Difference between Lock and Latch

Locks everywhere is a good start to understand how SQL Server provides logical consistency. Every operation has a lock and latch. What does Latch mean? Latch protects memory on Buffer Pool, is a method that provides physical consistency. SQL Server does operations in memory, that means, it read the page from disk and put that … Continue reading Difference between Lock and Latch

SQL Server threads architecture

SQL Server retrieves threads from Windows. The SQL Server configuration setting max worker threads (set at instance level) determines how many threads will be retrieved. SQL Server has its own internal scheduling system, independent of the scheduling performed by the operating system. Instead of using Windows threads directly, SQL Server creates a pool of worker … Continue reading SQL Server threads architecture

How SQL Server stores data?

SQL Server stores data in heaps or b-tree structures. Heaps are unordered set and balanced trees are ordered by their keys. Heaps and b-tree use collection of pages within the structure and it's called allocation units. IN_ROW_DATA -> contains all data. LOB_DATA -> structure for large objects used to stored in xml, varchar(max), nvarchar(max), varbinary(max) … Continue reading How SQL Server stores data?

Wait Statistics

The first post about waits on SQL Server was regarding what SQL Server Wait means, a brief explanation and the concept might be difficult to catch at first. When the task needs to wait for a resource, it is placed on a list until the resource is available. SQL Server keeps detailed internal records of … Continue reading Wait Statistics


Every database has a transaction log that records all transactions and the database modifications made by each transaction The transaction log is a critical component of the database. If there is a system failure, you will need that log to bring your database back to a consistent state. How that works? What steps SQL Server … Continue reading Logging

Wait wait wait…

Let's talk about why we have to wait and how to understand the wait types. Paul Randal in his post Wait statistics, or please tell me where it hurts said: A thread is using the CPU (called RUNNING) until it needs to wait for a resource. It then moves to an unordered list of threads … Continue reading Wait wait wait…

How Update works?

Update data in SQL Server is a simple task. Using the command update table set column = value where column = value. Don't forget the where clause :). But, what SQL Server does internally? Delete plus Insert? Modify? Well, the answer depends. Let's see examples how that works. First, create a database and table for … Continue reading How Update works?