Erlang：ets选择并匹配性能 [英] Erlang: ets select and match performance

问题描述

我发现函数ets：select / 2和mnesia：select / 3优于ets：match / 2，ets：match_object / 2和mnesia：match_object / 3 引用链接： http://www.erlang.org/doc/efficiency_guide/tablesDatabases.html

我读了一些有关选择和匹配之间进行比较的文章，得出结论，有一些因素会影响结果，例如表中的记录数量，选择/匹配主键与否，表类型（袋子，设置...）等。

And I'd read some essay about comparing between select and match, I conclude there are some factor effecting the result, such as records' amount in table, select/match a primary key or not, table kind(bag, set...), etc.

在我的测试中，我对所有功率为10W的表都适用记录和1W记录，并且只能选择/匹配非主键。

In my test, I do for all kind of table with 10W records and 1W records, and only select/match for a un-primary key.

以下代码：

select_ets_test(Times) ->
    MS = ets:fun2ms(fun(T) when T#ets_haoxian_template.count == 15 -> T end),
    T1 = timer:tc(?MODULE, todo, [fun() -> ets:select(haoxian_test_bag, MS) end, Times]),
    T2 = timer:tc(?MODULE, todo, [fun() -> ets:select(haoxian_test_set, MS) end, Times]),
    T3 = timer:tc(?MODULE, todo, [fun() -> ets:select(haoxian_test_ordered_set, MS) end, Times]),
    T4 = timer:tc(?MODULE, todo, [fun() -> ets:select(haoxian_test_duplicate_bag, MS) end, Times]),
    io:format("select bag           : ~p~n", [T1]),
    io:format("select set           : ~p~n", [T2]),
    io:format("select ordered_set   : ~p~n", [T3]),
    io:format("select duplicate bag : ~p~n", [T4]).

match_ets_test(Times) ->
    MS = #ets_haoxian_template{count = 15, _ = '_' },
    T1 = timer:tc(?MODULE, todo, [fun() -> ets:match_object(haoxian_test_bag, MS) end, Times]),
    T2 = timer:tc(?MODULE, todo, [fun() -> ets:match_object(haoxian_test_set, MS) end, Times]),
    T3 = timer:tc(?MODULE, todo, [fun() -> ets:match_object(haoxian_test_ordered_set, MS) end, Times]),
    T4 = timer:tc(?MODULE, todo, [fun() -> ets:match_object(haoxian_test_duplicate_bag, MS) end, Times]),
    io:format("match bag           : ~p~n", [T1]),
    io:format("match set           : ~p~n", [T2]),
    io:format("match ordered_set   : ~p~n", [T3]),
    io:format("match duplicate bag : ~p~n", [T4]).

todo(_Fun, 0) ->
    ok;
todo(Fun, Times) ->
    Fun(),
    todo(Fun, Times - 1).

记录如下：#ets_haoxian_template {type = X，count = Y，...}，关键姿势是类型。

the record would like: #ets_haoxian_template{type = X, count = Y, ...}, keypose is type.

以下结果：
1W测试：

the resule following: 1W test:

insert bag           : {324000,true}
insert set           : {221000,true}
insert ordered_set   : {108000,true}
insert duplicate bag : {173000,true}

select bag           : {284000,ok}
select set           : {255000,ok}
select ordered_set   : {221000,ok}
select duplicate bag : {252000,ok}

match bag           : {238000,ok}
match set           : {192000,ok}
match ordered_set   : {136000,ok}
match duplicate bag : {191000,ok}

10W测试：

insert bag           : {1654000,true}
insert set           : {1684000,true}
insert ordered_set   : {981000,true}
insert duplicate bag : {1769000,true}

select bag           : {3404000,ok}
select set           : {3433000,ok}
select ordered_set   : {2501000,ok}
select duplicate bag : {3678000,ok}

match bag           : {2749000,ok}
match set           : {2927000,ok}
match ordered_set   : {1748000,ok}
match duplicate bag : {2923000,ok}

似乎匹配比选择更好？还是我的测试有问题？

It seem match is better than select? Or my test something wrong???

推荐答案

match 函数使用特殊的元组语法（ match_pattern ）决定返回什么。

The match function employs a special tuple syntax (match_pattern) to decide what to return.

select 函数采用特殊的元组语法（< a href = http://erlang.org/doc/man/ets.html#type-match_spec rel = nofollow noreferrer> match_spec ）这是 match_pattern 的超集，能够指定后卫并从结果集中提取元素（而不仅仅是返回匹配的键）。

The select function employs a special tuple syntax (match_spec) that is a superset of match_pattern, with the ability to specify guards and extract elements from the result set (rather than just returning the matching keys).

我的理解是：

• select 会编译 match_spec 转换为匿名函数，从而加快了运行速度


• 为该函数提供防护的能力可以比以前更快地消除误报只需 match_pattern （si （因为它们将首先运行）


• 就地从结果集中提取元素的功能可以节省您以后要做的工作，而不是遍历返回的键来提取该数据

• select compiles the match_spec into an anonymous function, expediting how fast it runs
• the ability to provide guards to this function eliminates false positives quicker than is possible with just a match_pattern (since they will run first)
• the ability to extract elements from the result set in-place saves you work you would have to do later, rather than iterating over the returned keys to extract that data.

在琐碎的非特定用例中， select 只是一个匹配的很多工作。在非平凡的更常见的用例中， select 会给您您真正想要的更快。

In trivial non-specific use-cases, select is just a lot of work around match. In non-trivial more common use-cases, select will give you what you really want a lot quicker.

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