columns (0.016000)[0m [0mSHOW FIELDS FROM `posts`[0m
[4;36;1mPost Load (0.000000)[0m [0;1mSELECT * FROM `posts` WHERE (`posts`.`id` = 953125641) LIMIT 1[0m
[4;35;1mComment Columns (0.015000)[0m [0mSHOW FIELDS FROM `comments`[0m
[4;36;1mSQL (0.000000)[0m [0;1mSELECT count(*) AS count_all FROM `comments` WHERE (`comments`.post_id = 953125641) [0m
@post = Post.find_by_id(953125641)
@comments = @post.comments.find(:all)
@length = @comments.size产生了如下的SQL语句
[4;35;1mPost Columns (0.015000)[0m [0mSHOW FIELDS FROM `posts`[0m
[4;36;1mPost Load (0.032000)[0m [0;1mSELECT * FROM `posts` WHERE (`posts`.`id` = 953125641) LIMIT 1[0m
[4;35;1mComment Load (0.000000)[0m [0mSELECT * FROM `comments` WHERE (`comments`.post_id = 953125641) [0m
由以上的测试数据表明： #count方法总是会产生count(*)的SQL语句去查询数据库，而#length方法总是取查询出的集合的个数，它总是不会产生count(*)的查询，#size方法就比较好了，如果目标集合还没有取出来，它会像#count方法一样，产生count(*)的查询，如果记录已经取出来了，它就像#length方法一样，直接读取集合的个数了。
基于以上的分析，结论：虽然以上三种方法都可以取得数据记录的个数，但是还是要根据根据实际所需要，调用相应的方法，以达到优化。
参考资料：
我发的帖子： http://www.railsforum.com/viewtopic.php?id=19593
文章：count vs length vs size http://blog.hasmanythrough.com/2008/2/27/count-length-size
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