One thing that caught me out is that if you are doing an operation on a string like "lower(email)" then the query planner will not use an index on the email col

One thing that caught me out is that if you are doing an operation on a string like "lower(email)" then the query planner will not use an index on the email column, instead you would need an index on "lower(email)", which is fine if you always access it in the same way but otherwise requires multiple indexes to get coverage in all your scenarios. There are also plenty of other weird planner choices which I can't work out but which were much easier to understand in SQL Server. Sometimes, the smallest change will stop using an index e.g. using LIMIT on a query can completely bypass an index.