In the realm of SQL Server development, understanding the intricacies of query optimization can drastically impact the performance of your applications. A common scenario that developers encounter involves deciding between using SELECT * INTO to create and populate a temporary table at the beginning of a stored procedure versus first creating a temp table and then populating it with a SELECT followed by an INSERT. This article delves into the reasons why the latter approach often results in better performance, backed by practical T-SQL code examples.

Understanding the Differences

The SELECT * INTO statement is frequently used for its simplicity and convenience, as it creates a temporary table on the fly and inserts the resulting rows from the query. At first glance, this seems efficient. However, this method can lead to suboptimal performance for several reasons:

1. Lack of Indexing: SELECT * INTO creates a table without any indexes, except for a possible clustered index if the SELECT statement includes an ORDER BY clause. Indexes are crucial for query performance, especially in subsequent operations on the temporary table.
2. Data Type Inference: When using SELECT * INTO, SQL Server infers the data types of the columns in the new table based on the data returned. This inference might not always select the most efficient data types, potentially leading to larger than necessary column sizes and, subsequently, reduced performance.
3. Selectivity: SELECT * can inadvertently pull in more data than necessary, especially if not all columns are needed for subsequent operations. This excess data can increase I/O operations and memory usage, slowing down the query.

On the other hand, explicitly creating a temp table and then using a SELECT statement to specify the needed columns, followed by an INSERT INTO operation, offers several advantages:

1. Optimized Data Types: By manually creating the temp table, you can define each column with the most appropriate data type and size, ensuring a more efficient storage.
2. Indexing Opportunities: Manually creating a temp table allows for the explicit definition of indexes based on anticipated query patterns, significantly improving the performance of subsequent operations.
3. Selective Data Retrieval: Selecting only the necessary columns for insertion reduces the amount of data processed and stored, streamlining the performance.

Practical Examples

Consider a scenario where you need to perform operations on a subset of data from a larger table. Here’s how the two approaches compare:

Using SELECT * INTO:

SELECT * INTO #TempTable
FROM LargeTable;
-- Followed by additional operations on #TempTable

Using SELECT and INSERT INTO:

CREATE TABLE #TempTable (
    Column1 INT,
    Column2 VARCHAR(50)
    -- Define as needed
);
INSERT INTO #TempTable (Column1, Column2)
SELECT Column1, Column2
FROM LargeTable;
-- Indexes can be added here as needed
-- Followed by additional operations on #TempTable

Conclusion

While SELECT * INTO is undeniably convenient for quick data movement, the lack of control over data types, indexing, and data selectivity often leads to poorer performance compared to a more deliberate SELECT then INSERT INTO approach. By taking the time to define the structure of your temporary tables and carefully selecting the data to insert, you can achieve significant performance gains in your SQL Server applications.

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