> What is the impact of the inline CTEs? It depends greatly on the size of the data and the nature of the query inside the CTE. Sometimes it'll make no differe

> What is the impact of the inline CTEs? It depends greatly on the size of the data and the nature of the query inside the CTE. Sometimes it'll make no difference at all, of course. Sometimes it could mean the difference between scanning an entire table and being able to perform index seeks. As the result is materialised in earlier versions this could mean a fair chunk of write IO as well as read IO if the result is too large to just keep in RAM. Even worse for recursive CTEs the process could be repeated at every level of the recursive depth. Of course the query planner will still need to do that for bad queries or queries against structures that need better indexing, and there may be many situations out there where a better query/schema wasn't created because before v12 it would make no difference anyway so some rewriting is needed to take advantage of the new optimisation potential. If the innermost CTE contains filters that significantly limit the data and there are sufficient indexes to support that filtering, or if the data is small to start with, then this change will likely make little or no difference.