SQL: DATABASE UNAVAILABLE TO QUERY

Sometimes, an MSSQL database becomes inaccessible for executing queries while remaining accessible for viewing data.

Timing is crucial when a database is unavailable for querying. As long as this issue exists, application users cannot edit or update the database, which means operation disruption. In this case, it’s important to have a full backup available to restore the database to as close as possible to its previous state and reestablish querying functionality as soon as possible.

AimBetter recognizes this problem immediately when it happens, saving time and ensuring business continuity.

Symptoms

Users receive error messages when trying to work with the database’s tables. The system cannot accept new updates, inserts, deletions, and other commands.

Impact: Critical

Inaccessible databases imply working time and data loss, disrupting the business flow.

Expected behavior

Databases should not be in a situation where they are inaccessible for querying. This alert must be investigated, especially to check data integrity and functionality.

Find out how you can save hours of work when investigating the root cause of this issue.

Possible causes

1- Database corruption. Priority: Critical

Database corruption refers to errors within the database files or structures that prevent the database management system from accessing or manipulating data properly. Corruption can occur at various levels within the database, such as the database itself, indexes, tables, or data pages. A corrupted database might make it unavailable for running queries.

As a result, running queries with the corrupted database will cause failure, slowness, or time-outs. Transactions may be automatically rolled back. In severe cases of corruption, data loss may occur.

SQL Server may report that a table in the database cannot be accessed because it has become corrupted.

Problem identification:

Users might complain about immediate errors when trying to run processes. Several error messages of corrupted tables may be displayed during an operation.

Recommended action :

If possible, use utilities that allow you to repair corrupted data pages or structures. If unsuccessful, restore the database from the most recent backup file taken before the corruption occurred. This ensures that data integrity is restored to a known good state, yet there is a chance for data loss.

Before then, you can rebuild and examine indexes.

If the backup chosen to restore wasn’t close enough to date, implement other backup mechanisms that run more frequently for important production databases. A skilled DBA should do this.

2- Locked tables or Deadlocks. Priority: High

A Database’s table won’t be available for query in cases where transactions are open in the database or a query is blocking other queries.
This case is less urgent than cases when the whole database is not operable since usually, only one or a few tables are locked.
A deadlock occurs when two or more processes compete for exclusive access to a resource but cannot obtain it because some other process prevents it. As a result, one of the processes is chosen as the “victim” to do a rollback and to be terminated.

You can read more about how to investigate these issues in the following articles: SQL – Blocking Queries, SQL – Deadlock.

Recommended action :

Terminate problematic queries after identifying them and/or improve performance by reviewing the execution plan of the query and making it simpler so the running time decreases.

Make sure the scheduling of processes doesn’t collapse each other in the meaning of source use.

Ensure the database settings fit the SQL version and the required business application.

3-Overloaded or inefficient hardware resources. Priority: Medium

Overloaded server resources such as CPU, memory, disk I/O, or network can slow down query execution. It might happen due to a specific application or many processes running simultaneously.

This may lead to query execution bottlenecks, causing queries to timeout or fail.

Problem identification:

Check if CPU, memory, disk I/O, or network resources are highly utilized. Identify the source of the load and whether it is recurrent. Check if it happens when the database cannot be accessed.

Recommended action :

Cancel unnecessary programs that cause high hardware utilization and schedule non-urgent processes at times of lower user demand.

Consider upgrading hardware if possible.  Ensure proper network connectivity,

Monitoring would be useful for following up on this issue.

4-Maintenance tasks. Priority: Low

Maintenance tasks are essential for SQL server databases to operate well. They are important for the database’s functionality and integrity, performance optimization, and disaster recovery planning. These tasks might include index rebuilds, database backups and restores, statistics updates, integrity checks (DBCC CHECKDB commands), and more. 

Problem identification:

Check if maintenance tasks are running when DB is unavailable for querying.

Recommended action :

Maintenance tasks are important. You just have to make sure they’re done with enough system resources and outside business working hours so they won’t cause more harm than help.

5-Settings are not compatible with the required version of the SQL plan. Priority: Low

Incompatible versions in a database environment can cause queries not to run due to differences in features, syntax, or supported operations between different versions of the database software, client tools, or other components.

Problem identification:

Error messages about an unfit version should be raised when a query cannot be run because of an incompatible version.

Recommended action :

Ensure that the database version and client tools are compatible and support the features required by your queries. Upgrade what’s needed to fit the database server’s version. Review the queries’ code. If needed, set the database’s version compatibility.

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