The following best practices will provide you with techniques to develop privacy-centric and performant queries.
Privacy and data accuracy
Develop queries on sandbox data
Best practice: Only query production data when you’re in production.
Utilize sandbox data during your query development whenever possible. Jobs using sandbox data don’t introduce additional opportunities for differential privacy checks to filter your query results. Additionally, because of the lack of privacy checks, sandbox queries run faster, allowing for more rapid iteration during query development.
If you have to develop queries on your actual data (such as when using match tables), to make it less likely that you overlap rows, choose date ranges and other parameters which are unlikely to overlap for each iteration of your query. Finally, run your query over the desired range of data.
Carefully consider historical results
Best practice: Decrease the likelihood of result-set overlap between recently run queries.
Bear in mind that the rate of change between query results will have an effect on how likely it is that results are omitted later on due to privacy checks. A second results set that closely resembles a recently returned results set is likely to be dropped.
Instead, modify key parameters in your query, such as date ranges or campaign IDs, to decrease the likelihood of significant overlap.
Optimize and understand joins
Best practice: Use a
LEFT JOIN instead of an
INNER JOIN to join clicks or conversions to impressions.
Not all impressions are associated with clicks or conversions. Therefore, if you
INNER JOIN clicks or conversions on impressions, impressions that aren’t tied to clicks or conversions will be filtered from your results.
Join some final results in BigQuery
Best practice: Avoid Ads Data Hub queries that join aggregated results. Instead, write 2 separate queries and join the results in BigQuery.
Rows that don’t meet aggregation requirements are filtered from your results. Therefore, if your query joins an insufficiently aggregated row with a sufficiently aggregated row, the resultant row will be filtered. Additionally, queries with multiple aggregations are less performant in Ads Data Hub.
You can join results (in BigQuery) from multiple aggregation queries (from Ads Data Hub). Results computed using common queries will share final schemas.
The following query takes individual Ads Data Hub results (
campaign_data_456) and joins them in BigQuery:
SELECT t1.campaign_id, t1.city, t1.X, t2.Y FROM `campaign_data_123` AS t1 FULL JOIN `campaign_data_456` AS t2 USING (campaign_id, city)
Use filtered row summaries
Best practice: Add filtered row summaries to your queries.
Filtered row summaries tally data that was filtered due to privacy checks. Data from filtered rows is summed and added to a catch-all row. While the filtered data can't be further analyzed, it provides a summary of how much data was filtered from the results.
Account for zeroed user IDs
Best practice: Account for zeroed user IDs in your results.
An end-user’s ID may be set to 0 for a number of reasons, including: opting out of ads personalization, regulatory reasons, etc. As such, data originating from multiple users will be keyed to a
user_id of 0.
If you want to understand data totals, such as total impressions or clicks, you should include these events. However, this data won’t be useful for deriving insights on customers, and should be filtered if you’re doing such analysis.
You can exclude this data from your results by adding
WHERE user_id != 0 to your queries.
Best practice: Avoid multiple layers of aggregation across users.
Queries that combine results that have already been aggregated, such as in the case of a query with multiple
GROUP BYs, or nested aggregation, require more resources to process.
Often, queries with multiple layers of aggregation can be broken up, improving performance. You should attempt to keep rows at the event or user level while processing and then combine with a single aggregation.
The following patterns should be avoided:
SELECT SUM(count) FROM (SELECT campaign_id, COUNT(0) AS count FROM ... GROUP BY 1)
Queries that use multiple layers of aggregation should be rewritten to use a single layer of aggregation.
(SELECT ... GROUP BY ... ) JOIN USING (...) (SELECT ... GROUP BY ... )
Queries that can easily be broken up should be broken up. You can join results in BigQuery.
Optimize for BigQuery
Generally, queries that do less perform better. When evaluating query performance, the amount of work required depends on the following factors:
- Input data and data sources (I/O): How many bytes does your query read?
- Communication between nodes (shuffling): How many bytes does your query pass to the next stage?
- Computation: How much CPU work does your query require?
- Outputs (materialization): How many bytes does your query write?
- Query anti-patterns: Are your queries following SQL best practices?
If query execution isn’t meeting your service level agreements, or you’re encountering errors due to resource exhaustion or timeout, consider:
- Using the results from previous queries instead of recomputing. For example, your weekly total could be the sum computed in BigQuery of 7 single day aggregate queries.
- Decomposing queries into logical subqueries (such as splitting multiple joins into multiple queries), or otherwise restricting the set of data being processed. You can combine results from individual jobs into a single dataset in BigQuery. Although this may help with resource exhaustion, it may slow down your query.
- If you’re running into resources exceeded errors in BigQuery, try using temp tables to split your query into multiple BigQuery queries.
- Referencing fewer tables in a single query, as this uses large amounts of memory and can cause your query to fail.
- Rewriting your queries such that they join fewer user tables.
- Rewriting your queries to avoid joining the same table back on itself.