Reflection not working

Dremio University
1

I have a query and enabled raw reflection. However, I see this error message “Reflection cannot accelerate. 2 refresh jobs failed. Will reattempt”. Other queries are ok with reflection. How do I know what went wrong with this query?

2

@wz2020

Send us the job profile

Share a Dremio job profile

3

Here is the job profile642ab4db-5154-41fb-bf00-65d88e9947db.zip (11.0 KB)

4

@wz2020

You have sent me the job profile of the refresh job, you would probably need to send me the profile of the query that did not use this reflection

5

OK, how about this one? d61c313e-7b37-45fd-b012-19dee5b365f5.zip (9.9 KB)

6

@wz2020

I see the reflection is not considered but why are you running Dremio 3.2.4, its a very old version

7

Yes, I will upgrade Dremio shortly. But that aside, I just want to understand why the reflection works for some query not the other? How do I troubleshoot? Or are you implying this problem will go away once I upgrade?

8

Can you help me understand why reflection fails? Is there a document on reflection? How can I troubleshoot? Thanks.

9

Hi @wz2020

Sure, let me come back with some detailed explanation and what problem you are facing and how a more recent version can help

Thanks
@balaji.ramaswamy

10

@wz2020

You said there were other queries that used this reflection to accelerate, any chance you have that profile too? Want to understand your complete picture

Thanks

11

@wz2020

There are couple of phases in the reflection matching process. First, the reflection needs to match. This means the reflection should have all the required data to accelerate the query or a subset of the query. Then we do a costing algorithm to see if using the reflection is better than going back to the source

What I am not able to understand is that your query does not even consider the reflection, if you open the profile and click on the accelerations tab it says the below

Reflection Outcome

Query was NOT accelerated

Time To Find Reflections: 6 ms
Time To Canonicalize: 0 ms
Time To Match: 0 ms

I am wondering if your reflection refresh, even though the job was successful had an exception

Try the below,
New Query - select * from sys.reflections where reflection_id=‘00fecdf8-b637-4c2c-9a63-e47a8424822e’ and then click on the 3 dots (…) on the top right corner and click JSON and send us the file

We have completely changed the reflection algorithm since Dremio 3.2 and that is the reason I wanted you to upgrade so we are sure we are not hitting any old bug. In any case let us start here and then decide next steps

Thanks
Bali

Thanks
Bali

12

Using this reflection_id, the system returns nothing. It’s probably because I deleted that previous reflection. Here is a new one:

{“reflection_id”:“e3712173-f1fc-4c73-af94-61f0ce129960”,“name”:“Raw Reflection”,“type”:“RAW”,“status”:“CANNOT_ACCELERATE_SCHEDULED”,“num_failures”:1,“dataset”:“Insight.tc_vessel_shift”,“sortColumns”:“”,“partitionColumns”:“”,“distributionColumns”:“”,“dimensions”:“”,“measures”:“”,“displayColumns”:“ID$, PRIMARY_KEY_VAL$, TERM_ID$, CREATED_DATE$, SOURCE_SCN$, SQL_OPERATION$, BREAK_DURATION, BREAK_TIME, FINISH_DATE_TIME, MOVES_PER_HOUR, NAME, START_DATE_TIME, VESSEL_SHIFT_ID, VESSEL_VISIT_ID, dir0, dir1, dir2, dir3”,“externalReflection”:“”}

What I don’t understand is that Data Reflection is like creating an index, right? If so, whether or not it can help accelerate any query is irrelevant at the time of creation. It will be my job later to craft my query to take advantage of the reflection. But why did reflection creation fail in the first place?

13

@wz2020

I see the reflection creation failed (like the index creation failed), Do you have the profile of the REFRESH REFLECTION and also the LOAD MATERIALIZATION jobs? Also send us the server.log, right after the reflection creation job completed and there should be a clue there

Thanks
Bali

14

How do I get the profiles for REFRESH REFLECTION and also the LOAD MATERIALIZATION jobs?

My dremio instance doesn’t have a server.log file, only server.out. How do I enable server.log?

15

@wz2020

To get the profiles, do the below

Click on jobs, select filter UI, External Tools and uncheck everything other than “Accelerator”. You should find 2 profiles next to each other REFRESH REFLECTION & LOAD MATERIALIZATION. On the right side the dataset on which it was created would be displayed. Download profile on the right after clicking on the respective jobs

server.log might be going to standard out. Try using journalctl -u dremio.service

16

I only found the REFRESH REFLECTION885c5fbf-c0a3-452b-8b4b-2325defdd6b0.zip (11.0 KB) job and here is the profile.

17

Here is the server log. tc_vessel_shift.zip (6.0 KB)

18

@wz2020

This query ran at “2020-06-25 18:01:29” UTC but the logfile you sent me starts at “2020-06-25 18:55:10,580” UTC

19

It’s the same reflection I was trying to enable. I did several times. So the time may not match but the content should. Does it matter?

20

@wz2020

Got it, but the only issue is I am not sure if the error is for the right query ID, I do see a creation error so it could be from your other reflection creation job

In your jobs page search, can you search for “210b0ce7-71ff-ebe7-03f7-5d86aa273a00” and send me the profile

Thanks
Bali