SELECT from Clustered Columnstore 导致 LOB 读取

Question

我正在从以下包含 52 亿条记录的聚集列存储索引中执行 SELECT。当我查看 STATISTICS IO 的输出时，我看到的是 LOB 逻辑读取。

我的问题是当表中的数据类型都不是 LOB 值时，为什么查询执行 LOB 逻辑读取？

我在 SQL Server 2014 SP3 企业版环境中工作。

表定义：

CREATE TABLE [dbo].[ColumnstoreTableA](
    [RollupID] [bigint] IDENTITY(1,1) NOT NULL,
    [DT_DIMID] [int] NOT NULL,
    [TC_DIMID] [int] NOT NULL,
    [C_DIMID] [int] NOT NULL,
    [CD_DIMID] [int] NOT NULL,
    [AC_DIMID] [int] NOT NULL,
    [UA_DIMID] [int] NOT NULL,
    [UL_DIMID] [int] NOT NULL,
    [JP_DIMID] [int] NOT NULL,
    [RTT_DIMID] [int] NOT NULL,
    [RA] [int] NOT NULL,
    [Impressions] [int] NOT NULL,
    [Clicks] [int] NOT NULL,
    [Conversions] [int] NOT NULL,
    [Sends] [int] NOT NULL,
    [CreationDate] [datetime2](0) NOT NULL DEFAULT GETDATE(),
    [ModifyDate] [datetime2](0) NOT NULL DEFAULT GETDATE(),
    [Z_DIMID] [int] NOT NULL,
    [R_Actual] [money] NULL,
    [JP_DIMID2] [int] NULL
) ON [PRIMARY]

选择语句：

SELECT  [T].[Impressions]
FROM    [dbo].[ColumnstoreTableA] AS [T]

统计 IO 输出：

表 'ColumnstoreTableA'。扫描计数 5，逻辑读取 127，物理读取 0，预读读取 0，lob 逻辑读取 3719，lob 物理读取 0，lob 预读读取 0。

Answer 1

多个来源似乎表明 Columnstore Indexes 将其数据存储为 LOB 数据，这种情况正在发生。我的理解和收获是，在使用列存储索引时，应该始终期望发生 LOB 读取。

列存储索引的结构是什么？

如何在 SQL Server 中使用列存储索引

SQL Server 2012 列存储索引内部

SELECT      COUNT([partition_number]) AS [Number of Partitions],
            SUM([in_row_used_page_count]) AS [SUM_in_row_used_page_count],
            SUM([lob_used_page_count]) AS [SUM_lob_used_page_count],
            SUM([used_page_count]) AS [SUM_used_page_count],
            SUM([row_count]) AS [SUM_row_count]
FROM        [sys].[dm_db_partition_stats]
WHERE       [partition_id] IN   (
                                    SELECT  [p].[partition_id]
                                    FROM    [sys].[partitions] AS [p] 
                                    INNER JOIN  [sys].[objects] AS [o] ON [o].[object_id] = [p].[object_id]
                                    WHERE   [o].[object_id] = 971254615
                                );

结果：

Number of Partitions SUM_in_row_used_page_count SUM_lob_used_page_count SUM_used_page_count  SUM_row_count
-------------------- -------------------------- ----------------------- -------------------- --------------------
847                  0                          5630558                 5630558             5175958971