About This Blog

SQL Server internals information including my content from SQLBlog.com and SQLPerformance.com.

Sunday, 11 October 2020

sql_handle and the SQL Server batch text hash

sql_handle and the SQL Server batch text hash

This article describes the structure of a sql_handle and shows how the batch text hash component is calculated.

Thursday, 8 October 2020

Closest Match with Sort Rewinds

Closest Match with Sort Rewinds

In When Do SQL Server Sorts Rewind? I described how most sorts can only rewind when they contain at most one row. The exception is in-memory sorts, which can rewind at most 500 rows and 16KB of data.

These are certainly tight restrictions, but we can still make use of them on occasion.

To illustrate, I am going reuse a demo Itzik Ben-Gan provided in part one of his Closest Match series, specifically solution 2 (modified value range and indexing).

As Itzik’s title suggests, the task is to find the closest match for a value in one table in a second table.

As Itzik describes it:

The challenge is to match to each row from T1 the row from T2 where the absolute difference between T2.val and T1.val is the lowest. In case of ties (multiple matching rows in T2), match the top row based on val ascending, keycol ascending order.

That is, the row with the lowest value in the val column, and if you still have ties, the row with the lowest keycol value. The tiebreaker is used to guarantee determinism.