When is Using Multiple INSERT Statements Faster Than a Single INSERT with Multiple VALUES in SQL Server?

SQL Server: Multiple INSERT Statements vs. Single INSERT with Multiple VALUES – A Performance Paradox

The Problem: Benchmark tests surprisingly show that executing 1000 individual INSERT statements can be faster than a single INSERT statement with 1000 VALUES clauses.

The Reason: SQL Server's query processing involves a crucial "binding" or "algebrizing" phase after parsing. This phase detects and handles duplicate values within the INSERT statement, building an optimized execution plan. The time required for this phase increases dramatically with the number of unique values.

Performance Influencers:

• Data Size: Longer string values significantly increase comparison times during binding.
• Duplicate Data: Many duplicate values reduce binding time as fewer comparisons are needed.
• Automatic Parameterization: While beneficial for smaller INSERT statements, auto-parameterization can become a bottleneck with a large number of values, hindering performance.

Execution Plan Analysis:

• A single INSERT with 1000 VALUES shows a sharp increase in compilation time beyond 250 VALUES clauses, indicating a shift to a less efficient, non-parameterized plan.
• Compilation time and memory consumption rise sharply with increasing unique values.

Practical Implications:

• For large INSERT operations with short strings or high data redundancy, using multiple, separate INSERT statements might be faster.
• For INSERT operations with few unique values and long strings, a single INSERT with many VALUES might still be more efficient.
• While SQL Server 2012 offered some improvements, complete resolution of this issue may require newer versions.

Further Considerations:

• Optimizer Limitations: Query optimizers aren't always perfect, especially with massive datasets.
• Cached Plan Impact: The cached plan size remains unaffected by duplicate values or string length.
• UDF Overhead: Using User-Defined Functions (UDFs) within INSERT statements can negatively impact performance.

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