topic Re: Request to perform an external distributed computation has failed with error... in Data Warehouse

Request to perform an external distributed computation has failed with error...

JoshAsh — Mon, 19 Jun 2023 10:29:55 GMT

When trying to run a realistic production query on Fabric Warehouse that we currently run on SQL Server I get

Msg 15802, Level 16, State 16, Line 1
Request to perform an external distributed computation has failed with error "Assertion failed: Invalid child output DOP".

This was the first query we tried..

I have done some playing, if i remove any construct in the query using COUNT(DISTINCT ...) there is no failure.

Re: Request to perform an external distributed computation has failed with error...

justinj — Mon, 19 Jun 2023 16:13:26 GMT

Hi Josh,

The queries from SQL Server would not execute as is on Synapse WH as the TSQL has some changes.

do you have a sample of the script you are trying to execute?

Re: Request to perform an external distributed computation has failed with error...

JoshAsh — Mon, 19 Jun 2023 21:49:05 GMT

I've had to change column and table names for security reasons.

SELECT COUNT(DISTINCT CASE WHEN UPPER(fee.blah) LIKE '%AMEP%' THEN c.blah ELSE NULL END) as DISTINCT_BLAH_COUNT, MAX(c.blah) as blah FROM dbo.TABLE1 a INNER JOIN dbo.TABLE2 d ON a.blah = d.blah INNER JOIN dbo.TABLE3 b ON a.blah = b.blah INNER JOIN dbo.TABLE4 dt ON a.blah = dt.blah INNER JOIN dbo.TABLE5 att ON a.blah = att.blah INNER JOIN dbo.TABLE6 c ON a.blah = c.blah INNER JOIN dbo.TABLE7 fc ON a.blah = fc.blah INNER JOIN dbo.TABLE8 age ON a.blah = age.blah LEFT JOIN dbo.TABLE9 ft ON a.blah = ft.blah LEFT JOIN dbo.TABLE10 fee ON a.blah = fee.blah LEFT JOIN dbo.TABLE11 ccs ON a.blah = ccs.blah WHERE d.blah in ('Centre','Discontinued') AND dt.blah >= 2019 GROUP BY a.blah, c.blah, fc.blah, dt.blah, dt.blah

But this is the simplest version I can make that is erroring. It's not really a complex query as you can see

Another errror happening occasionally is

Request to perform an external distributed computation has failed with error "Assertion failed: Degree of partitioned parallelism can only be 1 when the child operator distribution is full or when the session dop is 1".

Re: Request to perform an external distributed computation has failed with error...

justinj — Tue, 20 Jun 2023 07:30:42 GMT

This is happening due to the COUNT(DISTINCT xyz) function as the data is distributed on Synapse Warehouse.

Can you try using APPROX_COUNT_DISTINCT instead and let me know if that works?

APPROX_COUNT_DISTINCT (Transact-SQL) - SQL Server | Microsoft Learn

Re: Request to perform an external distributed computation has failed with error...

KevinConanMSFT — Tue, 20 Jun 2023 13:52:32 GMT

JoshAsh could you open a support case for this please? We'd like to look deeper into this.

Re: Request to perform an external distributed computation has failed with error...

JoshAsh — Tue, 20 Jun 2023 18:59:49 GMT

Hey Kevin

Done - 2306200030005070