SQL Server Query Slowness: Causes & Fixes

There could be various reasons why a SQL Server query suddenly becomes slow, here are some common ones:

1. Index failure: If the table involved in the query does not have the correct index or the index fails, it will result in decreased query performance.
2. Hardware failure such as server hardware malfunction, insufficient disk space, and low memory can lead to decreased query performance.
3. An increase in data volume may lead to a decrease in query performance, especially without proper index support.
4. Inadequate query optimization: If query statements are not optimized or do not use appropriate indexes, slow performance can result.
5. Lock waiting occurs when a query is being held by another transaction, causing the query to wait for the lock to be released, thereby decreasing query performance.
6. Outdated Statistics: SQL Server’s query optimizer relies on table statistics to generate query plans. When statistics are outdated or inaccurate, it can lead to a decrease in query performance.
7. Long-running queries: Long-running queries will use up database resources and impact the performance of other queries.
8. Network issues such as latency and bandwidth limitations can also result in slower queries.

To enhance query performance, one can analyze execution plans, check index status, optimize query statements, and update statistics to address the reasons for the performance decline.