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BrianHFASPS
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When to refresh a SQL Table?

For performance reasons I am trying to understand better when it is necessary to refresh a SQL table exactly? Lets say I have a table that is only used to patch into and do Lookup / Filter from. I am not using it in a Gallery or dropdown. Further the table could be patched into via multiple users simultaneously potentially. My question is does the LookUp / Filter always go back to SQL server or does it use a local cached copy of data? Do I need to do a Refresh esssentially anytime data changes?

 

As a bonus question is there anyway to optimize speed of Refresh since I can't limit its query essentially? I know I can use a view but since I don't use the full data set that way wasn't sure if that was best idea?

 

Thanks.

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My understanding is that when you issue a refresh on the datasource, PowerApps will check all the formulas (filters) that reference it and only pull back what is necessary (what matches the filters). *This is if your filters are delegable* - look out of the little warning triangles that indicate a filter (or sort or search) is not delegable. If you have any non-delegable filters, PowerApps will bring back all the records in the table up to the delegation limit in App Settings (500 by default, 2000 max). Be sure to crush all those warning triangles in your apps!

Delegable filters are processed by SQL Server (or relevant datasource) so only relevant rows are returned and with the Explicit Column Selection turned on, only relevant columns are returned.

For anything you can't delegate (e.g. complex calculations and joins), I'd opt for a View, but if your filter is delegable I'd just let PowerApps handle it.

Of course when you use a delegable filter in PowerApps the connector will be building the SQL statement that returns this data. Building your own (via a View and using hints) *may* be more efficient (especially if the query plan can be cached and re-used), but I would guess this is only going to be significant for very large datasets.

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PaulD1
Community Champion
Community Champion

Hi @BrianHFASPS

When you Patch to the table, obvisouly PowerApps is aware of that change and updates the local cache accordingly (assuming the Patch is succesful). I *think* that if the table is below a certain number of rows, then Patching will trigger a full refresh (i.e. you will see changes from other users). Over a certain number of rows and the full refresh would cause a performance hit, so there is not a full refresh. I last checked this some months ago, so best to test!

I often use Views. As the PowerApps connector for SQL does not support updating a View you always have to update the underlying table(s). In this case you will have to refresh the View every time you modify a record as PowerApps does not know the data in the View and the table are linked and so the local cache of the View is not updated.

To optimize the speed of a refresh, pull back as few rows as possible. If you display the data in a Gallery and your formula is delegable, PowerApps will initially bring back just 100 rows, as you scroll towards the bottom of that recordset, PowerApps will fetch the next hundred rows automatically.

There is an Advanded Setting (in App Settings) for 'Explicit column selection'. Turn this on so that you are only pulling in the columns actually referenced in the App (e.g. your table may have 10 columns but you only use 3, so only those 3 get pulled from the server) - this is especially important if some of your unused columns are 'large' (image, varchar(Max), etc.).

Thanks for info. My confusion is that the Refresh() command doesn't have any controls. So when you say


@PaulD1 wrote:

Hi @BrianHFASPS

 

To optimize the speed of a refresh, pull back as few rows as possible.


How do I accomplish that? I can't apply a filter on a refresh right?

 

Also it seems like I can get better performance when refresh / viewing if I am using well built views right? Then SQL "internally" does the heavy lifting potentially of filtering out rows / columns I don't need rather than pulling in data to local PowerApps cache. Also saving on data transfer time since that appears to be most expensive activity.

My understanding is that when you issue a refresh on the datasource, PowerApps will check all the formulas (filters) that reference it and only pull back what is necessary (what matches the filters). *This is if your filters are delegable* - look out of the little warning triangles that indicate a filter (or sort or search) is not delegable. If you have any non-delegable filters, PowerApps will bring back all the records in the table up to the delegation limit in App Settings (500 by default, 2000 max). Be sure to crush all those warning triangles in your apps!

Delegable filters are processed by SQL Server (or relevant datasource) so only relevant rows are returned and with the Explicit Column Selection turned on, only relevant columns are returned.

For anything you can't delegate (e.g. complex calculations and joins), I'd opt for a View, but if your filter is delegable I'd just let PowerApps handle it.

Of course when you use a delegable filter in PowerApps the connector will be building the SQL statement that returns this data. Building your own (via a View and using hints) *may* be more efficient (especially if the query plan can be cached and re-used), but I would guess this is only going to be significant for very large datasets.

It seems like I may be over thinking in my attempts at optimization and I should just let PowerApps and SQL do their things. As long as I build my filters efficiently and use delegation always. Thanks.

Anonymous
Not applicable

@PaulD1 Sorry for bringing up this old discussion, but I am finding no good information on this.

 

It looks like Refresh([SqlTable]) is actually bringing the top X records from that table even if the only use of that table is through filtered collections. (X being the configured limit of records returned by PowerApps).

 

In a simple app, I created 

  • A connection to an Azure SQL table
  • A button that has ClearCollect(colLines,Filter('[SqlTable]',Id=1000)); 
  • A button that has Refresh('[SqlTable]')

I monitored the network traffic on a browser, and clicked the two buttons. As expected, the first button returned a single record. The second button returned 500 records...

 

This seems completely crazy to me and pretty much rules out PowerApps entirely for any private data solutions. Is there a way to "correctly" refresh SQL data? 

 

EDIT: I'm going to post this as a new topic for more visibility as it's not actually fully related to the question of this topic

PaulD1
Community Champion
Community Champion

Hi @Anonymous 

Sorry for delayed response...

I believe each time you hit your button that performs the ClearCollect, your App will go and get the (latest) data from the SQL table, so in your scenario there is no need to issue a refresh. The refresh will not alter the data in your collection colLines (once populated that will remain static regardless of any refreshes until/unless you explicitly modify the collection, for example, by re-running the ClearCollect).

Refresh will rerun any formulas (not actions, like OnSelect, but things like the Items property of a gallery) that reference the refreshed table. It may also return the first 500 records (or up to your delegation limit) but not sure that is always the case.

Rather than monitoring via the network traffic on the browser, try using the performance monitor tool (click the screwdriver icon in the left hand pane to reveal the option) - that is a much easier way to determine what the App is actually doing in terms of data requests.

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