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I have a table with around 2 million records in it. I created a spatial index, using the defaults other than the bounding box. I've been noticing that some queries are extremely fast, and some are extremely slow. The determining factor appears to the size of the polygon used in the query.

On larger search areas, using WITH(INDEX(SIX_FT5)) slows down the query considerably (from 0 seconds, to 15+ seconds). On smaller search areas, the exact opposite it true.

Here's some of the queries I'm testing with:

Fast:

SELECT TOP(1000) * FROM [FT5] WHERE (shape.STIntersects(geometry::STGeomFromText('POLYGON ((-133462.805381701 -668610.241000959, 2934415.68824241 -668610.241000959, 2934415.68824241 2200521.65831815, -133462.805381701 2200521.65831815, -133462.805381701 -668610.241000959))', 2264)) = 1) 

Slow:

SELECT TOP(1000) * FROM [FT5] WITH(INDEX(SIX_FT5)) WHERE (shape.STIntersects(geometry::STGeomFromText('POLYGON ((-133462.805381701 -668610.241000959, 2934415.68824241 -668610.241000959, 2934415.68824241 2200521.65831815, -133462.805381701 2200521.65831815, -133462.805381701 -668610.241000959))', 2264)) = 1) 

Why is that WITH clause slowing things down sometimes?

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