The index page cluster ratio is used to estimate the cost of large I/O for queries that need to read a large number of leaf-level pages from nonclustered indexes or clustered indexes on data-only-locked tables. Some examples of such queries are covered index scans and range queries that read a large number of rows.
On newly created indexes, the “Index page cluster ratio” is 1.0, or very close to 1.0, indicating optimal clustering of index leaf pages on extents. As index pages are split and new pages are allocated from additional extents, the ratio drops. A very low percentage may indicate that dropping and re-creating the index, or running reorg rebuild on it, would improve performance, especially if many queries perform covered scans.
