Incorrect Results with Parallel Eager Spools and Batch Mode
An incorrect results bug with batch mode hash joins and a parallel eager table spool used for Halloween Protection. Unfixed before SQL Server 2022.
Articles
Page 3 of 11
— showing articles
37 to
54
of 182
Understanding Execution Plan Operator Timings
How SQL Server actual execution plan operator timings work, and why parallel row mode plan timings can be misleading.
SQL Server batch text hash and the sql_handle
Describes the structure of a sql_handle and explains how the SQL Server batch text hash component is calculated. Provides T-SQL functions to replicate the calculations.
Closest Match with Sort Rewinds
A SQL Server sort operator can replay its results, but only when the sort operates on at most one row. This article employs a sort to avoid a table spool in a query designed to find the closest match to a given value in a second table.
When Do SQL Server Sorts Rewind?
The rare circumstances when a SQL Server sort operator inside a nested loop can rewind (replay) cached results to avoid re-executing child plan operators.
SQL Server 2019 Aggregate Splitting
Exploring a new query optimizer exploration rule in SQL Server 2019 can split large batch mode hash aggregates to avoid or reduce spilling to tempdb.
A bug with Halloween Protection and the OUTPUT Clause
SQL Server has a bug with Halloween Protection when using an OUTPUT clause on a DELETE statement.
How MAXDOP Really Works
A MAXDOP n hint does not limit a SQL Server execution plan to using a maximum of n CPU cores concurrently 🤯
Pulling Group By Above a Join
How and when the SQL Server query optimizer can pull a logical Group By operator above a logical Join.
New Metadata-Only Column Changes in SQL Server 2016
New instant column data type changes possible in SQL Server 2016 when using data compression.
Finding Distinct Values Quickly
A few different ways to retrieve distinct values from a table, including some big "it just runs faster" wins in SQL Server 2019.
The Eager Index Spool and The Optimizer
The SQL Server Eager Index Spool execution plan operator, and the optimizer rules capable of adding it into plans.
Nested Loops Joins and Performance Spools
Performance spools and how they can be used by the optimizer to reduce the estimated cost of nested loops joins.
Batch Mode Normalization and Performance
The internals of SQL Server batch mode data normalization and how it can affect execution plans and performance.
Batch Mode Bitmap Demos
Demo scripts for SQL Server batch mode hash bitmap selectivity estimates. Simple and complex bitmap types. Compiled and compressed bitmaps.
Batch Mode Bitmaps in SQL Server
A deep dive into SQL Server batch mode bitmaps revealing undocumented details that can greatly impact performance.
Some ANY Aggregate Transformations are Broken
Two cases where the introduction of the internal ANY aggregate by the optimizer may cause wrong results.
Apply versus Nested Loops Join
Understanding the differences between SQL Server APPLY and nested loops joins, including details of query optimizer transformations.