What is Sitecore Fast Query
Sitecore Fast Query is designed for retrieving and filtering items from the Sitecore database. Sitecore Fast Query uses the database engine to execute queries.
Known limitations :
- Fast query does not account for the context language
- Fast query does not support sorting
- In a fast query, attributes should not appear at the beginning of a predicate, directly after the opening square bracket character
- Fast query supports only the parent, child, ancestor, and descendant axes, the latter two only when the FastQueryDescendantsDisabled setting is false
- Fast query does not support standard values
- Fast query does not support functions, though it provides an alternative to the contains and therefore I assume the startswith() and endswith() functions
- Fast query does not support operators
- Fast query does not support subqueries, which are queries within predicates (square brackets –  characters)
- Fast Query works for data providers that inherit from the SqlDataProvider class and that support SQL queries generated by the QueryToSqlTranslator class
- Fast query does not support query catenation (using pipe delimiter to separate paths)
Should i continue using it ?
No, Fast Query has been deprecated in Sitecore 10.1. So, you shouldn’t use it anymore.
What it the alternative ?
- Content Search : use Sitecore indexes to search items.
- Sitecore Queries and Item API : use normal queries and Item API, both pass through application without using directly database engine to execute queries.