SQL，数字辅助表
对于某些类型的 sql 查询，辅助数字表可能非常有用.它可以创建为包含特定任务所需行数的表，也可以创建为返回每个查询所需行数的用户定义函数.
For certain types of sql queries, an auxiliary table of numbers can be very useful. It may be created as a table with as many rows as you need for a particular task or as a user defined function that returns the number of rows required in each query.
创建这样一个函数的最佳方法是什么?
What is the optimal way to create such a function?
推荐答案
嘿...抱歉我这么晚才回复旧帖子.而且，是的，我必须做出回应，因为该线程上最受欢迎的答案(当时，带有指向 14 种不同方法的链接的递归 CTE 答案)是，嗯……充其量是对性能的挑战.
Heh... sorry I'm so late responding to an old post. And, yeah, I had to respond because the most popular answer (at the time, the Recursive CTE answer with the link to 14 different methods) on this thread is, ummm... performance challenged at best.
首先，包含 14 种不同解决方案的文章非常适合查看动态创建 Numbers/Tally 表的不同方法，但正如文章和引用线程中所指出的那样，有一个 非常 重要的引述...
First, the article with the 14 different solutions is fine for seeing the different methods of creating a Numbers/Tally table on the fly but as pointed out in the article and in the cited thread, there's a very important quote...
"关于效率和性能往往是主观的.不管查询是如何进行的使用，物理实现决定查询的效率.因此，与其依赖有偏见的指导方针，势在必行您测试查询并确定哪个表现更好."
"suggestions regarding efficiency and performance are often subjective. Regardless of how a query is being used, the physical implementation determines the efficiency of a query. Therefore, rather than relying on biased guidelines, it is imperative that you test the query and determine which one performs better."
具有讽刺意味的是，文章本身包含许多主观陈述和有偏见的指导方针"，例如递归 CTE 可以生成一个数字列表相当有效"和"这是从 Itzik Ben-Gen 发布的新闻组中使用 WHILE 循环的一种有效的方法"(我确信他发布的目的只是为了比较).来吧，伙计们......仅仅提到Itzik的好名字可能会导致一些可怜的懒汉实际使用这种可怕的方法.作者应该实践他所宣扬的内容，并且应该在做出如此荒谬的错误陈述之前进行一些性能测试，尤其是在面对任何可扩展性时.
Ironically, the article itself contains many subjective statements and "biased guidelines" such as "a recursive CTE can generate a number listing pretty efficiently" and "This is an efficient method of using WHILE loop from a newsgroup posting by Itzik Ben-Gen" (which I'm sure he posted just for comparison purposes). C'mon folks... Just mentioning Itzik's good name may lead some poor slob into actually using that horrible method. The author should practice what (s)he preaches and should do a little performance testing before making such ridiculously incorrect statements especially in the face of any scalablility.
考虑到在对任何代码的功能或某人喜欢"什么做出任何主观声明之前实际进行一些测试，这里有一些代码您可以自己进行测试.为您运行测试的 SPID 设置探查器并自行检查...只需为您的最喜欢"号码执行数字 1000000 的Search'n'Replace"并查看...
With the thought of actually doing some testing before making any subjective claims about what any code does or what someone "likes", here's some code you can do your own testing with. Setup profiler for the SPID you're running the test from and check it out for yourself... just do a "Search'n'Replace" of the number 1000000 for your "favorite" number and see...
--===== Test for 1000000 rows ==================================
GO
--===== Traditional RECURSIVE CTE method
WITH Tally (N) AS
(
SELECT 1 UNION ALL
SELECT 1 + N FROM Tally WHERE N < 1000000
)
SELECT N
INTO #Tally1
FROM Tally
OPTION (MAXRECURSION 0);
GO
--===== Traditional WHILE LOOP method
CREATE TABLE #Tally2 (N INT);
SET NOCOUNT ON;
DECLARE @Index INT;
SET @Index = 1;
WHILE @Index <= 1000000
BEGIN
INSERT #Tally2 (N)
VALUES (@Index);
SET @Index = @Index + 1;
END;
GO
--===== Traditional CROSS JOIN table method
SELECT TOP (1000000)
ROW_NUMBER() OVER (ORDER BY (SELECT 1)) AS N
INTO #Tally3
FROM Master.sys.All_Columns ac1
CROSS JOIN Master.sys.ALL_Columns ac2;
GO
--===== Itzik's CROSS JOINED CTE method
WITH E00(N) AS (SELECT 1 UNION ALL SELECT 1),
E02(N) AS (SELECT 1 FROM E00 a, E00 b),
E04(N) AS (SELECT 1 FROM E02 a, E02 b),
E08(N) AS (SELECT 1 FROM E04 a, E04 b),
E16(N) AS (SELECT 1 FROM E08 a, E08 b),
E32(N) AS (SELECT 1 FROM E16 a, E16 b),
cteTally(N) AS (SELECT ROW_NUMBER() OVER (ORDER BY N) FROM E32)
SELECT N
INTO #Tally4
FROM cteTally
WHERE N <= 1000000;
GO
--===== Housekeeping
DROP TABLE #Tally1, #Tally2, #Tally3, #Tally4;
GO
当我们在这里的时候，这里是我从 SQL Profiler 中得到的数值，分别是 100、1000、10000、100000 和 1000000...
While we're at it, here's the numbers I get from SQL Profiler for the values of 100, 1000, 10000, 100000, and 1000000...
SPID TextData Dur(ms) CPU Reads Writes
---- ---------------------------------------- ------- ----- ------- ------
51 --===== Test for 100 rows ============== 8 0 0 0
51 --===== Traditional RECURSIVE CTE method 16 0 868 0
51 --===== Traditional WHILE LOOP method CR 73 16 175 2
51 --===== Traditional CROSS JOIN table met 11 0 80 0
51 --===== Itzik's CROSS JOINED CTE method 6 0 63 0
51 --===== Housekeeping DROP TABLE #Tally 35 31 401 0
51 --===== Test for 1000 rows ============= 0 0 0 0
51 --===== Traditional RECURSIVE CTE method 47 47 8074 0
51 --===== Traditional WHILE LOOP method CR 80 78 1085 0
51 --===== Traditional CROSS JOIN table met 5 0 98 0
51 --===== Itzik's CROSS JOINED CTE method 2 0 83 0
51 --===== Housekeeping DROP TABLE #Tally 6 15 426 0
51 --===== Test for 10000 rows ============ 0 0 0 0
51 --===== Traditional RECURSIVE CTE method 434 344 80230 10
51 --===== Traditional WHILE LOOP method CR 671 563 10240 9
51 --===== Traditional CROSS JOIN table met 25 31 302 15
51 --===== Itzik's CROSS JOINED CTE method 24 0 192 15
51 --===== Housekeeping DROP TABLE #Tally 7 15 531 0
51 --===== Test for 100000 rows =========== 0 0 0 0
51 --===== Traditional RECURSIVE CTE method 4143 3813 800260 154
51 --===== Traditional WHILE LOOP method CR 5820 5547 101380 161
51 --===== Traditional CROSS JOIN table met 160 140 479 211
51 --===== Itzik's CROSS JOINED CTE method 153 141 276 204
51 --===== Housekeeping DROP TABLE #Tally 10 15 761 0
51 --===== Test for 1000000 rows ========== 0 0 0 0
51 --===== Traditional RECURSIVE CTE method 41349 37437 8001048 1601
51 --===== Traditional WHILE LOOP method CR 59138 56141 1012785 1682
51 --===== Traditional CROSS JOIN table met 1224 1219 2429 2101
51 --===== Itzik's CROSS JOINED CTE method 1448 1328 1217 2095
51 --===== Housekeeping DROP TABLE #Tally 8 0 415 0
如您所见，递归 CTE 方法在 Duration 和 CPU 方面仅次于 While 循环，其内存压力是 While 循环的 8 倍.它是类固醇上的 RBAR，应该不惜一切代价避免任何单行计算，就像应该避免 While 循环一样.有些地方递归非常有价值，但这不是其中之一.
As you can see, the Recursive CTE method is the second worst only to the While Loop for Duration and CPU and has 8 times the memory pressure in the form of logical reads than the While Loop. It's RBAR on steroids and should be avoided, at all cost, for any single row calculations just as a While Loop should be avoided. There are places where recursion is quite valuable but this ISN'T one of them.
作为边栏，Denny 先生绝对是...正确大小的永久性 Numbers 或 Tally 表是处理大多数事情的方式.大小合适是什么意思?好吧，大多数人使用 Tally 表来生成日期或在 VARCHAR(8000) 上进行拆分.如果您使用N"上的正确聚集索引创建一个 11,000 行的 Tally 表，您将有足够的行来创建超过 30 年的日期(我经常使用抵押贷款，所以 30 年对我来说是一个关键数字) 并且肯定足以处理 VARCHAR(8000) 拆分.为什么正确的尺寸"如此重要?如果 Tally 表被大量使用，它很容易放入缓存中，这使得它非常快，根本不会对内存造成太大压力.
As a side bar, Mr. Denny is absolutely spot on... a correctly sized permanent Numbers or Tally table is the way to go for most things. What does correctly sized mean? Well, most people use a Tally table to generate dates or to do splits on VARCHAR(8000). If you create an 11,000 row Tally table with the correct clustered index on "N", you'll have enough rows to create more than 30 years worth of dates (I work with mortgages a fair bit so 30 years is a key number for me) and certainly enough to handle a VARCHAR(8000) split. Why is "right sizing" so important? If the Tally table is used a lot, it easily fits in cache which makes it blazingly fast without much pressure on memory at all.
最后但并非最不重要的一点是，每个人都知道，如果您创建一个永久的 Tally 表，那么您使用哪种方法来构建它并不重要，因为 1)它只会被制作一次，并且 2)如果它类似于一个 11,000 行的表，所有方法都将运行足够好".那么为什么我对使用哪种方法感到愤慨???
Last but not least, every one knows that if you create a permanent Tally table, it doesn't much matter which method you use to build it because 1) it's only going to be made once and 2) if it's something like an 11,000 row table, all of the methods are going to run "good enough". So why all the indigination on my part about which method to use???
答案是，一些不知道更好，只需要完成他或她的工作的可怜的家伙/gal 可能会看到类似递归 CTE 方法的东西，并决定将其用于更大、更频繁的事情使用而不是建立一个永久的 Tally 表，我试图保护这些人、他们的代码运行所在的服务器以及拥有这些服务器上数据的公司.是的……这有什么大不了的.它也应该适用于其他所有人.教导正确的做事方式，而不是足够好".在发布或使用帖子或书中的内容之前进行一些测试......事实上，您所拯救的生命可能是您自己的，特别是如果您认为递归 CTE 是实现此类目标的方法.;-)
The answer is that some poor guy/gal who doesn't know any better and just needs to get his or her job done might see something like the Recursive CTE method and decide to use it for something much larger and much more frequently used than building a permanent Tally table and I'm trying to protect those people, the servers their code runs on, and the company that owns the data on those servers. Yeah... it's that big a deal. It should be for everyone else, as well. Teach the right way to do things instead of "good enough". Do some testing before posting or using something from a post or book... the life you save may, in fact, be your own especially if you think a recursive CTE is the way to go for something like this. ;-)
感谢收听...
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