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

[JoshAsh]

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.

[KevinConanMSFT]

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

[JoshAsh]

Hey Kevin

Done - 2306200030005070

[justinj]

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? 

[JoshAsh]

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".

[justinj]

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