[SalesForce] Governor Limit on number of child records fetched

I provided a solution to this as an answer to a similar question: Top 5 Opportunities for each user.

The best way to achieve this is to define a duplicate Owner lookup field on the Opportunity object (kept in sync with true-owner by a trigger, perhaps). This lookup field creates the missing child relationship Opportunities__r needed for a SOQL subquery, which you can have a LIMIT 5 on to just pull in the top 5 Opportunity records for each User.

The API isn't the same as Apex Code. In the API, you've a hard limit of 50,000,000 rows as an administrator, and up to approximately 1,000,000 rows as a non-administrator (because of sharing calculations, depending on the object and the object's row count). The query editor is an API call, not an Apex Code call, and not subject to the limits of Apex Code. Specifying read-only on an Apex Class grants a new limit of 1,000,000 rows, but prohibits DML operations. Additionally, specifying read-only on the Visualforce page grants higher loop/collection limits, but restricts DML operations. Coincidentally, you can call the REST API through Apex Code to skirt around these limitations, assuming the running user has API access. The developer console uses executeAnonymous for the Execute Apex Code window, but uses the normal REST API for the query editor, which explains the difference in behavior.