May 31, 2017
This article is the third in a three-part series to help you utilize the functionality and data in Snowflake’s Information Schema to better understand and effectively Snowflake.
As a Customer Success Engineer, my daily job entails helping our customers get the most value from our service. In my first post, I discussed getting a handle on your utilization of compute resources by using various Information Schema views and functions to profile your virtual warehouse usage. In my second post, I showed you how to get a handle on your storage usage.
In this final post, I will deep-dive into understanding query profiling. To do this, I will show you examples using the QUERY_HISTORY family of functions. I will also show you a handy page in the UI that provides a graphical view of each query. Keep in mind that you’ll need warehouse MONITOR privileges to perform the tasks described in this post. Typically, the SYSADMIN role has the necessary warehouse MONITOR privileges across your entire account; however, other lower-level roles may also have the necessary privileges.
Ready to get started? Here we go!
Query History Profiling
Query profiling is perhaps one of the more popular topics I field questions about. Many customers are interested in improving their query performance. Although every development team should strive to periodically refactor their code, many find it challenging to determine where to start. Going through this analysis should help with identifying a good starting point.
Let’s look at some syntax, per our documentation for QUERY_HISTORY:
select * from table(information_schema.query_history(dateadd('hours',-1, current_timestamp()),current_timestamp())) order by start_time;
This query provides a view into all of the queries run by the current user in the past hour:
We can also leverage the QUERY_HISTORY companion functions to narrow down your focus:
These are particularly useful if you have identified specific workflow issues you need to address.
Profiling Tip #1: Using HASH()
Now for a particularly useful tip: utilizing HASH on the QUERY_TEXT column can help you consolidate and group on similar queries (the HASH function will return the same result if any queries are exactly the same). As a general rule, identifying query groups and finding the max and min query runtime should help you sort through specific workflows. In the example below, I’m doing an analysis on average compile and execution time. Additionally, I’m collecting a count of the queries with the same syntax:
select hash(query_text), query_text, count(*), avg(compilation_time), avg(execution_time) from table(information_schema.query_history(dateadd('hours',-1,current_timestamp()),current_timestamp())) group by hash(query_text), query_text order by count(*) desc;
Using the HASH function further allows a user to easily query a particular instance of this query from the QUERY_HISTORY function. In the example above, I could check for specific queries where the HASH of the query text converted to the value
-102792116783286838. For example:
select * from table(information_schema.query_history()) where hash(query_text) = -102792116783286838 order by start_time;
The above result shows you all of the times you have issued this particular query (going back 7 days). Pay specific attention to the following columns:
- QUEUED (times)
If a query is spending more time compiling (COMPILATION_TIME) than executing (EXECUTION_TIME), perhaps it is time to review the complexity of the query. Snowflake’s query compiler will optimize your query and identify all of the resources required to perform the query in the most efficient manner. If a query is overly complex, the compiler needs to spend more time sorting through the query logic. Take a look at your query and see if there are many nested subqueries or unnecessary joins. Additionally, if there are more columns being selected than required, then perhaps be more specific in your SELECT statement by specifying certain columns.
QUEUED time is interesting because it could be an indicator about your warehouse size and the amount of workload you’ve placed on the warehouse. Snowflake is able to run concurrent queries and it does a very good job in doing so. However, there will be times when a particularly large query will require more resources and, thus, cause other queries to queue as they wait for compute resources to be freed up. If you see a lot of queries spending a long time in queue, you could either:
- Dedicate a warehouse to these large complex running queries, or
- Utilize Snowflake’s multi-clustering warehouse feature to allow more parallel execution of the queries. For more information about multi-cluster warehouses, see the Snowflake documentation.
In the recent updates to our QUERY_HISTORY_* Information Schema functions, we have added more metadata references to the results and now you should have a range of metadata at your disposal:
- USER_NAME , ROLE_NAME
- DATABASE_NAME , SCHEMA_NAME
- WAREHOUSE_NAME , WAREHOUSE_SIZE , WAREHOUSE_TYPE
These columns will help you identify the origin of the queries and help you fine tune your workflow. A simple example would be to find the warehouse with the longest-running queries. Or, find the user who typically issues these queries.
Profiling Tip #2: Using the UI
Once you have identified a particular query you would like to review, you can copy its QUERY_ID value and use this value to view its query plan in Snowflake’s Query Profile. To do this, click on the History icon, add a QUERY ID filter, and paste the QUERY_ID in question. For example:
Hint: If you don’t see a result, make sure you are using a role with the necessary warehouse MONITOR privilege (e.g. SYSADMIN or ACCOUNTADMIN) and you’ve selected the correct QUERY ID.
Once the search is complete, you should be able to click on the link provided under the Query ID column to go to the query’s detail page:
Now click on the Profile tab.
You should see a visualization of the Query Profile. In my example, Snowflake shows that this particular query executed in two steps:
The online documentation provides in-depth details on how to interpret this view. Pay particular attention to the orange bar in this view. It indicates the percentage of overall query time spent on this particular process. In this instance, we can see our query spent most of the time reading data from the table. If we run this query often enough, we should see this time decrease because we’ll be reading the data from cache instead of disk.
By utilizing the UI and the Information Schema functions and views described in this post, you can profile your queries to help you understand your current workflow and identify queries that can be better optimized. This will help save you money in the long run and also improve your user experience. Snowflake will continue to invest in tools like these to help our users better understand and use our platform.
To dig in some more on this subject, check out our online documentation:
I hope this article and this series gave you some inspiration for how you would like to manage your Snowflake instance. There are a lot of options to play with and they’re all intended to provide you with the flexibility and control you need to best use Snowflake. Please share your thoughts with us! We would love to help you on your journey to the cloud.
For more information, please feel free to reach out to us at email@example.com. And keep an eye on this blog or follow us on Twitter (@snowflakedb) to keep up with all the news and happenings here at Snowflake Computing.