Why Are Multiple INSERT Statements Faster Than a Single INSERT with Multiple VALUES in SQL Server?

Multiple INSERT statements outperform single INSERT with multiple VALUES due to the auto parameterization process in SQL Server.

In case of multiple INSERT statements, each statement is compiled and executed independently, resulting in minimal overhead. However, with a single INSERT statement containing multiple VALUES, the query optimizer must parse and compile the entire statement before execution, which can significantly increase compile time.

The cut-off for auto parameterization occurs around 250 VALUES clauses. Beyond this point, the cached plan size drops abruptly, compile time increases drastically, and the efficiency of processing value clauses decreases linearly.

This behavior stems from a change in plan type. When using auto parameterization, the plan is specific to the literal values being inserted. When it exceeds the limit, the plan becomes non-parameterized, and it must perform activities that do not scale linearly, such as sorting, which results in longer compile times.

The impact of this can be significant, especially when dealing with longer strings or unique values. As demonstrated by the graphs, increasing string length or eliminating duplicates leads to worse performance for the single INSERT statement with multiple VALUES method.

Conversely, introducing duplicates improves performance because SQL Server can identify them during the algebrizing stage and avoid unnecessary sorting.

Therefore, for inserting multiple rows, using multiple INSERT statements is generally more efficient than a single INSERT statement with multiple VALUES, particularly when dealing with longer strings or unique values.

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