In which situation is it impossible to create a custom query?

Creating a custom query is not feasible when needing to reference Apex or SOQL because these coding languages have their own specific syntax and structure which are not compatible with the standard way of creating a custom query. Apex is a strongly typed, object-oriented programming language that allows developers to execute flow and transaction control statements on the Salesforce.com server in conjunction with calls to the API. SOQL, or Salesforce Object Query Language, is specifically meant for querying data from Salesforce objects. Both of these require precise coding, which contrasts with the flexibility needed in a custom query format.

In contrast, simple filters allow for straightforward manipulation of data sets without needing custom query construction, smaller data sets don't inherently limit query creation, and standard report visualizations typically rely on defined structures rather than custom queries. Thus, the requirement to reference Apex or SOQL introduces a constraint that makes custom query creation impossible in those scenarios.