SQLServerBang

SQL Server does not sniff for variable, it just simply uses the fixed value.   I'm going to use AdventureWorks 2019 in this post. --------------------Please run this script--------------- use AdventureWorks2019 go IF exists(select 1 from sys.tables where name='SalesOrderDetail' and schema_id=schema_id('dbo'))       drop table SalesOrderDetail go select * into SalesOrderDetail from [Sales].[SalesOrderDetail] go create statistics iProductID ON SalesOrderDetail(productid) with fullscan go dbcc traceon(3604,2363)---trace flag 2363 displays more detail about the selectivity go --------------------Please run this script---------------     Equality(=): all density DECLARE @pid INT = 0 SELECT * FROM SalesOrderDetail WHERE ProductID = @pid     456= 'All density'*card =0.003759399*121317(note, if there new rows inserted or deleted, you have to replace the 121317 with current card)   DBCC execution completed. If DBCC printed error messages, contact your system administ

The threshold of temp table in stored procedure is little bit tricky, it doesn’t exactly follow the regular threshold. First of all, you can’t use the modification_counter to determine when auto update statistics happens. Because the data in tempdb is deleted every time after execution, replying on the data that that already deleted is pointless. Secondly, the algorithm is different: Here is the original threshold of temp table that are not in stored procedure.   Here is the new algorithm. (note, compatibility of user database decides old threshold or sublinear threshold, the compatibility of tempdb does not matter) Table cardinality ( n ) Old threshold Sublinear threshold n  < 6 >=n+6 >=n+6 6 <=  n  <= 500 >=n+500 >=n+500 500<n  <=19682 >= n+(500 +0.2*n) or <= n-(500+0.2*n) >= n+(500 +0.2*n) or <= n-(500+0.2*n) n  >19682 500 + (0.20 *  n ) >= n+sqrt(1000*n) or <=n-sqrt(1000*n)     Here is an example in compatibility level 150 in SQL 2019

SQL Server IAM page Hi Guys, Welcome to SQL Server allocation series, hope you enjoy the  post1 and post2 . Today, I’m going to show you the detail of IAM page. An  Index Allocation Map (IAM)  page maps the extents in a 4-GB part of a database file used by an allocation unit. An allocation unit is one of three types: IN_ROW_DATA Holds a partition of a heap or index. LOB_DATA Holds large object (LOB) data types, such as XML, VARBINARY(max), and VARCHAR(max). ROW_OVERFLOW_DATA Holds variable length data stored in VARCHAR, NVARCHAR, VARBINARY, or SQL_VARIANT columns that exceed the 8,060 byte row size limit. Each partition of a heap or index contains at least an IN_ROW_DATA allocation unit. It may also contain a LOB_DATA or ROW_OVERFLOW_DATA allocation unit, depending on the heap or index schema. An IAM page covers a 4-GB range in a file and is the same coverage as a GAM or SGAM page. If the allocation unit contains extents from more than one file, or more than one 4-GB range of a file, t

In previous post , we discussed the PFS, GAM and SGAM pages. Today, I’m going show you two related corruptions on these pages.   SQL Server checks the bit in PFS, GAM, SGAM and IAM are checked when new pages are allocated. It guarantees that it does not store data to a wrong page/extent. These pages are crucial to SQL Server, SQL Server considers it as a corruption if they are messed up. I’m going to use the same database I used in previous article . You may restore the   backup  f ile and give it a try. Please note, restoring database will consume some pages and change the PFS, GAM, SGAM and IAM. You may get a different view from my mine.   Let me show you some of the content of GAM and SGAM before the corruption samples. Here are the GAM and SGAM page result: 1.GAM 2.SGAM 3.We can tell that the extent of page(1:352) and extents greater than that have not been allocated. 4.Let’s take a dive into the GAM page. 5.‘ 00000000 00f0 ’ interpretation: 5)Pa