从varbinary（max）中清空数据后缩小DB的最佳方法？

我们有一个数据库，其中有大量数据存储在varbinary（max）类型的字段中。在某些时候，我们可以清除大多数行（但不是全部）的数据。我们的计划是使该字段可为空，并在不再需要该数据时将其清空。完成后，我们希望减少数据库的大小。做到这一点的最佳方法是什么？

如果没有一种利用当前设置回收空间的好方法，那么我的一个想法就是将数据字段移动到只有两列的单独表中：主表的键和数据字段。然后，当不再需要这些行时，我们可以简单地删除它们。（然后进行某种缩小。）但是，与简单地使现有字段可为空相比，这将是一件困难得多的更改。

注意：实际上，我不太关心使数据库文件变小，但是我关心新释放的空间变得可重用。

这一字段占数据库大小的90％以上。我已经在3TB了。

— TTT
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在我看来，仅更新列即可NULL释放页面以供重复使用。这是一个VeryScottish®演示，庆祝它是美国东部时间下午5点左右。

USE tempdb;

DROP TABLE IF EXISTS dbo.RobertBurns;

CREATE TABLE dbo.RobertBurns
(
    Id INT IDENTITY(1, 1) PRIMARY KEY CLUSTERED,
    Scotch VARCHAR(50),
    HaggisAddress VARBINARY(MAX)
);

DECLARE @AddressToAVarbinaryHaggis VARBINARY(MAX); 
DECLARE @AddressToAHaggis NVARCHAR(MAX) = N'
Good luck to you and your honest, plump face,
Great chieftain of the pudding race!
Above them all you take your place,
        gut, stomach-lining, or intestine,
You''re well worth a grace
        as long as my arm.

The overloaded serving tray there you fill,
Your buttocks shaped like a distant hilltop,
Your wooden skewer could be used to fix a mill
         if need be,
While through your pores your juices drip
         like liquid gold.

His knife see the serving-man clean,
And then cut you up with great skill,
Making a trench in your bright, gushing guts
        To form a ditch,
And then, 0h! What a glorious sight!
        Warm, steaming, and rich!

Then, spoonful after spoonful, they eagerly eat,
The devil will get the last bit, on they go,
Until all their well-stretched stomachs, by-and-by,
        are bent like drums,
Then the head of the family, about to burst,
        murmurs “Thank the Lord".

Is there a pretentious soul who, over his French ragout,
Or Italian cuisine that would make a pig sick,
Or French stew that would make that same pig ill
        with complete and utter disgust,
Looks down with a sneering, scornful attitude,
        on such a meal? (as Haggis)

Poor devil! See him over his trash!
As feeble as a withered bullrush,
His skinny leg no thicker than a thin rope,
        His fist the size of a nut,
Through a river or field to travel,
        Completely unfit!

But look at the healthy, Haggis-fed person!
The trembling earth respects him as a man!
Put a knife in his fist,
        He''ll make it work!
And legs, and arms, and heads will come off,
        Like the tops of thistle.

You Powers who look after mankind,
And dish out his bill of fare,
Old Scotland wants no watery, wimpy stuff
        That splashes about in little wooden bowls!
But, if You will grant her a grateful prayer,
        Give her a Haggis!';


INSERT dbo.RobertBurns (Scotch, HaggisAddress )
SELECT TOP 1000 
CASE WHEN x.c % 15 = 0 THEN 'Laphroaig'
     WHEN x.c % 5 = 0 THEN 'Lagavulin'
     WHEN x.c % 3 = 0 THEN 'Port Ellen'
     ELSE 'Ardbeg'
END AS Scotch, 
CONVERT(VARBINARY(MAX), REPLICATE(@AddressToAHaggis, x.c % 20 + 1))
FROM (
SELECT ROW_NUMBER() OVER (ORDER BY @@ROWCOUNT) AS c
FROM sys.messages AS m
) AS x;

CREATE INDEX ix_novarbinary ON  dbo.RobertBurns (Scotch, Id);
CREATE INDEX ix_yesvarbinary ON dbo.RobertBurns (Scotch, Id) INCLUDE (HaggisAddress);

插入行后，让我们检查一下索引页。

SELECT   OBJECT_NAME(i.object_id) AS table_name,
         i.name AS index_name,
         MAX(a.used_pages) AS leaf_me_alone
FROM     sys.indexes AS i
JOIN     sys.partitions AS p
ON p.object_id = i.object_id
   AND p.index_id = i.index_id
JOIN     sys.allocation_units AS a
ON a.container_id = p.partition_id
WHERE OBJECT_NAME(i.object_id) = 'RobertBurns'
GROUP BY i.object_id, i.index_id, i.name
ORDER BY OBJECT_NAME(i.object_id), i.index_id;

插入后，我得到了。实际页面可能会因您而异。

table_name  index_name                      leaf_me_alone
RobertBurns PK__RobertBu__3214EC074BE633A2  5587
RobertBurns ix_novarbinary                  10
RobertBurns ix_yesvarbinary                 5581

让我们NULL出去一些行！

UPDATE rb
    SET rb.HaggisAddress = NULL
FROM dbo.RobertBurns AS rb
WHERE rb.Id % 15 = 0;

并重新登录我们的页面：

table_name  index_name                      leaf_me_alone
RobertBurns PK__RobertBu__3214EC074BE633A2  5300
RobertBurns ix_novarbinary                  10
RobertBurns ix_yesvarbinary                 5273

因此减少了页数。头晕！对于涉及我们VARBINARY数据的两个索引，它们丢失了很多页面。这意味着它们又重新流通，供其他对象使用。由于我在tempdb中，它们可能很快就被这里发生的所有垃圾事件所吞噬。

现在，让我们将一些数据放回去：

INSERT dbo.RobertBurns (Scotch, HaggisAddress )
SELECT TOP 10 rb.Scotch, rb.HaggisAddress
FROM dbo.RobertBurns AS rb;

然后签入：

table_name  index_name                      leaf_me_alone
RobertBurns PK__RobertBu__3214EC074BE633A2  5330
RobertBurns ix_novarbinary                  11
RobertBurns ix_yesvarbinary                 5305

页面计数略微增加。

因此，看起来您不必做任何疯狂的事情，甚至不必收缩数据库即可重复使用空间。我认为您正在混淆删除列的行为，并需要根据DBCC CLEANTABLE实际操作来运行。

希望这可以帮助！

— 埃里克·达林（Erik Darling）
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