增加小数精度突然增加了表的大小

Question

我有一张像：

CREATE TABLE grid_rows(
    [grid_row_id] [int] NOT NULL,
    [grid_column_id] [smallint] NOT NULL,
    [decimal_val] [decimal](18, 6) NULL,
    [datetime_val] [datetime] NULL,
    [integer_val] [int] NULL,
    [string_val] [varchar](1024) NULL
)

这张桌子有一些 1,037,560 rows

exec sp_spaceused "grid_rows" gives:
rows         reserved    data
1,037,560    461,768KB  302,648KB`

---

将精度从 (18, 6) 更改为 (24,6) 后即 ALTER TABLE grid_rows ALTER COLUMN decimal_val decimal(24, 6)

exec sp_spaceused "grid_rows" gives:
rows         reserved     data
1,037,560    641,352KB  560,832KB

根据所分配的空间decimal(18,6)就是9 bytes那的(24, 6)是13 bytes。MSDN参考

The reserved space has increased by around 179,584 KB and data space by 260,000KB. Shouldn't it be increased by 1,037,560 * 4/1024 = 4052 KB

Answer 1

我只是使用DBCC IND和从存储随后修改为数据的数据DBCC PAGE的页面中读取物理数据。旧字段现在列为，另一个字段包含新数据。DECIMAL(4,2)DECIMAL(8,4)DROPPED

        CREATE TABLE TestDec
        (
            DecimalValue DECIMAL(4,2)
        );

        INSERT INTO TestDec VALUES (16.25);
        SELECT * FROM TestDec;

        EXEC sp_spaceused 'TestDec';

        DBCC IND('Test', 'TestDec', -1)

                    /* 20267501 happens to be the page in my database, yours WILL 
                       be different */
        DBCC TRACEON(3604) /* display output from DBCC PAGE */
        DBCC PAGE('Test',1,20267501,3) WITH TABLERESULTS

        ALTER TABLE TestDec ALTER COLUMN DecimalValue DECIMAL(8,4)
        DBCC PAGE('Test',1,20267501,3) WITH TABLERESULTS

第 46 行和第 47 行显示相关项目。

这表示当小数字段大小更改时，现有数据将复制到新的物理位置。