Adaptive Join : Threshold rows

sakuraime 2,321 Reputation points
2021-10-04T06:35:56.82+00:00

Just interesting how to determine the threshold rows of the adaptive join which switch between hash or nested loop ?

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SQL Server
SQL Server
A family of Microsoft relational database management and analysis systems for e-commerce, line-of-business, and data warehousing solutions.
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  1. AmeliaGu-MSFT 13,971 Reputation points Microsoft Vendor
    2021-10-06T07:18:26.077+00:00

    Hi sakuraime,
    Please also check this doc:

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    Best Regards,
    Amelia


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  1. AmeliaGu-MSFT 13,971 Reputation points Microsoft Vendor
    2021-10-05T02:41:23.33+00:00

    Hi sakuraime,

    If the Adaptive Threshold Rows is more than an actual number of rows, SQL Server Engine has preferred nested loop join. If the Adaptive Threshold Rows is less than an actual number of rows, SQL Server Engine has preferred Hash Match join.
    Please refer to this article.

    Best Regards,
    Amelia


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  2. Erland Sommarskog 106K Reputation points MVP
    2021-10-05T21:17:42.373+00:00

    I mean how this number comes out ?

    >

    half of the actual rows or what?

    I have not checked, but I can't but see that this is a number which is determined at compile time. That is, the optimizer estimates that if the actual number of rows coming into the Adaptive Join operator is lower than the threshold, the nested loops will be the cheapest join method, and but the hash is estimated to be better if the number is above the threshold.