If execution plans are the brain of SQL Server, statistics are the eyes. Without accurate statistics, SQL Server makes bad guesses, leading to slow queries, table scans, and poor stored procedure performance.
This blog explains what statistics are, how they work, and how to manage them properly.
1️⃣ What Are Statistics in SQL Server?
Statistics are metadata that describe the distribution of data in a column or index.
They help SQL Server estimate:
- Number of rows
- Data distribution
- Selectivity of queries
📌 Better statistics → Better execution plans
2️⃣ What Information Do Statistics Contain?
A statistics object includes:
✔ Histogram (data distribution)
✔ Density vector
✔ Row count information
3️⃣ How SQL Server Uses Statistics
When a query runs, SQL Server:
- Reads statistics
- Estimates rows
- Chooses indexes
- Builds an execution plan
📌 Wrong estimate = Wrong plan
4️⃣ Auto Create & Auto Update Statistics
Default Behavior
AUTO_CREATE_STATISTICS ON
AUTO_UPDATE_STATISTICS ON
✔ SQL Server creates statistics automatically
✔ Updates when data changes significantly
5️⃣ When Statistics Become Outdated
Statistics can become stale when:
❌ Large data changes
❌ Bulk inserts
❌ Truncate + reload
❌ Data skew
6️⃣ Common Symptoms of Bad Statistics
❌ Index scan instead of seek
❌ Bad join order
❌ Slow stored procedures
❌ Sudden performance degradation
7️⃣ Viewing Statistics Details
DBCC SHOW_STATISTICS ('Users', 'IX_Users_UserId');
Look at:
- Histogram
- Density
- Updated date
8️⃣ Updating Statistics Manually
Update Single Table
UPDATE STATISTICS Users;
Update Full Database
EXEC sp_updatestats;
📌 Use during low-traffic periods.
9️⃣ Statistics vs Indexes
| Index | Statistics |
|---|---|
| Physical structure | Metadata |
| Speeds up reads | Guides optimizer |
| Needs maintenance | Needs refresh |
📌 Index without statistics = Blind index
🔟 Statistics and Stored Procedures
Common Issue
Stored procedure suddenly slow.
Root Cause
- Old statistics
- Wrong row estimation
- Poor cached plan
Solution
UPDATE STATISTICS TableName;
EXEC dbo.ProcedureName;
1️⃣1️⃣ FULLSCAN vs SAMPLE
❌ Default
Uses sampling (faster, less accurate)
✅ FULLSCAN
UPDATE STATISTICS Users WITH FULLSCAN;
✔ More accurate
❌ More time
Use for:
- Critical tables
- Skewed data
1️⃣2️⃣ Statistics and Parameter Sniffing
Bad statistics + parameter sniffing = disaster.
📌 Fresh statistics reduce sniffing problems.
1️⃣3️⃣ Best Practices for Statistics
✔ Keep auto update ON
✔ Update stats after bulk operations
✔ Use FULLSCAN selectively
✔ Monitor query performance
✔ Don’t disable statistics
📌 Statistics-Friendly Stored Procedure Example
CREATE PROCEDURE dbo.GetUsersByStatus
@Status VARCHAR(20)
AS
BEGIN
SET NOCOUNT ON;
SELECT UserId, UserName
FROM Users
WHERE Status = @Status;
END
✍️ Conclusion
Statistics are silent performance drivers. When queries suddenly slow down, statistics are often the real culprit, not the code.
Understanding statistics helps you predict performance issues and fix them before users complain.