SQL Server Estimates don't use AVG_RANGE_ROWS for Uniqueidentifer Parameter

I'm trying to debug a very weird query row estimation. The query is very simple. I have a table OrderItems that contains for each Order (column OrderId) the items of the order.

SELECT count(*)
FROM orders.OrderItem 
WHERE OrderId = '5a7e53c4-fc70-f011-8dca-000d3a3aa5e1'

According to the statistics from IX_OrderItem_FK_OrderId (that's just a normal unfiltered foreign key index CREATE INDEX IX_OrderItem_FK_OrderId on orders.OrderId(OrderId), the density is 1.2620972E-06 with 7423048 rows, so about ~9.3 items per order (if we ignore the items with OrderId = NULL, if we include them there are even less). The statistics are created with FULLSCAN, and are only slightly out of date (around ~0.2% new rows since the last recompute). | Name | Updated | Rows | Rows Sampled | Steps | Density | Average key length | String Index | Filter Expression | Unfiltered Rows | Persisted Sample Percent | | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | | IX_OrderItem_FK_OrderId | Aug 3 2025 4:36PM | 7423048 | 7423048 | 198 | 0.1649756 |26.443027 | "NO " | NULL | 7423048 | 100 | | All density | Average Length | Columns | | --- | --- | --- | | 1.2620972E-06 | 10.443027 | OrderId | | 1.3471555E-07 | 26.443027 | OrderId, Id | The query plan however expects, that the query returns 205.496 items. And in reality there are actually 0 results - because the orderId doesn't exist. Detailed Query Plan: https://www.brentozar.com/pastetheplan/?id=hVKYNLmXSU It probably uses the histogram for coming up with the estimate. It should fall into following bucket with RANGE_HI_KEY = 'a39932d8-aa2c-f011-8b3d-000d3a440098'. But that estimate should then be 6.87 according to the AVG_RANGE_ROWS. It somehow looks like it uses the EQ_ROWS from the previous bucket (but 205 might also just be by accident). | RANGE_HI_KEY | RANGE_ROWS | EQ_ROWS | DISTINCT_RANGE_ROWS | AVG_RANGE_ROWS | | --- | --- | --- | --- | --- | --- | | 9d2e2bea-aa6e-f011-8dca-000d3a3aa5e1 | 12889 | 205 | 2412 | 5.343698 | | a39932d8-aa2c-f011-8b3d-000d3a440098 | 21923 | 107 | 3191 | 6.8702602 | OPTION(RECOMPILE) does not help. Can somebody explain how SQL Server (in particularly Azure SQL) is coming up with that number? - Does it really think that the parameter is close enough to the bucket start, and just takes the EQ_ROWS value even though the AVG_RANGE_ROWS is a lot smaller? - Does it not understand the parameter because it's defined as VARCHAR? If I replace it with DECLARE @OrderId UNIQUEIDENTIFIER = '5a7e...'; WHERE OrderId = @OrderId the estimate is down to 6. But if that's the reason, from where is the estimate 205?