Common table expressions, aka CTEs, aka WITH queries, are not only the gateway to writing recursive SQL queries, but also help developers write maintainable SQL. WITH query clauses can help developers who are more comfortable writing in imperative languages to feel more comfortable writing SQL, as well as help reduce writing redundant code by reusing a particular common table expressions multiple times in a query.
A new patch, scheduled to be a part of PostgreSQL 12 major release later in the year, introduces the ability, under certain conditions, to inline common table expressions within a query. This is a huge feature: many developers could suddenly see their existing queries speed up significantly, and the ability to explicitly specify when to inline (i.e. the planner "substitutes" a reference to the CTE in the main query and can then optimize further) or, conversely, materialize (i.e. place the CTE into memory but lose out on certain planning & execution optimizations).
But why is this a big deal? Before we look into the future, first let's understand how WITH queries currently work in PostgreSQL.