Our application uses a popular 3rd-party extension to Entity Framework that allows for valuable things that native Entity Framework is not capable of or designed to do, like bulk deletes and updates.
It seems that the query pattern produced by the bulk delete API method from this library is not optimized well by the SQL Server query optimizer.
I am using SQL Server 2014. Consider the following table and index (names have been changed to protect the innocent):
CREATE TABLE [dbo].[a_staging_table] ( [staging_ky] [BIGINT] IDENTITY(1,1) NOT NULL, [col_a] [INT] NOT NULL, [col_b] [VARCHAR](15) NOT NULL, [col_c] [VARCHAR](256) NOT NULL, [col_d] [INT] NULL, [col_e] [INT] NOT NULL, [col_f] [VARCHAR](3) NOT NULL, [col_g] [VARCHAR](3) NULL, [col_h] [VARCHAR](20) NULL, [col_i] [CHAR](4) NOT NULL, [col_j] [INT] NOT NULL, [col_k] [VARCHAR](10) NULL, [col_l] [DATETIME] NOT NULL, [col_m] [VARCHAR](25) NOT NULL, [col_n] [DATETIME] NOT NULL, [col_o] [VARCHAR](25) NOT NULL, [col_p] [BIGINT] NOT NULL, [col_q] [BIT] NOT NULL, [col_r] [DATETIME] NULL, [col_s] [VARCHAR](25) NULL, PRIMARY KEY CLUSTERED ([staging_ky] ASC) ); GO CREATE NONCLUSTERED INDEX [IDX_dbo_a_staging_table_col_p] ON [dbo].[a_staging_table]([col_p] ASC) WITH (FILLFACTOR = 100); GO
The application is trying to delete all records from the table where col_p is equal to a certain value, and the generated SQL query from Entity Framework looks like this:
DELETE [dbo].[a_staging_table] FROM [dbo].[a_staging_table] AS j0 INNER JOIN ( SELECT 1 AS [C1], [Extent1].[staging_ky] AS [staging_ky] FROM [dbo].[a_staging_table] AS [Extent1] WHERE [Extent1].[col_p] = @p__linq__0 ) AS j1 ON (j0.[staging_ky] = j1.[staging_ky])
Looking at the query plan, SQL Server does not optimize away the join between the table and itself, and instead, the plan does an index scan with a nested loops join and seek to the same index, followed by a table spool, prior to the delete.
A more traditional and optimal delete statement is something like this statement, which just does an index seek, prior to the clustered index delete:
DELETE [dbo].[a_staging_table] FROM [dbo].[a_staging_table] AS j0 WHERE [j0].[col_p] = @p__linq__0
The actual query plans for both statements are posted at:https://www.brentozar.com/pastetheplan/?id=SygZoD6um
Is this query pattern generated by this library a known query anti-pattern for a delete statement in SQL Server?