Power BI: Import vs DirectQuery

The differences between selecting Import and DirectQuery are the following:

Import – the selected tables and columns are imported into Power BI Desktop. As you create or interact with a visualization, Power BI Desktop uses the imported data. You must refresh the data, which imports the full data set again, to see any changes that occurred to the underlying data since the initial import or the most recent refresh.

DirectQuery – no data is imported or copied into Power BI Desktop. For relational sources, the selected tables and columns appear in the Fields list. For multi-dimensional sources like SAP Business Warehouse, the dimensions and measures of the selected cube appear in the Fields list. As you create or interact with a visualization, Power BI Desktop queries the underlying data source, which means you’re always viewing current data.

Many data modeling and data transformations are available when using DirectQuery, though with some limitations. When creating or interacting with a visualization, the underlying source must be queried and the time necessary to refresh the visualization is dependent on the performance of the underlying data source. When the data necessary to service the request has recently been requested, Power BI Desktop uses recent data to reduce the time required to display the visualization. Selecting Refresh from the Home ribbon will ensure all visualizations are refreshed with current data.

The Power BI and DirectQuery article describes DirectQuery in detail. Also, see the following sections for more information about benefits, limitations, and important considerations when using DirectQuery.

Benefits of using DirectQuery

There are a few benefits to using DirectQuery:

• DirectQuery lets you build visualizations over very large datasets, where it would otherwise be unfeasible to first import all of the data with pre-aggregation
• Underlying data changes can require a refresh of data, and for some reports, the need to display current data can require large data transfers, making re-importing data unfeasible. By contrast, DirectQuery reports always use current data
• The 1-GB dataset limitation does not apply to DirectQuery

Limitations of DirectQuery

There are currently a few limitations to using DirectQuery:

• All tables must come from a single database, unless using composite models
• If the Query Editor query is overly complex, an error will occur. To remedy the error you must either delete the problematic step in Query Editor, or Import the data instead of using DirectQuery. For multi-dimensional sources like SAP Business Warehouse, there is no Query Editor
• Relationship filtering is limited to a single direction, rather than both directions (though it is possible to enable cross filtering in both directions for DirectQuery as a Preview feature). For multi-dimensional sources like SAP Business Warehouse, there are no relationships defined in the model
• Time intelligence capabilities are not available in DirectQuery. For example, special treatment of date columns (year, quarter, month, day, so on) is not supported in DirectQuery mode.
• By default, limitations are placed on DAX expressions allowed in measures; see the following paragraph (after this bulleted list) for more information
• There is a one-million-row limit for returning data when using DirectQuery. The limit does not affect aggregations or calculations used to create the dataset returned using DirectQuery, only the rows returned. For example, you can aggregate 10 million rows with your query that runs on the data source, and accurately return the results of that aggregation to Power BI using DirectQuery as long as the data returned to Power BI is less than 1 million rows. If more than 1 million rows would be returned from DirectQuery, Power BI returns an error.

To ensure that queries sent to the underlying data source have acceptable performance, limitations are imposed on measures by default. Advanced users can choose to bypass this limitation by selecting File > Options and settings > Options and then DirectQuery, then selecting the option Allow unrestricted measures in DirectQuery mode. When that option is selected, any DAX expression that is valid for a measure can be used. Users must be aware, however, that some expressions that perform very well when the data is imported may result in very slow queries to the backend source when in DirectQuery mode.

Important considerations when using DirectQuery

The following three points should be taken into consideration when using DirectQuery:

• Performance and load – All DirectQuery requests are sent to the source database, so the time required to refresh a visual is dependent on how long that back-end source takes to respond with the results from the query (or queries). The recommended response time (with requested data being returned) for using DirectQuery for visuals is five seconds or less, with a maximum recommended results response time of 30 seconds. Any longer, and the experience of a user consuming the report becomes unacceptably poor. In addition, once a report is published to the Power BI service, any query that takes longer than a few minutes will time out, and the user will receive an error.Load on the source database should also be considered, based on the number of Power BI users who will consume the published report. Using Row Level Security (RLS) can have a significant impact as well; a non-RLS dashboard tile shared by multiple users results in a single query to the database, but using RLS on a dashboard tile usually means the refresh of a tile requires one query per user, thus significantly increasing load on the source database and potentially impacting performance.Power BI creates queries that are as efficient as possible. Under certain situations however, the generated query may not be efficient enough to avoid refresh that would fail. One example of this situation is when a generated query would retrieve an excessively large number of rows (more than 1 million) from the back-end data source, in which case the following error occurs:CopyThe resultset of a query to external data source has exceeded the maximum allowed size of '1000000' rows. This situation can occur with a simple chart that includes a very high cardinality column, with the aggregation option set to Don’t Summarize. The visual needs to only have columns with a cardinality below 1 million, or must have appropriate filters applied.
• Security – All users who consume a published report connect to the back-end data source using the credentials entered after publication to the Power BI service. This is the same situation as data that is imported: all users see the same data, irrespective of any security rules defined in the backend source. Customers who want per-user security implemented with DirectQuery sources should use RLS. Learn more about RLS.
• Supported features – Not all features in Power BI Desktop are supported in DirectQuery mode, or have some limitations. In addition, there are some capabilities in the Power BI service (such as Quick Insights) that are not available for datasets using DirectQuery. As such, the limitation of such features when using DirectQuery should be taken into consideration when determining whether to use DirectQuery.

Sources:

https://docs.microsoft.com/en-us/power-bi/desktop-use-directquery