Blog
Mert Özen's blog posts on software development, backend, frontend, and DevOps.
Reading Older SQL Servers: Engine Evolution from 2016 to 2025 and a Compatibility Level Upgrade Strategy
The engine version and the compatibility level are different things: in a database running at level 130 on SQL Server 2025, a table variable is still estimated at 1 row, an MSTVF at 100 rows, a scalar UDF is called 8,916 times and REGEXP_LIKE isn't recognized. A 2016 to 2025 feature table, Microsoft Lifecycle dates, and a real run of the flow: upgrade the engine at the old level, collect with Query Store, raise the level, force the plan on a regression.
A Query's Journey Inside SQL Server: Relational Engine, Storage Engine, Buffer Pool and Transaction Log
A query reaches SQL Server over TDS, is parsed, bound and optimized in the Relational Engine and lands in the plan cache; the Storage Engine reads pages through the buffer pool, and changes go to the transaction log first (WAL) and to the data file later via checkpoint. Also: the deterministic 5.6-million-row Bookstore lab data used across the series, the 2025 editions and how compatibility level differs from engine version.
What SQL Server 2022 Added to T-SQL: IS DISTINCT FROM, GREATEST/LEAST, WINDOW, GENERATE_SERIES, DATE_BUCKET and JSON Functions
SQL Server 2022 brought IS DISTINCT FROM for NULL-safe comparison, GREATEST/LEAST, GENERATE_SERIES, DATETRUNC, DATE_BUCKET, TRIM with characters, ordered STRING_SPLIT, JSON_OBJECT/JSON_ARRAY and APPROX_PERCENTILE_CONT. We try each with before/after code and show with real error messages why GENERATE_SERIES, WINDOW and TRIM LEADING fail at compatibility level 150.