SA0046 : Consider creating statistics on all composite index columns

When a composite index is created in SQL Server, statistics are automatically maintained only for the first column of the index. This can be problematic because the remaining columns in the composite index might also influence query performance but lack direct statistics to inform the query optimizer. Insufficient statistical insights can result in poor query execution plans.

For example:

1-- Example with a composite index on two columns
2CREATE INDEX idx_composite ON TableName (ColumnA, ColumnB);

In this case, SQL Server only generates statistics for ColumnA. Queries that primarily filter or sort on ColumnB might not perform efficiently, as the query optimizer doesn’t have enough information about its distribution and relation to the dataset.

• Potentially leads to inefficient execution plans, as the query optimizer lacks statistical data for other key columns.

• May cause increased resource consumption and slower query performance, especially for queries not targeting the leading column.

Ensure adequate statistics for composite indexes to improve query performance.

Follow these steps to address the issue:

1. Identify the composite indexes in your database. Use sys.indexes and sys.index_columns to find indexes with multiple columns.

2. Determine key columns of these indexes that lack statistics. Focus on columns beyond the first key column.

3. Create statistics for these key columns manually to provide the query optimizer with more comprehensive data.

For example:

1-- Example of creating statistics for a non-leading column in a composite index
2CREATE STATISTICS stat_ColumnB ON TableName (ColumnB);

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