💎SQL Reloaded: Monitoring the Synapse serverless SQL pool with Dynamics Management Views II

Identifying the SQL Server DMVs which are accessible for the Serverless SQL pool (see previous post), allowed me to identify besides sys.dm_exec_requests_history three more DMVs with statistics on the statements executed on the server: sys.dm_request_phases, sys.dm_request_phases_task_group_stats and sys.dm_request_phases_exec_task_stats. Untofurtunately, there seems to be no documentation available on these DMVs, and, at the time the post was written, there were also no further hits on google.com or bing.com found on the same.

sys.dm_request_phases

sys.dm_request_phases  provides insights in the phases an execution statement goes through, and seems to summarize the other two views:

-- Azure Serverless SQL pool: request phases
SELECT TOP (100) dist_statement_id
, RPH.dist_request_id
, TRY_CAST(RPH.id as bigint) id
, TRY_CAST(RPH.parent_ids as bigint) parent_ids
, RPH.start_time
, RPH.end_time
--, RPH.total_elapsed_time_ms
--, RPH.total_elapsed_time_ms/1000.0 total_elapsed_time_sec
--, RPH.min_time_ms
--, RPH.min_time_ms/1000.0 min_time_sec
--, RPH.max_time_ms
--, RPH.max_time_ms/1000.0 max_time_sec
--, RPH.avg_time_ms
, RPH.avg_time_ms/1000.0 avg_time_sec
--, RPH.stdev_time_ms
--, RPH.stdev_time_ms/1000.0 stdev_time_sec -- it has no values
--, RPH.min_rows
--, RPH.max_rows
--, RPH.avg_rows
--, RPH.stdev_rows -- it has no values
, RPH.total_rows
--, RPH.total_bytes_processed
, RPH.total_bytes_processed/1028.0 total_kb_processed
, RPH.state_desc
, RPH.operation_type
, RPH.input_dop
, RPH.output_dop
, RPH.task_retries
, RPH.error_id
FROM sys.dm_request_phases RPH
ORDER BY Id

dist_statement_id	dist_request_id	id	parent_ids	start_time	end_time	avg_time_sec	total_rows	total_kb_processed	state_desc	operation_type	input_dop	output_dop	task_retries	error_id
8C4386DC...	820E9FC6...	1	2	...09:58:34.213	...09:58:36.337	1.031	2030	2343.310311	succeeded	Shuffle	1	1	0	0
8C4386DC...	820E9FC6...	2	0	...09:58:36.447	...09:58:39.713	1.891	9	7145.193579	succeeded	Return	1	1	0	0
C9524971...	680DCB55...	3	4	...10:05:46.747	...10:05:47.057	0.203	2030	2343.310311	succeeded	Shuffle	1	1	0	0
C9524971...	680DCB55...	4	0	...10:05:47.057	...10:05:48.480	1.406	0	6630.101167	succeeded	Return	1	1	0	0
FD2D17AD...	C9453EF2...	5	6	...11:58:54.060	...11:58:55.297	0.547	10	1534.098249	succeeded	ComputeToControlNode	1	1	0	0
FD2D17AD...	C9453EF2...	6	0	...11:58:55.297	...11:58:55.420	0.125	10	4.074902	succeeded	Return	1	1	0	0
9FB0A268...	CAA533DE...	7	8	...11:59:16.483	...11:59:16.700	0.203	2030	2343.310311	succeeded	Shuffle	1	1	0	0
9FB0A268...	CAA533DE...	8	0	...11:59:16.700	...11:59:18.640	1.922	6	7143.673151	succeeded	Return	1	1	0	0
1732AB0D...	AC1A4F10...	9	10	...11:59:25.950	...11:59:26.140	0.172	2030	2343.310311	succeeded	Shuffle	1	1	0	0
1732AB0D...	AC1A4F10...	10	0	...11:59:26.140	...11:59:27.450	1.297	9	6635.185797	succeeded	Return	1	1	0	0

Notes:
1) The foreign keys and dates (in the above and below queries) were truncated to accomodate all the important attributes in the snapshot of the values returned.
2) Based on the exisitng queries, there are two records for each executed statement, a Shuffle or ComputeToControlNode followed by a Return (see operation_type). In more complex scenario there are several Shuffles and Broadcasts and a Return. According to the Microsoft team, even if for serverless SQL pools there's no Data Movement Service (DMS), there's a similar algorithm responsible for moving the data between the nodes.
3) Because in serverless SQL pool each query has its own distribution statement id, the min, max, avg and total values will have the sames values across the columns. Therefore, the columns with redundant values were commented.
4) The Id of the request phase seems to have numeric values despite being defined as alphanumeric. I tried to cast the values to bigint for sorting purposes.

sys.dm_request_phases_task_group_stats

sys.dm_request_phases_task_group_stats stores metadata about the requests breakdown at task group:

-- Azure Serverless SQL pool: request phases breakdown at task group
SELECT TOP (100) RPT.dist_request_id
, TRY_CAST(RPT.id as bigint) id
, TRY_CAST(RPT.parent_ids as bigint) parent_ids
, RPT.dist_statement_id
, RPT.state_desc
, RPT.start_time
, RPT.end_time
, RPT.input_dop
, RPT.output_dop
, RPT.operation_type
, RPT.task_retries
FROM sys.dm_request_phases_task_group_stats RPT
ORDER BY id

dist_request_id	id	parent_ids	dist_statement_id	state_desc	start_time	end_time	input_dop	output_dop	operation_type	task_retries
820E9FC6...	1	2	8C4386DC...	succeeded	638098055142132551	638098055163382693	1	1	Shuffle	0
820E9FC6...	2	0	8C4386DC...	succeeded	638098055164476163	638098055197133001	1	1	Return	0
680DCB55...	3	4	C9524971...	succeeded	638098059467450021	638098059470574953	1	1	Shuffle	0
680DCB55...	4	0	C9524971...	succeeded	638098059470574953	638098059484793682	1	1	Return	0
C9453EF2...	5	6	FD2D17AD...	succeeded	638098127340607112	638098127352951067	1	1	ComputeToControlNode	0
C9453EF2...	6	0	FD2D17AD...	succeeded	638098127352951067	638098127354202970	1	1	Return	0
CAA533DE...	7	8	9FB0A268...	succeeded	638098127564826084	638098127567013504	1	1	Shuffle	0
CAA533DE...	8	0	9FB0A268...	succeeded	638098127567013504	638098127586388549	1	1	Return	0
AC1A4F10...	9	10	1732AB0D...	succeeded	638098127659513620	638098127661388514	1	1	Shuffle	0
AC1A4F10...	10	0	1732AB0D...	succeeded	638098127661388514	638098127674513601	1	1	Return	0

Notes:
1) The DVM seems to return the same number of records as sys.dm_request_phases.
2) Observe the format of the start_time and end_time, probably the timestamps come from the Spark cluster and were not translated into an SQL Server data type.

sys.dm_request_phases_exec_task_stats

sys.dm_request_phases_exec_task_stats stores metadata about the requests breakdown at task level:

-- Azure Serverless SQL pool: request phases breakdown at task
SELECT TOP (100) RPE.dist_request_id
, TRY_CAST(RPE.id as bigint) id
--, RPE.min_time_ms
--, RPE.max_time_ms
, RPE.avg_time_ms/1000.0 avg_time_sec
--, RPE.stdev_time_ms
, RPE.total_bytes_processed
--, RPE.min_rows
--, RPE.max_rows
--, RPE.avg_rows
--, RPE.stdev_rows
, RPE.total_rows
, RPE.error_id
FROM sys.dm_request_phases_exec_task_stats RPE
ORDER BY id

dist_request_id	id	avg_time_sec	total_kb_processed	total_rows	error_id
820E9FC6...	1	1.031	2343.310311	2030	0
820E9FC6...	2	1.891	7145.193579	9	0
680DCB55...	3	0.203	2343.310311	2030	0
680DCB55...	4	1.406	6630.101167	0	0
C9453EF2...	5	0.547	1534.098249	10	0
C9453EF2...	6	0.125	4.074902	10	0
CAA533DE...	7	0.203	2343.310311	2030	0
CAA533DE...	8	1.922	7143.673151	6	0
AC1A4F10...	9	0.172	2343.310311	2030	0
AC1A4F10...	10	1.297	6635.185797	9	0

What does all this mean?

The lack of documentation makes it challenging to interpret the values of the views besides the data and metadata they offer. In a paper on POLARIS, the code given to the serveless SQL pool engine, a taks is defined as "a careful packaging of data and query processing into units [...] that can be readily moved across compute nodes and re-started at the task level" [1]. Therefore, one can assume that this is the level targetted by the sys.dm_request_phases_exec_task_stats DMV. Further on, the tasks are grouped at phase level according to the sys.dm_request_phases_task_group_stats, the metadata from the two DMVs being further combined into sys.dm_request_phases DMV. 

If the meaning is kept from dedicated SQL pools, a shuffle operation indicates that data is moved between the frontend and backend nodes to satisfy a request, while a Result represents the operation of returning the result selt to client. The "ComputeToControlNode" operation involves a simple select (e.g. SELECT top 10) from a CETA and therefore no "Shuffle" is needed.

Requests' history

Further on, one can use the "Distributed statement id" to join the execution request phases with the request history, however matches will be found only for a small subset of the records (probably the executions since the pool started):

 

-- Azure Serverless SQL pool: requests history with request phase info
SELECT top 100 ERH.status
, ERH.transaction_Id
, ERH.distributed_statement_Id 
, ERH.query_hash 
--, ERH.login_name 
, ERH.start_time
, ERH.end_time 
, ERH.command 
, ERH.query_text 
--, ERH.total_elapsed_time_ms
, ERH.total_elapsed_time_ms/1000.0 total_elapsed_time_sec
--, ERH.data_processed_mb
, ERH.data_processed_mb
, RPH.avg_time_ms/1000.0 avg_time_sec
, RPH.total_rows
, RPH.total_bytes_processed/1028.0/1028.0 total_mb_processed
, RPH.state_desc
, RPH.operation_type
, RPH.input_dop
, RPH.output_dop
, RPH.task_retries
, RPH.error_id
, ERH.error
, ERH.error_code 
FROM sys.dm_exec_requests_history ERH
     JOIN sys.dm_request_phases RPH
	   ON ERH.distributed_statement_Id = RPH.dist_statement_id
	  --AND RPH.parent_ids = 0 -- only the parent
ORDER BY RPH.Id DESC

Here's a subset of the result set focusing only on the statistical values:

 

distr_statement_Id	start_time	end_time	total_elapsed_time_sec	data_processed_mb	avg_time_sec	total_rows	total_mb_processed	operation_type	id	parent_ids
{8C4386D...	...8:24.4300000	...8:39.8266666	15.396	10	1.031	2030	2.279484738326	Shuffle	1	2
{8C4386D...	...8:24.4300000	...8:39.8266666	15.396	10	1.891	9	6.950577411478	Return	2	0
{C952497...	...5:45.2100000	...5:48.4933333	3.283	10	0.203	2030	2.279484738326	Shuffle	3	4
{C952497...	...5:45.2100000	...5:48.4933333	3.283	10	1.406	0	6.449514753891	Return	4	0
{FD2D17A...	...8:52.1400000	...8:55.4166666	3.276	10	0.547	10	1.492313471789	ComputeToControlNode	5	6
{FD2D17A...	...8:52.1400000	...8:55.4166666	3.276	10	0.125	10	0.003963912451	Return	6	0
{9FB0A26...	...9:15.1300000	...9:18.6366666	3.506	10	0.203	2030	2.279484738326	Shuffle	7	8
{9FB0A26...	...9:15.1300000	...9:18.6366666	3.506	10	1.922	6	6.949098395914	Return	8	0
{1732AB0...	...9:24.6900000	...9:27.4500000	2.76	10	0.172	2030	2.279484738326	Shuffle	9	10
{1732AB0...	...9:24.6900000	...9:27.4500000	2.76	10	1.297	9	6.454460892023	Return	10	0

Notes:
As can be seen, the volume of data processed and the elapsed time values don't match between the two tables, though they are close. The differences probably result from further steps occuring in the process. 

Happy coding!

Previous Post  <<||>>  Next Post

References:
[1] Josep Aguilar-Saborit, Raghu Ramakrishnan et al, "POLARIS: The Distributed SQL Engine in Azure Synapse", VLDB Conferences. PVLDB, 13(12): 3204 – 3216, 2020, DOI: https://doi.org/10.14778/3415478.3415545