Introduction: Why Temporary Structures Matter in SQL Server

SQL Server provides several ways to store temporary data: temp tables, global temp tables, and table variables. Many developers use them without understanding how they behave internally, especially how they affect execution plans, statistics, locking, and tempdb usage. Choosing the wrong one can slow down queries, increase CPU usage, or even cause blocking across sessions.

This guide breaks down the differences so you can make smarter performance decisions.

What Are Temp Tables?

Temp tables (created with #MyTable) are local to the current session and stored physically in tempdb.

Characteristics

• Exist only for the session or stored procedure
• Support indexes, constraints, and statistics
• Can store large volumes of data efficiently
• Fully participate in SQL Server’s optimizer

Creation Example

CREATE TABLE #Employees
(
    EmpID INT,
    Name NVARCHAR(100)
);

Temp tables behave like normal tables but disappear automatically when the session ends.

What Are Global Temp Tables?

Global temp tables use double hash marks (##MyTable) and are visible to all sessions.

Key Characteristics

• Temporary, but shared across connections
• Useful for data exchange across sessions
• Dropped when the last session referencing it ends
• Must be carefully named to avoid collisions

Example

CREATE TABLE ##TempSales
(
    OrderID INT,
    Amount DECIMAL(10,2)
);

Global temp tables behave most like regular tables, but their scope is server-wide.

What Are Table Variables?

Table variables are declared with an @ prefix and typically used inside functions or small operations.

Key Characteristics

• Lifetime exists within the batch or function
• Stored in tempdb (not memory as many assume)
• Do not support full statistics
• Poor cardinality estimation for large datasets
• Great for small row counts

Example

DECLARE @Orders TABLE
(
    OrderID INT,
    Amount DECIMAL(10,2)
);

Table variables are lightweight but not always efficient.

Execution Behavior: Temp Table vs Global Temp Table vs Table Variable

Aspect	Temp Table	Global Temp Table	Table Variable
Stored In	tempdb	tempdb	tempdb
Scope	Session	Server-wide	Batch / Proc
Statistics	Yes	Yes	Limited
Indexing	Full	Full	Limited
Transaction Support	Full	Full	Limited
Best For	Medium–large data	Cross-session sharing	Small datasets

Performance Considerations of Temp Tables

Temp tables are usually the most optimizer-friendly temporary structure.

Advantages

• Full statistics help the engine estimate row counts accurately
• Good join performance
• Ideal for large operations or complex transformations
• Indexing improves speed dramatically

Typical Use Cases

• ETL pipelines
• Stored procedures with heavy logic
• Multi-step calculations

Performance Considerations of Global Temp Tables

Global temp tables behave like regular temp tables, but with wider visibility.

Advantages

• Ideal for loading data once and using across multiple sessions
• Helpful in long-running reporting pipelines
• Good for shared caching scenarios

Drawbacks

• Risk of naming conflicts
• Potential blocking across sessions
• Must handle concurrency carefully

Performance Considerations of Table Variables

Table variables often perform worse for larger datasets due to bad statistics.

Key Limitations

• SQL Server assumes 1 row unless using modern cardinality estimation
• Joins perform poorly due to missing statistics
• No automatic indexing (must declare inline)

Best Use Cases

• Small tables (under 100 rows)
• Parameter passing
• Conditional logic where indexing isn’t needed

Statistics & Query Optimization Differences

Temp Tables

• Have statistics that auto-update
• Optimizer produces high-quality plans

Table Variables

• Historically had no statistics
• Newer SQL versions generate limited stats, but still inferior
• Can cause inefficient joins and scans

Global Temp Tables

• Same statistics capability as temp tables
• But impacted by concurrency

Real Execution Examples

Example: Temp Table is Faster

CREATE TABLE #Temp (ID INT);
INSERT INTO #Temp SELECT TOP 500000 ID FROM BigTable;

SELECT t.*
FROM #Temp t
JOIN Orders o ON t.ID = o.CustomerID;

Temp table wins because statistics help generate better join plans.

Example: Table Variable is Faster

DECLARE @Ids TABLE (ID INT);
INSERT INTO @Ids VALUES (1), (2);

SELECT *
FROM Orders
WHERE CustomerID IN (SELECT ID FROM @Ids);

For tiny datasets, a table variable reduces overhead.

Global Temp Tables in Multi-User Environments

Global temp tables shine when shared access is required.

Challenges

• Naming collisions (##Temp already exists)
• Blocking when multiple sessions modify the table
• Cleanup timing depends on session connections

Best Practices

• Use unique prefixes (##RptSession_123)
• Avoid heavy writes from multiple sessions
• Monitor with sys.objects in tempdb

Memory Use & TempDB Impact

Feature	Temp Table	Global Temp Table	Table Variable
TempDB Usage	Medium–High	Medium–High	Low–Medium
Memory Spills	Possible	Possible	Rare
Logging	Logged in tempdb	Logged in tempdb	Minimal

Table variables consume fewer tempdb resources—but only because they don’t generate statistics or heavy metadata.

Error Handling & Transactions

Temp Tables

• Fully respect transactions
• Rollback reverts modifications

Global Temp Tables

• Same as temp tables

Table Variables

• Do not rollback fully
• Ideal for transactional isolation inside functions

Best Practices for Choosing the Right Temporary Structure

Use Temp Tables When:

• Handling large resultsets
• Needing statistics and indexes
• Doing multi-step operations

Use Global Temp Tables When:

• Sharing data across sessions
• Running server-wide processes

Use Table Variables When:

• Storing very small sets
• Inside functions or simple procedures
• Temporary row-level logic

Comparison Table: Summary

Feature	Temp Table	Global Temp Table	Table Variable
Scope	Session	Server	Batch/Proc
Statistics	Full	Full	Limited
Indexing	Yes	Yes	Partial
Performance (small data)	Good	Good	Excellent
Performance (large data)	Excellent	Excellent	Poor
Concurrency Safety	High	Medium	High
Usage Difficulty	Easy	Medium	Easy

Frequently Asked Questions

1. Are table variables stored in memory?

No. They are stored in tempdb, though they use less metadata.

2. Are temp tables faster than table variables?

Yes, especially for large datasets, due to full statistics.

3. Do table variables support indexing?

Yes—but only through inline declarations, not traditional CREATE INDEX.

4. When should I use global temp tables?

When multiple sessions require the same temporary data.

5. Can global temp tables cause conflicts?

Yes, if two processes attempt to create the same table name.

6. Do temp tables lock tempdb?

They can, especially under heavy load.

Conclusion: Choosing the Right Temporary Object

Understanding temp table vs global temp table vs temp variable helps you write faster, more efficient SQL. Temp tables provide the best optimizer support, global temp tables allow cross-session sharing, and table variables shine in small, lightweight tasks.

Pick the one that aligns with your query size, concurrency needs, and performance goals.