Change the execution plan of query in postgresql manually?

Question

Is it possible to change the order of the operations of an execution plan manually in postgresql? E.g. if I always want to have the ordering operation before a filtering (although it doesn't make sense in a normal use of postgresql), is it possible to enforce that manually by e.g. changing the internal costs of an operation?

What about if I implement my own function? Is it possible to have such a function always being executed at the very end of the sql statement?

Answer 1

Use a subquery or CTE to force certain operations first. Like:

SELECT *
FROM  (
   SELECT *
   FROM   tbl
   LIMIT  10
   ) x
ORDER  BY 1;

You need to understand what you are doing, of course. In the example, I select 10 arbitrary rows and then order them by the first column.
You can use multiple layers of subqueries or multiple CTEs in a row.

Same example as CTE:

WITH x AS (
   SELECT *
   FROM   tbl
   LIMIT  10
   )
SELECT *
FROM   x
ORDER  BY 1;

A subquery is usually faster for simple queries, a CTE offers additional features (like reusing the same CTE in multiple places on different query levels).